1. Ankush Kumar, N S Vidhyadhiraja and G U Kulkarni,
Current distribution in conducting wire networks,
Journal of Applied Physics,
(2017).
In Press.
2. Wasim Raja Mondal, N S Vidhyadhiraja*, T Berlijn, Juana Moreno and Mark Jarrell,
Localization of phonons in mass disordered alloys A typical medium dynamical cluster approach,
Physical Review B,
96, 014203 (2017).
, Read abstract. The effect of disorder on lattice vibrational modes has been a topic of interest for several decades. In this work, we employ a Green's function based approach, namely the dynamical cluster approximation (DCA), to investigate phonons in mass disordered systems. Detailed benchmarks with previous exact calculations are used to validate the method in a wide parameter space. An extension of the method, namely the typical medium DCA (TMDCA), is used to study Anderson localization of phonons in three dimensions. We show that, for binary isotopic disorder, lighter impurities induce localized modes beyond the bandwidth of the host system, while heavier impurities lead to a partial localization of the low frequency acoustic modes. For a uniform (box) distribution of masses, the physical spectrum is shown to develop long tails comprising mostly localized modes. The mobility edge separating extended and localized modes, obtained through the TMDCA, agrees well with results from the transfer matrix method. A reentrance behavior of the mobility edge with increasing disorder is found that is similar to, but somewhat more pronounced than, the behavior in disordered electronic systems. Our work establishes a new computational approach, which recovers the thermodynamic limit, is versatile and computationally inexpensive, to investigate lattice vibrations in disordered lattice systems.
3. Y. Zhang, Y. F. Zhang, S. X. Yang, K.M. Tam, N. S. Vidhyadhiraja and M. Jarrell,
Calculation of twoparticle quantities in the typical medium dynamical cluster approximation,
Physical Review B,
95, 144208 (2017).
, Read abstract. The meanfield theory for disordered electron systems without interaction is widely and successfully used to describe equilibrium properties of materials over the whole range of disorder strengths. However, it fails to take into account the effects of quantum coherence and information of localization. Vertex corrections due to multiple backscatterings may drive the electrical conductivity to zero and make expansions around the mean field in strong disorder problematic. Here, we present a method for the calculation of twoparticle quantities which enables us to characterize the metalinsulator transitions in disordered electron systems by using the typical medium dynamical cluster approximation. We show how to include vertex corrections and information about the mobility edge in the typical meanfield theory. We successfully demonstrate the application of the developed method by showing that the conductivity formulated in this way properly characterizes the metalinsulator transition in disordered systems.
4. Nagamalleswararrao Dasari, N. S. Vidhyadhiraja, Mark Jarrell and Ross. H. Mckenzie,
Quantum critical local spin dynamics near the Mott metalinsulator transition in infinite dimensions,
Physical Review B,
95, 165105 (2017).
, Read abstract. Finding microscopic models for metallic states that exhibit quantum critical properties such as ω/T scaling is a major theoretical challenge. We calculate the local dynamical spin susceptibility χ(T,ω) for a Hubbard model at half filling using Dynamical MeanField Theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite temperature critical point. The signature of the latter is ω/T scaling where T is the temperature. Our results are consistent with previous results showing scaling of the dc electrical conductivity and are relevant to experiments on organic charge transfer salts.
5. Yi Zhang, R. Nelson, Elisha Siddiqui, K.M. Tam, U. Yu, T. Berlijn, W. Ku, N. S. Vidhyadhiraja, J. Moreno and M. Jarrell,
Generalized multiband typical medium dynamical cluster approximation: Application to (Ga,Mn)N,
Physical Review B,
94, 224208 (2016).
, Read abstract. We generalize the multiband typical medium dynamical cluster approximation and the formalism introduced by Blackman, Esterling, and Berk so that it can deal with localization in multiband disordered systems with both diagonal and offdiagonal disorder with complicated potentials. We also introduce an ansatz for the momentumresolved typical density of states that greatly improves the numerical stability of the method while preserving the independence of scattering events at different frequencies. Starting from the firstprinciples effective Hamiltonian, we apply this method to the diluted magnetic semiconductor Ga1−xMnxN, and find the impurity band is completely localized for Mn concentrations x<0.03, while for 0.03<x<0.10 the impurity band has delocalized states but the chemical potential resides at or above the mobility edge. So, the system is always insulating within the experimental compositional limit (x≈0.10) due to Anderson localization. However, for 0.03<x<0.10 hole doping could make the system metallic, allowing doubleexchange mediated, or enhanced, ferromagnetism. The developed method is expected to have a large impact on firstprinciples studies of Anderson localization.
6. Sudeshna Sen, Hanna Terletska, Juana Moreno, N. S. Vidhyadhiraja* and Mark Jarrell,
Local theory for MottAnderson localization,
Physical Review B,
94, 235104 (2016).
, Read abstract. The paramagnetic metallic phase of the AndersonHubbard model (AHM) is investigated using a nonperturbative local moment approach within the framework of dynamical meanfield theory with a typical medium. Our focus is on the breakdown of the metallic phase near the metalinsulators transition as seen in the singleparticle spectra, scattering rates, and the associated distribution of Kondo scales. We demonstrate the emergence of a universal, underlying lowenergy scale, TpeakK. This lies close to the peak of the distribution of Kondo scales obtained within the metallic phase of the paramagnetic AHM. Spectral dynamics for energies ω≲TpeakK display Fermi liquid universality crossing over to an incoherent universal dynamics for ω≫TpeakK in the scaling regime. Such universal dynamics indicate that within a local theory the low to moderately lowenergy physics is governed by an effective, disorder renormalized Kondo screening.
