arXiv subjects

T. Maitra

Publications and source records attributed to T. Maitra.

At least 37 records · Page 2Linked to original sources

arXiv2018

Electronic structure of metallic tetra-boride $\textrm{TmB}_{\textrm{4}}$: An LDA+DMFT study

Recent experimental observations of magnetization plateau in metallic tetraboride $\textrm{TmB}_{4}$ have created a lot of interest in these class of materials. Hysteretic longitudinal resistance and anomalous Hall Effect are other remarkable features in the rare-earth tetraborides which represent experimental realizations of Archimedean Shastry-Sutherland (SSL) lattice. Electronic band structures, calculated under GGA and GGA+SO approximations, show that $\textrm{TmB}_{4}$ is a narrow band system with considerable correlation in its f-level. Strong correlation effects in this system are studied under single-site dynamical mean field theory (DMFT) [LDA+DMFT scheme] using multi-orbital generalization of iterated perturbation theory (MO-IPT). Pseudo-gap behaviour in spectral function and non-Fermi liquid behaviour of self-energy shows non-trivial strong correlation effects present in this geometrically frustrated metallic magnets. We also consider the extant, heather-to-neglected, strong atomic spin-orbit coupling (SOC) effects. While there is a significant change in the topology of the Fermi surface in the presence of SOC, the non-Fermi liquid behavior survives. The system can be modelled by an effective two orbital spinless Falicov-Kimball model together with two free band like states.

cond-mat.str-el↗

arXiv2017

Nature of the spiral state, electric polarisation and magnetic transitions in Sr-doped YBaCuFeO$_5$: A first-principles study

Contradictory results on the ferroelectric response of type II multiferroic YBaCuFeO$_{5}$, in its incommensurate phase, has of late, opened up a lively debate. There are ambiguous reports on the nature of the spiral magnetic state. Using first-principles DFT calculations for the parent compound within LSDA+U+SO approximation, the multiferroic response and the nature of spiral state is revealed. The helical spiral is found to be more stable below the transition temperature as spins prefer to lie in ab plane. The Dzyaloshinskii-Moriya (DM) interaction and the spin current mechanism were earlier invoked to account for the electric polarisation in this system. However, the DM interaction is found to be absent, spin current mechanism is not valid in the helical spiral state and there is no electric polarisation thereof. These results are in good agreement with the recent single-crystal data. We also investigate the magnetic transitions in YBa$_{1-x}$Sr$_x$CuFeO$_5$ for the entire range $0\le x\le 1$ of doping. The exchange interactions are estimated as a function of doping and a quantum Monte Carlo (QMC) calculation on an effective spin Hamiltonian shows that the paramagnetic to commensurate phase transition temperature increases with doping till $x=0.5$ and decreases beyond. Our observations are consistent with experimental findings.

cond-mat.mtrl-sci↗

arXiv2017

Hidden Fermi Liquidity and Topological Criticality in the Finite Temperature Kitaev Model

The fate of exotic spin liquid states with fractionalized excitations at finite temperature ($T$) is of great interest, since signatures of fractionalization manifest in finite-temperature ($T$) dynamics in real systems, above the tiny magnetic ordering scales. Here, we study a Jordan-Wigner fermionized Kitaev spin liquid at finite $T$ employing combined Exact diagonalization and Monte Carlo simulation methods. We uncover $(i)$ checkerboard or stripy-ordered flux crystals depending on density of flux, and $(ii)$ establish, surprisingly, that: $(a)$ the finite-$T$ version of the $T=0$ transition from a gapless to gapped phases in the Kitaev model is a Mott transition of the fermions, belonging to the two-dimensional Ising universality class. These transitions correspond to a topological transition between a string condensate and a dilute closed string state $(b)$ the Mott "insulator" phase is a precise realization of Laughlin's gossamer (here, p-wave) superconductor (g-SC), and $(c)$ the Kitaev Toric Code phase (TC) is a {\it fully} Gutzwiller-projected p-wave SC. These findings establish the finite-$T$ QSL phases in the $d = 2$ to be {\it hidden} Fermi liquid(s) of neutral fermions.

cond-mat.str-el↗

arXiv2016

Spin reorientation in NdFe$_{0.5}$Mn$_{0.5}$O$_{3}$: Neutron scattering and \emph{Ab-initio} study

The structural, magnetic, and electronic properties of NdFe$_{0.5}$Mn$_{0.5}$O$_3$ have been studied in detail using bulk magnetization, neutron/x-ray diffraction and first principles density functional theory calculations. The material crystallizes in the orthorhombic $Pbnm$ structure, where both Mn and Fe occupy the same crystallographic site ($4b$). Mn/Fe sublattice of the compound orders in to a G-type antiferromagnetic phase close to 250\,K where the magnetic structure belongs to $Γ_{1}$ irreducible representation with spins aligned along the crystallographic $b$ direction. This is unconventional in the sense that most of the orthoferrites and orthochromites order in the $Γ_{4}$ representation below the Néel temperature.This magnetic structure then undergoes a complete spin reorientation transition with temperature in the range 75\,K$\gtrsim$ T $\gtrsim$ 25\,K where the magnetic structure exists as a sum of two irreducible representations ($Γ_{1}$+$Γ_{2}$) as seen from neutron diffraction measurements. At 6\,K, the magnetic structure belongs entirely to $Γ_{2}$ representation with spins aligned antiferromagnetically along the crystallographic $c$ direction having a small ferromagnetic component ($F_x$). The unusual spin reorientation and correlation between magnetic ground state and electronic structure have been investigated using first principles calculations within GGA+U and GGA+U+SO formalisms.

