arXiv subjects

N. Mohanta

Publications and source records attributed to N. Mohanta.

14 recordsLinked to original sources

arXiv2022

Interplay of magnetism and band topology in Eu$_{1-x}$Ca$_x$Mg$_2$Bi$_2$ (x=0, 0.5) from first principles study

Recent discovery of the time reversal symmetry breaking magnetic Weyl semimetals has created a huge surge of activities in the field of quantum topological materials. In this work, we have studied systematically the ground state magnetic order, electronic structure and the interplay between the magnetic order and band topology in one such materials, EuMg$_2$Bi$_2$ (EMB) and its Ca doped variant using first principles method within the framework of density functional theory (DFT). The detailed investigation unravels the existence of different topological phases in this single material which can be tuned by an external probe such as magnetic field or chemical substitution. Our DFT calculations including Coulomb correlation (U) and spin-orbit (SO) interaction within GGA+U+SO approximation confirms that the magnetic ground state of EMB is A-type Antiferromagnetic (A-AFM) with Eu magnetic moments aligned along the crystallographic $a$ or $b$ direction. Although the ground state of EMB is A-AFM, the Ferromagnetic (FM) state lies very close in energy. We observe a single pair of Weyl points connecting valence and conduction band very close to the Fermi level (FL) along $Γ$-A direction in the FM state of EuMg$_2$Bi$_2$ with Eu moments aligned along crystallographic $c$ direction. On doping 50\% Ca at Eu sites, we observe single pair of Weyl points moving closer to the FL which is highly desirable for application purposes. Further we observe that the separation between the Weyl points in the pair decreases in doped compound compared to that in the parent compound which has direct consequence on anomalous Hall conductivity (AHC). Our first principles calculation of AHC shows high peak values exactly at these Weyl points and the peak height decreases when we dope the system with Ca. Therefore, Ca doping can be a good external handle to tune AHC in this system.

cond-mat.mtrl-sci↗

arXiv2022

Spin sensitive transport in RuCl_3/Pt heterostructures: revealing a field-transverse spin anisotropy in a spin liquid candidate

Alpha-phase (a-) RuCl_3 has emerged as a possible candidate for a quantum spin liquid (QSL) that promises exotic quasi-particles relevant for fault-tolerant quantum computation. Here, we report spin-sensitive transport measurements using a proximal spin Hall metal -- platinum (Pt) -- to probe magnetic moments in the insulator a-RuCl_3. We observe a spin Hall magnetoresistance (SMR) where both the transverse and longitudinal resistivities of a-RuCl_3/Pt exhibit oscillations as functions of the angle between the in-plane magnetic field and the current, arising from the interplay between the spin Hall effect in Pt and local magnetic moments in a-RuCl3. The oscillations are observed from 1.5 T to 18 T, spanning the range of magnetic fields where zig-zag antiferromagnetic phase, the putative QSL phase, and the partially field-polarized states are reported in a-RuCl_3. The phase of the SMR oscillations suggest that the local moments, regardless of static or fluctuating, in a-RuCl_3 develop an anisotropy with an in-plane quantization axis that aligns largely transverse to the magnetic field for all fields above 1.5 T. Temperature dependance of the SMR revealed that the spin anisotropy has a similar energy scale to reported QSL signatures in a-RuCl_3. The coupling between the spin states within a-RuCl_3 and Pt demonstrated in our experiment opens a transport route to exploring exotic spin phases and device functionalities of QSL materials.

cond-mat.str-el↗

arXiv2020

Charge Density Wave and Superconductivity in Transition Metal Dichalcogenides

Competing orders in condensed matter give rise to the emergence of fascinating, new phenomena. Here, we investigate the competition between superconductivity and charge density wave in the context of layered-metallic compounds, transition metal dichalcogenides, in which the superconducting state is usually suppressed by the charge density wave. We show, using real-space self-consistent Bogoliubov-de Gennes calculations and momentum-space calculations involving density-functional theory and dynamical mean-field theory, that there is a surprising reappearance of superconductivity in the presence of non-magnetic disorder fluctuations, as observed in recent experiments.

cond-mat.str-el↗

arXiv2017

Observation of transient superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

We report the observation of a magnetic field assisted transient superconducting state in the two dimensional electron gas existing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The co-existence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism.

cond-mat.str-el↗

arXiv2016

Anomalous transport near the Lifshitz transition at the LaAlO$_3$/SrTiO$_3$ interface

The two-dimensional electron liquid, at the (001) interface between band insulators {L}a{A}l{O}$_3$ and {S}r{T}i{O}$_3$, undergoes Lifshitz transition as the interface is doped with carriers. At a critical carrier density, two new orbitals populate at the Fermi level, with a concomitant change in the Fermi surface topology. Using dynamical mean-field theory, formulated within a realistic three-orbital model, we study the influence of the Lifshitz transition and local electron correlations on the transport properties. We look at the thermal conductivity, optical conductivity, Seebeck coefficient and angle resolved photoemission spectra and find that at a critical density, both the thermal and dc conductivities rise sharply to higher values while the Seebeck coefficient shows a cusp. The inter-orbital electron-electron interaction transfers spectral weight near the $Γ$ point towards lower energy, thereby reducing the critical density. In the presence of external magnetic field, the critical density further reduces due to exchange splitting. Beyond a sufficiently large field, multiple cusps appear in the Seebeck coefficient revealing multiple Lifshitz transitions.