7. Nagamalleswararao Dasari, S. R. K. C. Sharma Yamijala, Manish Jain, T. Saha Dasgupta, Juana Moreno, Mark Jarrell and N. S. Vidhyadhiraja,
A first principles investigation of cubic BaRuO3: A Hund's metal,
Physical Review B,
94, 085143 (2016).
, Read abstract. A firstprinciples investigation of cubicBaRuO3, by combining density functional theory with dynamical meanfield theory and a hybridization expansion continuous time quantum MonteCarlo solver, has been carried out. Nonmagnetic calculations with appropriately chosen onsite Coulomb repulsion, U and Hund's exchange, J, for singleparticle dynamics and static susceptibility show that cubicBaRuO3 is in a spinfrozen(SF) state at temperatures above the ferromagnetic transition point. A strong red shift with increasing J of the peak in the real frequency dynamical susceptibility indicates a dramatic suppression of the Fermi liquid coherence scale as compared to the bare parameters in cubicBaRuO3. The selfenergy also shows clear deviation from Fermi liquid behaviour that manifests in the singleparticle spectrum. Such a clean separation of energy scales in this system provides scope for an incoherent SF phase, that extends over a wide temperature range, to manifest in nonFermi liquid behaviour and to be the precursor for the magnetically ordered ground state.
8. Nagamalleswararao Dasari, Wasim Raja Mondal, Peng Zhang, Juana Moreno, Mark Jarrell and N. S. Vidhyadhiraja,
A multiorbital iterated perturbation theory for model Hamiltonians and real materialspecific calculations of correlated systems,
European Physical Journal B,
89, 202 (2016).
pdf
, Read abstract. Perturbative schemes utilizing a spectral moment expansion are well known and extensively used for investigating the physics of model Hamiltonians and real material systems. The advantages they offer, in terms of being computationally inexpensive, with real frequency output at zero and finite temperatures, compensate for their deficiencies and offer a quick, qualitative analysis of the system behavior. In this work, we have developed a method, that can be classified as a multiorbital iterative perturbation theory (MOIPT) to study Nfold degenerate and non degenerate Anderson impurity models. As applications of the solver, we have combined the method with dynamical mean field theory to explore lattice models like the single orbital Hubbard model, covalent band insulator and the multiorbital Hubbard model for densitydensity type interactions in different parameter regimes. The Hund's coupling effects in case of multiple orbitals is also studied. The limitations and quality of results are gauged through extensive comparison with data from the numerically exact continuous time quantum Monte Carlo method (hybridization expansion CTQMC). In general we observe that the agreement with CTQMC results gets better as we move away from particlehole symmetry. We have integrated MOIPT with density functional theory based electronic structure methods to study real material systems. As a test case, we have studied the classic, strongly correlated electronic material, SrVO3. A comparison of density of states and photo emission spectrum (PES) with results obtained from different impurity solvers and experiments yields good agreement.
9. Sudeshna Sen and N. S. Vidhyadhiraja,
Quantum critical Mott transitions in a bilayer Kondo insulatormetal model system,
Physical Review B,
95, 155136 (2016).
, Read abstract. A bilayer system comprising a Kondo insulator coupled to a simple metal (KIM) is considered. Employing the framework of dynamical meanfield theory, the model system is shown to exhibit a surface of quantum critical points (QCPs) that separates a Kondo screened, Fermi liquid phase from a local moment, Mott insulating phase. The quantum critical nature of these Mott transitions is characterized by the vanishing of (a) the coherence scale on the Fermi liquid side, and (b) the Mott gap on the MI side. In contrast to the usual “largetosmall” Fermi surface (FS) QCPs in heavyfermion systems, the bilayer KIM system exhibits a complete FS destruction.
10. Swagata Acharya, Amal Medhi, N S Vidhyadhiraja and A Taraphder,
Feasibility of a metamagnetic transition in correlated systems,
Journal of Physics: Condensed Matter,
28, 116001 (2016).
, Read abstract. The longstanding issue of the competition between the magnetic field and the Kondo effect, favoring, respectively, triplet and singlet ground states, is addressed using a cluster slaverotor meanfield theory for the Hubbard model and its spincorrelated, spinfrustrated extensions in two dimensions. The metamagnetic jump is established and compared with earlier results of dynamical meanfield theory. This approach also reproduces the emergent superexchange energy scale in the insulating side. A scaling is found for the critical Zeeman field in terms of the intrinsic coherence scale just below the metal–insulator transition, where the critical spin fluctuations are soft. The conditions required for metamagnetism to appear at a reasonable field are also underlined. Gutzwiller analysis on the twodimensional Hubbard model and a quantum Monte Carlo calculation on the Heisenberg spin system are performed to check the limiting cases of the cluster slaverotor results for the Hubbard model. Lowfield scaling features for magnetization are discussed.
11. Shuo Liu, B. Phillabaum, E. W. Carlson, K. A. Dahmen, N. S. Vidhyadhiraja, M. M. Qazilbash and D. N. Basov,
Random Field Driven Spatial Complexity at the Mott Transition in VO_{2},
Physical Review letters,
116, 036401 (2016).
, Read abstract. We report the first application of critical cluster techniques to the Mott metalinsulator transition in vanadium dioxide. We show that the geometric universal properties of the metallic and insulating puddles observed by scanning nearfield infrared microscopy are consistent with the system passing near criticality of the random field Ising model as temperature is varied. The resulting large barriers to equilibrium may be the source of the unusually robust hysteresis phenomena associated with the metalinsulator transition in this system.
12. C. E. Ekuma, S.X. Yang, H. Terletska, K.M. Tam, N. S. Vidhyadhiraja, J. Moreno and M. Jarrell,
Metalinsulator transition in a weakly interacting disordered electron system,
Physical Review B,
92, 201114 (2015).