cond-mat.str-el↗

arXiv2016

First-Principles Correlated Approach to the Normal State of Strontium Ruthenate

The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered Sr$_{2}$RuO$_{4}$. This mandates a detailed revisit of the normal state and, in particular, the $T$-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr$_{2}$RuO$_{4}$ and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund's metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system.

cond-mat.str-el↗

arXiv2015

Orbital Ordering in Fe$_{1-x}$Mn$_x$V$_2$O$_4$: A First Principles Study

Long range orbital order has been investigated in Fe$_{1-x}$Mn$_x$V$_2$O$_4$ as a function of doping (x) using first principles density functional theory calculations including the effects of Coulomb correlation and spin-orbit interaction within GGA+U and GGA+U+SO approximations. Through a detailed analysis of corresponding Wannier orbital projections of the Vanadium d bands, we have clearly established that for x$\le$0.6, the orbital order at V sites consists of a linear superposition of d$_{xz}$ and d$_{yz}$ orbitals of the type d$_{xz}\pm$d$_{yz}$. Within each ab-plane a ferro-orbital ordering of either d$_{xz}$+d$_{yz}$ or d$_{xz}$-d$_{yz}$ is observed which alternates in successive ab-planes along the c-direction. On the contrary, for x$>$0.6, it is the d$_{xz}$ or d$_{yz}$ orbital which orders at V sites in successive ab-planes along c-direction (so called A-type ordering). At Fe sites, we observe an orbital ordering of d$_{x^2-y^2}$ orbitals for x$\le$0.6 and d$_{z^2}$ orbitals for x$>$0.6. Effect of spin-orbit interaction on orbital ordering is found to be not significant in the entire range of doping studied.

cond-mat.str-el↗

arXiv2015

Study of ground state phases for spin-1/2 Falicov-Kimball model on a triangular lattice

The spin-dependent Falicov-Kimball model (FKM) is studied on a triangular lattice using numerical diagonalization technique and Monte-Carlo simulation algorithm. Magnetic properties have been explored for different values of parameters: on-site Coulomb correlation $U$, exchange interaction $J$ and filling of electrons. We have found that the ground state configurations exhibit long range Neèl order, ferromagnetism or a mixture of both as $J$ is varied. The magnetic moments of itinerant ($d$) and localized ($f$) electrons are also studied. For the one-fourth filling case we found no magnetic moment from $d$- and $f$-electrons for $U$ less than a critical value.

cond-mat.str-el↗

arXiv2014

Competing electronic states in high temperature phase of NaTiO$_2$

First principle density functional theory (DFT) calculations on the high temperature phase of layered triangular lattice system NaTiO$_2$ have revealed that there exists a collective electronic state energetically close to the ground state but with competing transport properties: the latter is metallic with partially occupied doubly degenerate e$_g^{\prime}$ orbitals whereas the former is insulating with a$_{1g}$ orbital fully occupied. Significant occupation of this excited state is possible at non zero temperature either thermally or thanks to very soft (large amplitude) oxygen vibrations. Possible explanations of the experimental low conductivity based on competing orbital transport and of the specific heat jump at a structural transition based on orbital entropy are discussed.

cond-mat.str-el↗

arXiv2013

Phase transitions in a spinless, extended Falicov-Kimball model on the triangular lattice

A numerical diagonalization technique with canonical Monte-Carlo simulation algorithm is used to study the phase transitions from low temperature (ordered) phase to high temperature (disordered) phase of spinless Falicov-Kimball model on a triangular lattice with correlated hopping ($t^{\prime}$). It is observed that the low temperature ordered phases (i.e. regular, bounded and segregated) persist up to a finite critical temperature ($T_{c}$). In addition, we observe that the critical temperature decreases with increasing the correlated hopping in regular and bounded phases whereas it increases in the segregated phase. Single and multi peak patterns seen in the temperature dependence of specific heat ($C_v$) and charge susceptibility ($χ$) for different values of parameters like on-site Coulomb correlation strength ($U$), correlated hopping ($t^{\prime}$) and filling of localized electrons ($n_{f}$) are also discussed.

cond-mat.str-el↗

arXiv2012

Effect of spin-orbit coupling on magnetic and orbital order in MgV$_2$O$_4$

Recent measurements on MgV$_2$O$_4$ single crystal have reignited the debate on the role of spin-orbit (SO) coupling in dictating the orbital order in Vanadium spinel systems. Density functional theory calculations were performed using the full-potential linearized augmented-plane-wave method within the local spin density approximation (LSDA), Coulomb correlated LSDA+U, and with SO interaction (LSDA+U+SO) to study the magnetic and orbital ordering in low temperature phase of MgV$_2$O$_4$. It is observed that the spin-orbit coupling in the experimentally observed antiferromagnetic phase, affects the orbital order differently in alternate V-atom chains along c-axis. This observation is found to be consistent with the experimental prediction that the effect of spin-orbit coupling is intermediate between that in case of ZnV$_2$O$_4$ and MnV$_2$O$_4$.