, Rapid Communication
13. C.E.Ekuma, C.Moore, H.Terletska, K.M.Tam, N.S.Vidhyadhiraja, J.Moreno and M.Jarrell,
Finite Cluster Typical Medium Theory for Disordered Electronic Systems,
Physical Review B,
92, 014209 (2015).
, Read abstract. We use the recently developed typical medium dynamical cluster (TMDCA) approach~[Ekuma \etal,~\textit{Phys. Rev. B \textbf{89}, 081107 (2014)}] to perform a detailed study of the Anderson localization transition in three dimensions for the Box, Gaussian, Lorentzian, and Binary disorder distributions, and benchmark them with exact numerical results. Utilizing the nonlocal hybridization function and the momentum resolved typical spectra to characterize the localization transition in three dimensions, we demonstrate the importance of both spatial correlations and a typical environment for the proper characterization of the localization transition in all the disorder distributions studied. As a function of increasing cluster size, the TMDCA systematically recovers the reentrance behavior of the mobility edge for disorder distributions with finite variance, obtaining the correct critical disorder strengths, and shows that the order parameter critical exponent for the Anderson localization transition is universal. The TMDCA is computationally efficient, requiring only a small cluster to obtain qualitative and quantitative data in good agreement with numerical exact results at a fraction of the computational cost. Our results demonstrate that the TMDCA provides a consistent and systematic description of the Anderson localization transition.
14. Sudeshna Sen, Juana Moreno, Mark Jarrell and N. S. Vidhyadhiraja,
Spectral changes in layered felectron systems induced by Kondo hole substitution in the boundary layer,
Physical Review B,
91, 155146 (2015).
, Read abstract. We investigate the effect of disorder on the dynamical spectrum of layered felectron systems. With random dilution of f sites in a single Kondo insulating layer, we explore the range and extent to which Kondo hole incoherence can penetrate into adjacent layers. We consider three cases of neighboring layers: band insulator, Kondo insulator, and simple metal. The disorderinduced spectral weight transfer, used here for quantification of the proximity effect, decays algebraically with distance from the boundary layer. Further, we show that the spectral weight transfer is highly dependent on the frequency range considered as well as the presence of interactions in the clean adjacent layers. The changes in the lowfrequency spectrum are very similar when the adjacent layers are either metallic or Kondo insulating, and hence are independent of interactions. In stark contrast, a distinct picture emerges for the spectral weight transfers across large energy scales. The spectral weight transfer over all energy scales is much higher when the adjacent layers are noninteracting as compared to when they are strongly interacting Kondo insulators. Thus, over all scales, interactions screen the disorder effects significantly. We discuss the possibility of a crossover from nonFermiliquid to Fermiliquid behavior upon increasing the ratio of clean to disordered layers in particlehole asymmetric systems.
15. Naushad Ahmad Kamar and N. S. Vidhyadhiraja,
Random local attraction driven metalsuperconductor transitions,
Journal of Physical Society of Japan,
84, 014704 (2015).
pdf
, Read abstract. In this paper, we investigate the disordered attractive Hubbard model
by combining dynamical mean field theory, coherent potential approximation and iterated perturbation theory for superconductivity as an impurity solver.
Disorder is introduced in the local attraction U. We assume that U is distributed according to a bimodal probability distribution, wherein an $x$ fraction of sites are pairing centers (U\neq0) and (1x) fraction of sites are noninteracting (U=0). It is found that beyond a critical x=x_{c}, a first order metalsuperconductor phase transition leads to
superconductivity being induced in the interacting as well as noninteracting sites.
16. Pramod Kumar and N. S. Vidhyadhiraja,
Kondohole substitution in heavy fermions: Dynamics and transport,
Physical Review B,
90, 235133 (2014).
, Read abstract. Kondohole substitution is a unique probe for exploring the interplay of
interactions, felectron dilution and disorder in heavy fermion materials. Within the diluted periodic Anderson model, we investigate the changes in singleparticle dynamics as well as response functions, as a function of Kondo hole concentration (x) and temperature. We show that the spectral weight transfers due to Kondo hole substitution has characteristics that are different from those induced by temperature; The dc resistivity crosses over from a highly nonmonotonic form with a coherence peak in the x→0 limit to a monotonic singleimpurity like form that saturates at low temperature. The thermopower exhibits a characteristic maximum as a function of temperature, the value of which changes sign with increasing x, and its location is shown to correspond to a low energy scale of the system. The Hall coefficient also changes sign with increasing x at zero temperature and is highly temperature dependent for all x. As x is increased beyond a certain x_{c}, the Drude peak and the midinfrared peak in the optical conductivity vanish almost completely; A peak in the optical scattering rate melts and disappears eventually. We discuss the abovementioned changes in the properties in terms of a crossover from coherent, Kondo lattice behaviour to single impurity like, incoherent behaviour with increasing x. A comparison of theory with experiments carried out for the dc resistivity and the thermopower of Ce_{1x}La_{x}B_{6} yields good agreement.
17. S. Koley, M. S. Laad, N. S. Vidhyadhiraja and A. Taraphder,
Preformed excitons, orbital selectivity, and chargedensitywave order,
Physical Review B,
90, 115146 (2014).
pdf
, Read abstract. Traditional routes to Charge Density Wave in transition metal dichalcogenides, relying
on Fermi surface nesting or JahnTeller instabilities have recently been
brought into question. While this calls for exploration of alternative
views, a paucity of theoretical guidance sustains lively controversy on the
origin of, and interplay between CDW and superconductive orders in
transition metal dichalcogenides. Here, we explore a preformed excitonic liquid
route, heavily supplemented by modern correlated electronic structure
calculations, to an excitonic CDW order in 1TTiSe_{2}. We show that
orbitalselective dynamical localisation arising from preformed excitonic liquid
correlations is somewhat reminiscent of states proposed for $d$ and $f$ band quantum
criticality at the border of magnetism. Excellent quantitative
explication of a wide range of spectral and transport
responses in both normal and CDW phases provides strong support
for our scenario, and suggests that soft excitonic liquid
fluctuations mediate superconductivity in a broad class of
transition metal dichalcogenides on the border of CDW. This brings the
transition metal dichalcogenides closer
to the bad actors (where the metallic state is clearly not a Fermi liquid)
in d and f band systems, where anomalously
soft fluctuations of electronic origin are believed to mediate
unconventional superconductivity on the border of magnetism.
18. Naushad Ahmad Kamar and N. S. Vidhyadhiraja,
Sitedisorder driven superconductorinsulator transition: A dynamical mean field study,
Journal of Physics: Condensed Matter,
26, 095701 (2014).
pdf
, Read abstract. We investigate the effect of sitedisorder on the superconducting (SC) state in the
attractive Hubbard model within the framework of dynamical mean field theory.
For a fixed interaction strength (U), the SC order parameter (OP) decreases monotonically with increasing disorder (x), while the singleparticle spectral gap (SG) decreases for small x,
reaches a minimum and keeps increasing for larger x. Thus, the system remains gapped beyond the destruction of the superconducting state, indicating a disorderdriven superconductorinsulator transition. We investigate this transition
in depth considering the effects of weak and strong disorder for a range of
interaction strengths.
In the clean case, the orderparameter is known to increase monotonically with increasing interaction, saturating
at a finite value asymptotically for U > \infty.
The presence of disorder results in destruction of superconductivity at large U, thus
drastically modifying the clean case behaviour. A physical understanding of our findings is obtained by invoking particlehole asymmetry and the probability distributions of the order parameter and spectral gap.
19. Pramod Kumar and N. S. Vidhyadhiraja,
Dynamics of valence fluctuations in the extended periodic Anderson model,
Journal of Physical Society of Japan (Suppl.),
(2013).
In Press. , Read abstract. We have investigated the valence transition in the extended periodic Anderson model (EPAM) within the
framework of dynamical mean field theory. Previous theoretical studies have indicated the presence of this transition in parameter ranges that are experimentally inaccessible. In this work, we show that, inclusion of dynamics beyond static meanfield brings the valence transition in physically relevant regimes. We show the strong dependence of the quasiparticleweight scale across the sharp valence crossover. The density of states also exhibit concomitant large scale spectral weight transfers, which can be observed in experiments.
20. N. S. Vidhyadhiraja and Pramod Kumar,
NonFermiliquid behavior from dynamical effects of impurity scattering in correlated Fermi liquids,
Physical Review B,
88, 195120 (2013).
pdf
, Read abstract. The interplay of disorder and interactions is a subject of perennial interest. In this work, we have investigated the effect of disorder due to chemical substitution on the dynamics and transport properties of correlated Fermi liquids. A low frequency analysis in the concentrated and dilute limits shows that the dynamical local potentials arising through disorder averaging generate a linear (in frequency) term
in the scattering rate. Such nonFermi liquid behaviour (nFL) is investigated in detail for Kondo hole substitution in heavy fermions within dynamical mean field theory (DMFT). We find closed form expressions for the dependence of the static and linear terms in the scattering rate on substitutional
disorder and model parameters. We argue that the low temperature resistivity will acquire a linear
in temperature term, and show that the Drude peak structure in the optical conductivity will disappear beyond a certain disorder p_{c}, that marks the crossover from lattice coherent to singleimpurity behaviour. A full numerical solution of the DMFT equations reveals that the nFL term will show up significantly only in certain regimes, although it is present for any nonzero disorder concentration in principle. We highlight the dramatic changes that occur in the quasiparticle scattering rate in the proximity of p_{c}. Remarkably, we find that the nFL behaviour due to dynamical effects of impurity scattering has features that are distinct from those arising through Griffiths singularities or distribution of Kondo scales. Relevance of our findings to experiments on alloyed correlated systems is pointed out.
21. Robert A. Sayer, Jeffrey D. Engerer, N. S. Vidhyadhiraja and Timothy S. Fisher,
Length and temperature dependent 1/f noise in vertical singlewalled carbon nanotube arrays
,
Journal of Applied Physics,
113, 144306 (2013).
pdf
, Read abstract. We report measurements of temperature and lengthdependent 1/f noise in vertical singlewalled
carbon nanotube (SWCNT) arrays. Carbon nanotubes are synthesized in a porous anodic alumina
template with submicrometer channel lengths ranging from 100 to 700 nm. A significant
difference is observed in the 1/f noise magnitude of quasiballistic and diffusive SWCNT devices,
with quasiballistic devices exhibiting 1/f noise levels that are one to two orders of magnitude less
than diffusively conducting devices. Furthermore, 1/f noise was measured from 90 to 400 K, and
the noise prefactor was found decreased significantly at temperatures below 250 K.
22. Sudeshna Sen and N. S. Vidhyadhiraja,
Protocols for characterising quantum transport through nanostructures,
Applied Physics Letters,
101, 133106 (2012).
pdf
, Read abstract. In this work, we have analysed the exact closedform solutions for transport quantities through a mesoscopic region which may be characterised by a polynomial functional of resonant transmission functions. These are then utilized to develop considerably improved protocols for parameters relevant for quantum transport through molecular junctions and quantum dots. The protocols are shown to be experimentally feasible and should yield the parameters at much higher resolution than the previously proposed ones.