cond-mat.str-el↗

arXiv2011

Thermodynamic studies of the two dimensional Falicov-Kimball model on a triangular lattice

Thermodynamic properties of the spinless Falicov-Kimball model are studied on a triangular lattice using numerical diagonalization technique with Monte-Carlo simulation algorithm. Discontinuous metal-insulator transition is observed at finite temperature. Unlike the case of square lattice, here we observe that the finite temperature effect is not able to smear out the discontinuous metal-insulator transition seen in the ground state. Calculation of specific heat (C_v) shows single and double peak structures for different values of parameters like on-site correlation strength (U), f-electron energy (E_f) and temperature.

cond-mat.str-el↗

arXiv2011

Orbital order in NaTiO$_2$ : A first principles study

The debate over the orbital order in layered triangular lattice system NaTiO$_2$ has been rekindled by the recent experiments of McQueen, et al. \cite{cava} on NaVO$_2$ ({\em Phys. Rev. Lett.} {\bf 101}, 166402 (2008)). In view of this, the nature of orbital ordering, in both high and low temperature states, is studied using an ab-initio electronic structure calculation. The orbital order observed in our calculations in the low temperature structure of NaTiO$_2$ is consistent with the predictions of McQueen, et. al. An LDA plus dynamical mean-field calculation shows considerable transfer of spectral weight from the Fermi level but no metal- insulator transition, confirming the poor metallic behaviour observed in transport measurements.

cond-mat.str-el↗

arXiv2010

An extended Falicov-Kimball model on a triangular lattice

The combined effect of frustration and correlation in electrons is a matter of considerable interest of late. In this context a Falicov-Kimball model on a triangular lattice with two localized states, relevant for certain correlated systems, is considered. Making use of the local symmetries of the model, our numerical study reveals a number of orbital ordered ground states, tuned by the small changes in parameters while quantum fluctuations between the localized and extended states produce homogeneous mixed valence. The inversion symmetry of the Hamiltonian is broken by most of these ordered states leading to orbitally driven ferroelectricity. We demonstrate that there is no spontaneous symmetry breaking when the ground state is inhomogeneous. The study could be relevant for frustrated systems like $GdI_2$, $NaTiO_2$ (in its low temperature C2/m phase) where two Mott localized states couple to a conduction band.

cond-mat.str-el↗

arXiv2010

Comparative study of FeCr2S4 and FeSc2S4: Spinels with orbitally active A site

Using first-principles density functional calculations, we perform a comparative study of two Fe based spinel compounds, FeCr2S4 and FeSc2S4. Though both systems contain an orbitally active A site with an Fe2+ ion, their properties are rather dissimilar. Our study unravels the microscopic origin of their behavior driven by the differences in hybridization of Fe d states with Cr/Sc d states and S p states in the two cases. This leads to important differences in the nature of the magnetic exchanges as well as the nearest versus next nearest neighbor exchange parameter ratios, resulting into significant frustration effects in FeSc2S4 which are absent in FeCr2S4.

cond-mat.str-el↗

arXiv2010

On the question of metallicity and ferromagnetism in charge ordered insulating Nd0.5Sr0.5MnO3 nanoparticles

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We, revisit this issue and discuss the overwhelming evidence from experiments on nano and bulk manganites as well as the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological understanding on the basis of enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanoparticles of Nd0.5Sr0.5MnO3. We also provide supportive evidence from an ab-initio electron structure calculation and a recent numerical simulation and argue that the mechanism is universal in all nanosize charge ordered manganites.

cond-mat.str-el↗

arXiv2010

Ground state phase diagram of a spinless, extended Falicov-Kimball model on the triangular lattice

Correlated systems with hexagonal layered structures have come to fore with renewed interest in Cobaltates, transition-metal dichalcogenides and GdI2. While superconductivity, unusual metal and possible exotic states (prevented from long range order by strong local fluctuations) appear to come from frustration and correlation working in tandem in such systems, they freeze at lower temperature to crystalline states. The underlying effective Hamiltonian in some of these systems is believed to be the Falicov-Kimball model and therefore, a thorough study of the ground state of this model and its extended version on a non-bipartite lattice is important. Using a Monte Carlo search algorithm, we identify a large number of different possible ground states with charge order as well as valence and metal-insulator transitions. Such competing states, close in energy, give rise to the complex charge order and other broken symmetry structures as well as phase segregations observed in the ground state of these systems.

cond-mat.str-el↗

arXiv2009

Proposed Orbital Ordering in MnV2O4 from First-principles Calculations

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation (LSDA)+U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favours a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

cond-mat.str-el↗

arXiv2006

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2 and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.

cond-mat.other↗