23. Monojit Bag, N. S. Vidhyadhiraja and K. S. Narayan,
Fluctuations in Photocurrent of Bulk Heterojunction Polymer Solar Cells – A Valuable Tool to Understand Microscopic and Degradation Processes,
Applied Physics Letters,
101, 043903 (2012).
pdf
, Commentary: The first experimental observations of current noise in organic bulkheterojunction solar cells are reported and analysed. The noise spectrum has a usual 1/f^{a} form at low frequencies. At frequencies of ~ 810kHz, a broad maximum shows up. This intriguing feature is a direct evidence of hidden cyclostationary processes in the system. The maximum depends strongly on the background intensity, the extent of degradation and temperature. A kinetic Monte Carlo (KMC) simulation of charge transport in a disordered system has been carried out to understand the results. The inclusion of a transport level or a mobility edge is the new feature of simulations. The flicker noise is well reproduced, and the maximum is argued to arise from spacecharge effects. , Read abstract.
We report electrical noise measurements from illuminated bulk heterojunction (BHJ) polymer based solar cells (PSC). These noisestudies of BHJPSCs provide considerable insight into the underlying charge transport processes. The power spectrum reveals a flicker noise of the form 1/f^{a} at low frequencies ( f < 1 kHz) while an unusual lognormal feature is observed in the fregime > 5 kHz. Photocurrent fluctuations were analyzed at different temperature, light intensity and device conditions. A theoretical description employing kinetic MonteCarlo simulations points to the importance of trap distribution and kinetics in the understanding of fluctuations in the low f regime.
24. Nagamalleswararao Dasari, P Mandal, A Sundaresan and N S Vidhyadhiraja,
Weak ferromagnetism and magnetization reversal in YFe_{1x}Cr_{x}O_{3},
Europhysics Letters,
99, 17008 (2012).
pdf
, Commentary: YFe_{1x}Cr_{x}O_{3}_{ }is an unusual system. It is a solid solution of the Yttrium orthoferrite and orthochromite materials. The end points (YFeO_{3} and YCrO_{3}) are boring canted antiferromagnets with a monotonic temperature dependent magnetization. The alloy, however, shows rich and unexpected behaviour such as magnetization reversal and compensation point(s). It turns out that the origin of such surprising behaviour is a threeway DzyaloshinskiiMoriya interaction between FeFe, CrCr and FeCr. Given that the first two are in the same direction, but the third is opposite, the whole range of experimental behaviour can be accounted for, in a quantitative way, using a simple minimalist theoretical approach. Read on. , Read abstract. We present combined experimental and theoretical studies on the magnetic properties of a solid solution between yttrium orthoferrite and yttrium orthochromite systems, YFe_{1x}Cr_{x}O_{3} (0 <= x <= 1) where Fe^{3+} and Cr^{3+} ions are distributed randomly at the same crystallographic site (4b). We found that all the compositions exhibit weak ferromagnetism below the N\'eel temperature that decreases nonlinearly with increasing x, while certain intermediate compositions (x = 0.4,0.5) show a compensation point and magnetization reversal. This unusual behavior is explained based on a simple model comprising the isotropic superexchange and the antisymmetric DzyaloshinskiiMoriya interactions. This model explains the magnetization behavior in the entire range of doping and temperature including the magnetization reversal which results from an interplay of various DM interactions such as, FeOFe, CrOCr and FeOCr.
25. Pramod Kumar and N. S. Vidhyadhiraja,
From mixed valence to the Kondo lattice regime,
Journal of Physics: Condensed Matter,
23, 485601 (2011).
pdf
, Read abstract. Many heavy fermion materials are known to cross over from the Kondo lattice regime to the mixed valence regime or vice versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valence regime: a low temperature coherence peak and a high temperature 'Hubbard band' peak. These two peaks merge, yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity likewise exhibits an unusual absorption feature (shoulder) in the deep midinfrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport and of the effective flevel in the optical conductivity are shown to be responsible for the anomalous transport properties. A twoband hybridizationgap model, which neglects incoherent effects due to manybody scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed valence regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn_{5}, Ce_{2}Ni_{3}Si_{5}, CeFeGe_{3} and YbIr_{2}Si_{2}, (b) pressure dependent resistivity of YbInAu_{2} and CeCu_{6}, and (c) optical conductivity measurements in YbIr_{2}Si_{2} yields excellent agreement.
26. A. Taraphder, S. Koley, N. S. Vidhyadhiraja and Mukul. S. Laad,
Preformed excitonic liquid route to charge density wave in 2HTaSe 2,
Physical Review letters,
106, 236405 (2011).
pdf
, Read abstract. Recent experiments on 2HTaSe2 contradict the longheld view of the charge density wave arising from
a nested band structure. An intrinsically strong coupling view, involving a charge density wave state
arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated
alternative. Using the local density approximation plus multiorbital dynamic mean field theory, we show
that this scenario agrees with a variety of normal state data for 2HTaSe2 . Based thereupon, the ordered
states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic
liquid.
27. K. Mallikarjuna Rao, Bhuvana Thiruvelu, Radha. B, Narendra Kurra, N. S. Vidhyadhiraja and G. U. Kulkarni,
Metallic Conduction in NiS2 Nanocrystalline Structures.,
Journal of Physical Chemistry,
115, 10462  10467 (2011).
pdf
, Read abstract. Thermolysis of Ni butanethiolate in a sulfur atmosphere has been carried
out at 300 and 600 °C to produce NiS2 nanocrystalline films with mean crystallite sizes
of 16 and 192 nm and resistivity values of ∼451 and 3540 μΩ 3 m, respectively; the
resistivity values are ∼2 orders of magnitude less compared to the bulk value (12000
μΩ 3 m). This is a significant observation since NiS2 is an insulator in the bulk.
Combining electrostatic force microscopy, transport, and surface degradation experi
ments, we establish conclusively that the surface layer of NiS2 is metallic. The density of
the surface conduction states computed through the variable range hopping model is
found to be comparable to the bulk density of states of good metals. The precursor is
also amenable for patterning by electron beam and molding.
28. H. Barman and N. S. Vidhyadhiraja,
Transport and Spectra in the halffilled Hubbard model: A dynamical meanfield study.,
International Journal of Modern Physics B,
25, 2461  2479 (2011).
pdf
, Read abstract. We present an improved numerical implementation of the iterated perturbation theory,
for use as an impurity solver for lattice models within dynamical mean field theory
(DMFT). We demonstrate higher resolution of spectral and transport features and a re
duced computational expense. Using this implementation, we study the issues of scaling
and universality in spectral and transport properties of the particlehole symmetric Hub
bard model within DMFT. We reexamine experimental results for pressuredependent
resistivity in Selenium doped NiS2 and thermal hysteresis on V2 O3 and find qualita
tive agreement. A systematic study of spectral weight transfer in optical conductivity is
carried out.
29. Sabyasachi Mukhopadhyay, Rakesh Voggu, C. N. R. Rao, N. S. Vidhyadhiraja and K. S. Narayan,
Lateral photocurrent scanning of donor and acceptor polymers on graphene coated substrates,
Japanese Journal of Applied Physics,
50, 061602 (2011).
pdf
, Read abstract. Graphene provides a twodimensional surface which can be utilized to interface with a variety of molecular species to modify optoelectronic
processes. We use a scanning photocurrent technique to study the effect of graphenecoated anode substrates in semiconducting polymer device
structures. The approach involves the measurement of the spatially varying photocurrent generated by an active semiconductingpolymer film
sandwiched between a patterned substrate and a top electrode, where the narrowincident light beam scans regions beyond the overlapping
electrodes. We observe substantial difference in the spatial decay profile of the photocurrent upon introduction of graphene layers in the structure.
Using simple circuit model and spreading impedance analysis we discuss the modification in carrier transport and recombination processes by
underlying graphene layer in solar cell devices.
30. D. Parihari, N. S. Vidhyadhiraja and A. Taraphder,
Field dependent dynamics in the metallic regime of the halffilled Hubbard model,
Journal of Physics: Condensed Matter,
23, 055602 (2011).
pdf
, Read abstract. A systematic study of the effect of magnetic field (h) on the Hubbard model has been carried
out at halffilling within dynamical mean field theory. In agreement with previous studies, we
find a zero temperature itinerant metamagnetic transition, reflected in the discontinuous changes
in magnetization as well as in the hysteresis, from a paramagnetic (PM) metallic state to a
polarized quasiferromagnetic (QFM) state, at intermediate and large interaction strengths (U).
The jump in magnetization vanishes smoothly with decreasing interaction strength, and at a
critical U, the transition becomes continuous. The region of ‘coexistence’ of the PM and QFM
solutions in the field–U plane obtained in this study agrees quantitatively with recent numerical
renormalization group calculations, thus providing an important benchmark. We highlight the
changes in dynamics and quasiparticle weight across this transition. The effective mass
increases sharply as the transition is approached, exhibiting a cusplike singularity at the critical
field, and decreases with field monotonically beyond the transition. We conjecture that the first
order metamagnetic transition is a result of the competition between Kondo screening, that tries
to quench the local moments, and Zeeman coupling, which induces polarization and hence
promotes local moment formation. A comparison of our theoretical results with experiments on
3He indicate that a theory of 3He based on the halffilled Hubbard model places it in a regime of
intermediate interaction strength.
31. Abhay. K. Tiwari and N. S. Vidhyadhiraja,
Transient and steady state lateral charge transport in polymeric semiconductors.
,
Optical and Quantum Electronics (Springer),
41, 787  793 (2009).
pdf
32. D. Gupta, N. S. Vidhyadhiraja and K. S. Narayan,
Transport of Photogenerated Charge Carriers in Polymer Semiconductors,
Proceedings of the IEEE,
97, 1558  1569 (2009).
pdf
, Read abstract. The electrical transport of photogenerated charge carriers in disordered polymer semiconductors is reviewed. We emphasize that the mobility parameter in these disordered semiconducting systems is not a welldefined quantity. We highlight the utility of scanning probe photocurrent technique on variety of polymers in an asymmetricelectrode patterned configuration. The multiple length and time scales present in carrier transport processes are indicated by the large observed decay length scales in these systems.
33. N. S. Vidhyadhiraja, A. Macridin, C. Sen, M. Jarrell and Michael Ma,
Quantum Critical Point at Finite Doping in the 2D Hubbard Model: A Dynamical Cluster Quantum Monte Carlo Study,
Physical Review Letters,
102, 206407 (2009).
pdf
, Read abstract. We explore the Matsubara quasiparticle fraction and the pseudogap of the twodimensional Hubbard model with the dynamical cluster quantum Monte Carlo method. The character of the quasiparticle fraction changes from nonFermiliquid, to marginal Fermi liquid, to Fermi liquid as a function of doping, indicating the presence of a quantum critical point separating nonFermiliquid from Fermiliquid character. Marginal Fermiliquid character is found at low temperatures at a very narrow range of doping where the singleparticle density of states is also symmetric. At higher doping the character of the quasiparticle fraction is seen to cross over from Fermi liquid to marginal Fermi liquid as the temperature increases.
34. Debabrata Parihari, N S Vidhyadhiraja,
Magnetoresistance in paramagnetic heavy fermion metals,
Journal of Physics: Condensed Matter,
21, 405602 (2009).
pdf
, Read abstract. A theoretical study of magnetic field (h) effects on singleparticle spectra and the transport quantities of heavy fermion metals in the paramagnetic phase is carried out. We have employed a nonperturbative local moment approach (LMA) to the asymmetric periodic Anderson model within the dynamical mean field framework. The lattice coherence scale ω_{L}, which is proportional within the LMA to the spinflip energy scale, and has been shown in earlier studies to be the energy scale at which crossover to singleimpurity physics occurs, increases monotonically with increasing magnetic field. The many body Kondo resonance in the density of states at the Fermi level splits into two, with the splitting being proportional to the field itself. For h≥0, we demonstrate adiabatic continuity from the strongly interacting case to a corresponding noninteracting limit, thus establishing Fermi liquid behaviour for heavy fermion metals in the presence of a magnetic field. In the Kondo lattice regime, the theoretically computed magnetoresistance is found to be negative in the entire temperature range. We argue that such a result could be understood at by fieldinduced suppression of spinflip scattering and at through lattice coherence. The coherence peak in the heavy fermion resistivity diminishes and moves to higher temperatures with increasing field. Direct comparison of the theoretical results to the field dependent resistivity measurements in CeB_{6} yields good agreement.
35. Debabrata Parihari, N. S. Vidhyadhiraja and David E Logan,
Interplay between strong correlations and magnetic field in the symmetric periodic Anderson model,
Physical Review B,
78, 035128 (2008).
pdf
, Read abstract. Magneticfield effects in Kondo insulators are studied theoretically, using a localmoment approach to the
periodic Anderson model within the framework of dynamical meanfield theory. Our main focus is on field
induced changes in singleparticle dynamics and the associated hybridization gap in the density of states.
Particular emphasis is given to the strongly correlated regime, where the dynamics is found to exhibit universal
scaling in terms of a fielddependent lowenergy coherence scale. Although the bare applied field is globally
uniform, the effective fields experienced by the conduction electrons and the f electrons differ because of
correlation effects. A continuous insulatormetal transition is found to occur on increasing the applied field,
closure of the hybridization gap reflecting competition between Zeeman splitting, and screening of the
felectron local moments. For intermediate interaction strengths, the hybridization gap depends nonlinearly on
the applied field, while in strong coupling its field dependence is found to be linear. For the classic Kondo
insulator YbB12, good agreement is found upon direct comparison of the field evolution of the experimental
transport gap with the theoretical hybridization gap in the density of states.
36. D Kabra, J. Verma, N. S. Vidhyadhiraja and K. S. Narayan,
Model for Studies of Lateral Photovoltaic Effect in Polymeric Semiconductors,
IEEE Sensors Journal,
8, 1663  1671 (2008).
pdf
, Read abstract. We present here a discrete circuit spreading
impedance network model for highly disordered semicon
ducting polymeric Schottky devices. Qualitative arguments are
presented to correlate the parameters of the spreading function,
which represents the network connectivity in the model, to the
underlying morphology of the polymer. A detailed theoretical
study of the lateral charge transport in organic Schottky devices
is carried out using the model. We observe and analyze the
dependence of the lateral photovoltage (LPV) on the polymer
morphology, incident power, material specific Schottky junc
tion parameters and the modulation frequency. The model is
shown to account for a variety of experimental observations on
poly(3hexylthiophene) and poly[2methoxy,5(2ethylhexoxy)
1, 4phenylene vinylene]based position sensitive devices, while
being physically transparent and computationally efficient. Using
the model, we highlight those parameter regimes that would be
optimal for position sensing applications.
37. N S Vidhyadhiraja,
On the specific heat of heavy fermion systems using the periodic Anderson model,
Europhysics Letters,
77, 36001 (2007).
pdf
, Read abstract. The periodic Anderson model (PAM) represents a paradigm for understanding the
physics of heavyfermion (HF) compounds. In this paper, we study the specific heat of HF systems
using the PAM within the framework of dynamical meanfield theory. By examining the systematic
variation of specific heat as a function of particlehole asymmetry and coupling strength, with the
natural focus being on strong coupling, it is demonstrated that experimentally measured specific
heat of a variety of paramagnetic Kondo insulators such as YbB12 , SmB6 , FeSi, CeRhSb and HF
metals such as CeFeGe3 , YbAl3 and CeAl3 can be understood semiquantitatively using the PAM.
38. Anne Gilbert, N S Vidhyadhiraja and David E Logan,
Interaction effects in mixedvalent Kondo insulators,
Journal of Physics: Condensed Matter,
19, 106220 (2007).
pdf
, Read abstract. We study theoretically the class of mixedvalent Kondo insulators, employing
a recently developed local moment approach to heavy Fermion systems using
the asymmetric periodic Anderson model (PAM). Novel features in spectra
and transport, observable experimentally but lying outside the scope of the
symmetric PAM or the Kondo lattice model, emerge naturally within the present
theory. We argue in particular that a shoulderlike feature in the optical
conductivity, that is distinct from the usual midinfrared or direct gap peak
and has been observed experimentally in mixedvalent compounds such as
CeOs4 Sb12 and YbAl3 , is of intrinsic origin. Detailed comparison is made
between the resultant theory and transport/optical experiments on the filled
skutterudite compound CeOs4 Sb12 , and good agreement is obtained.
39. D. Kabra, S. Shriram, N. S. Vidhyadhiraja and K. S. Narayan,
Charge carrier dynamics in organic semiconductors by position dependent optical probing,
Journal of Applied Physics,
101, 64510 (2007).
pdf
, Read abstract. We investigate charge carrier transport in poly͑3hexylthiophene͒ films where sizable lateral
photovoltaic effects are observed. Spatiotemporal measurements of the lateral photovoltage ͑LPV͒
using a local optical probe are carried out on device structures consisting of the semiconducting
polymer film with a Schottkytype backcontact and a front pair of Ohmic contacts. We employ a
spreading impedance approach in the context of a discrete circuit element model to obtain a
quantitative understanding of the spatial dependence and the frequency response of the lateral
photovoltage. An excellent agreement between theoretical model and experiment is found, leading
to an understanding of the origin of LPV in organic systems.
40. D. Kabra, N. S. Vidhyadhiraja and K. S. Narayan,
Lateral Photovoltaic Effect in Conjugated Polymers Based Structures for Position Sensitive Detectors,
CintelliqOrganics Electronics Conference and Exhibition,
(2006).
41. N S Vidhyadhiraja and David E Logan,
Optical and transport properties of heavy fermions: theory compared to experiment,
Journal of Physics: Condensed Matter,
17, 2959  2976 (2005).
pdf
, IOP Top papers 2005 showcase  http://ej.iop.org/pdf/jpcm/2005_top_papers.pdf , Read abstract. Employing a local moment approach to the periodic Anderson model within
the framework of dynamical meanfield theory, direct comparison is made
between theory and experiment for the dc transport and optical conductivities of
paramagnetic heavy fermion and intermediate valence metals. Four materials,
exhibiting a diverse range of behaviour in their transport/optics, are analysed in
detail: CeB6 , YbAl3 , CeAl3 and CeCoIn5. Good agreement between theory and
experiment is in general found, even quantitatively, and a mutually consistent
picture of transport and optics results.
42. David E Logan and N S Vidhyadhiraja,
Dynamics and transport properties of heavy fermions: theory,
Journal of Physics: Condensed Matter,
17, 2935  2958 (2005).
pdf
, IOP Top papers 2005 showcase  http://ej.iop.org/pdf/jpcm/2005_top_papers.pdf , Read abstract. The paramagnetic phase of heavy fermion systems is investigated, using a
nonperturbative local moment approach to the asymmetric periodic Anderson
model within the framework of dynamical meanfield theory. The natural focus
is on the strong coupling Kondo lattice regime wherein singleparticle spectra,
scattering rates, dc transport and optics are found to exhibit (ω/ωL , T /ωL )
scaling in terms of a single underlying lowenergy coherence scale ωL .
Dynamics/transport on all relevant (ω, T ) scales are encompassed, from the
lowenergy behaviour characteristic of the lattice coherent Fermi liquid,through
incoherent effective singleimpurity physics likewise found to arise in the
universal scaling regime, to nonuniversal highenergy scales; and which
description in turn enables viable quantitative comparison to experiment.
43. N S Vidhyadhiraja and David E Logan,
Dynamics and scaling in the periodic Anderson model,
European Physical Journal B,
39, 313  334 (2004).
pdf
, Read abstract. The periodic Anderson model (PAM) captures the essential physics of heavy fermion materials.
Yet even for the paramagnetic metallic phase, a practicable manybody theory that can simultaneously
handle all energy scales while respecting the dictates of Fermi liquid theory at low energies, and all in
teraction strengths from the strongly correlated Kondo lattice through to weak coupling, has remained
quite elusive. Aspects of this problem are considered in the present paper where a nonperturbative local
moment approach (LMA) to singleparticle dynamics of the asymmetric PAM is developed within the
general framework of dynamical meanfield theory. All interaction strengths and energy scales are encom
passed, although our natural focus is the Kondo lattice regime of essentially localized f spins but general
conduction band filling, characterised by an exponentially small lattice coherence scale ωL . Particular em
phasis is given to the resultant universal scaling behaviour of dynamics in the Kondo lattice regime as an
entire function of ω = ω/ωL , including its dependence on conduction band filling, f level asymmetry and
lattice type. A rich description arises, encompassing both coherent Fermi liquid behaviour at lowω and
the crossover to effective singleimpurity scaling physics at higher energies — but still in the ω/ωL scaling
regime, and as such incompatible with the presence of twoscale ‘exhaustion’ physics, which is likewise
discussed.
44. N S Vidhyadhiraja, Victoria E Smith, David E Logan and H R Krishnamurthy,
Dynamics and transport properties of Kondo insulators,
Journal of Physics: Condensed Matter,
15, 4045  4087 (2003).
pdf
, Read abstract. A manybody theory of paramagnetic Kondo insulators is described, focusing
specifically on singleparticle dynamics, scattering rates, dc transport and
optical conductivities. This is achieved by development of a nonperturbative
local moment approach to the symmetric periodic Anderson model within
the framework of dynamical meanfield theory. Our natural focus is the
strongcoupling, Kondo lattice regime, in particular the resultant ‘universal’
scaling behaviour in terms of the single, exponentially small lowenergy scale
characteristic of the problem. Dynamics/transport on all relevant (ω, T )scales
are considered, from the gapped/activated behaviour characteristic of the low
temperature insulator through to explicit connection to singleimpurity physics
at high ω and/or T ; and for optical conductivities emphasis is given to the
nature of the optical gap, the temperature scale responsible for its destruction
and the consequent clear distinction between indirect and direct gap scales.
Using scaling, explicit comparison is also made to experimental results for dc
transport and optical conductivities of Ce3 Bi4 Pt3 , SmB6 and YbB12 . Good
agreement is found, even quantitatively; and a mutually consistent picture of
transport and optics results.
45. N S Vidhyadhiraja, A N TahvildarZadeh, M Jarrell and H R Krishnamurthy,
Exhaustion physics in the periodic Anderson model from iterated perturbation theory,
Europhysics Letters,
49, 459  465 (2000).
pdf
, Read abstract. We discuss the "exhaustion" problem in the context of the Periodic Anderson Model using Iterated Perturbation Theory(IPT) within the Dynamical Mean Field Theory. We find that, despite its limitations, IPT captures the exhaustion physics, which manifests itself as a dramatic, strongly energy dependent suppression of the effective Anderson impurity problem. As a consequence, low energy scales in the lattice case are strongly suppressed compared to the "Kondo scale" in the singleimpurity picture. The IPT results are in qualitative agreement with recent Quantum Monte Carlo results for the same problem.