cond-mat.str-el↗

arXiv2016

Correlated Non-Gaussian phase fluctuations in LaAlO$_3$/SrTiO$_3$ heterointerface

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two-dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT-transition in two-dimensional inhomogeneous superconductors.

cond-mat.supr-con↗

arXiv2016

Influence of structural disorder and Coulomb interactions in the superconductor-insulator transition applied to boron doped diamond

The influence of disorder, both structural (non-diagonal) and on-site (diagonal), is studied through the inhomogeneous Bogoliubov-de Gennes (BdG) theory in narrow-band disordered superconductors with a view towards understanding superconductivity in boron doped diamond (BDD) and boron- doped nanocrystalline diamond (BNCD) films. We employ the attractive Hubbard model within the mean field approximation, including the Coulomb interaction between holes in the narrow acceptor band. We study substitutional boron incorporation in a triangular lattice, with disorder in the form of random potential fluctuations at the boron sites. The role of structural disorder was studied through non-uniform variation of the tight-binding coupling parameter where, following ex- perimental findings, we incorporate the concurrent increase in structural disorder with increasing boron concentration. We illustrate stark differences between the effects of structural and on-site disorder and show that structural disorder has a much greater effect on the density of states, mean pairing amplitude and superfluid density than on-site potential disorder. We show that structural disorder can increase the mean pairing amplitude while the spectral gap in the density of states decreases with states eventually appearing within the spectral gap for high levels of disorder. This study illustrates how the effects of structural disorder can explain some of the features found in superconducting BDD and BNCD films such as a tendency towards saturation of the T_{c} with boron doping and deviations from the expected BCS theory in the temperature dependence of the pairing amplitude and spectral gap.

cond-mat.supr-con↗

arXiv2016

Edwards polaron formation : From one to three dimension

Employing a self-consistent (optimized) variational diagonalization scheme, we investigate the formation of polaronic quasiparticles in a spinless fermion-boson transport model that couples the movement of charge carriers to fluctuations and correlations of a background medium. The background is parameterized by bosonic degrees of freedom. The variational fermion-boson Hilbert space is constructed to achieve high accuracy in one to three spatial dimensions with modest computational requirements. To characterize the ground-state properties of the Edwards model in the single-particle sector, we present exact numerical results for the polaron band dispersion, quasiparticle weight, Drude weight, mass enhancement, and the particle-boson correlations in a wide parameter regime. In the Edwards model, transport will be quasifree, diffusive or boson-assisted in the weakly fermion-boson coupled, fluctuation-dominated or strongly correlated regimes, respectively. Thereby correlated transport is not only assisted but also limited by the bosonic excitations. As a result, the Drude weight remains finite even in the limit of very small boson frequencies. For a strongly correlated background, closed loops are important, in any dimension, to generate a finite effective particle mass even when the free fermion has an infinite mass.

cond-mat.str-el↗

arXiv2015

Multi-band theory of superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

We present a multi-band model for superconductivity at the metallic interface between insulating oxides LaAlO$_3$ and SrTiO$_3$ (001). Using a self-consistent Bogoliubov-de Gennes theory, formulated with the realistic bands at the interface, we investigate the spin-singlet and spin-triplet pairings in intra-band and inter-band channels. We find that the Rashba and atomic spin-orbit interactions at the interface induce singlet pairing in the inter-band channel and triplet pairing in both the intra-band and inter-band channels when the pairing amplitude in the singlet intra-band channel is finite. The gate-voltage variation of superconductivity is resolved in different pairing channels, compared with experimental results and found to match quite well. Interestingly, an enhancement of the superconducting transition temperature by external in-plane magnetic field is found revealing the existence of a hidden superconducting state above the observed one. As the interface is known to possess high level of inhomogeneity, we explore the role of non-magnetic disorder incorporating thermal phase fluctuations by using a Monte-Carlo method. We show that even after the transition to the non-superconducting phase, driven by temperature or magnetic field, the interface possesses localized Cooper pairs whose signature was observed in previous experiments.

cond-mat.supr-con↗

arXiv2014

Topological superconductivity and Majorana bound states at the LaAlO$_3$/SrTiO$_3$ interface

The interface between two band insulators LaAlO$_3$ and SrTiO$_3$ exhibits low-temperature superconductivity coexisting with an in-plane ferromagnetic order. We show that topological superconductivity hosting Majorana bound states can be induced at the interface by applying a magnetic field perpendicular to the interface. We find that the dephasing effect of the in-plane magnetization on the topological superconducting state can be overcomed by tuning a gate-voltage. We analyze the vortex-core excitations showing the zero-energy Majorana bound states and the effect of non-magnetic disorder on them. Finally, we propose an experimental geometry where such topological excitations can be realized.

cond-mat.supr-con↗

arXiv2014

Emergent Spin Hall phase at a Lifshitz transition from competing orders

The effects of competing orders, such as superconductivity and ferromagnetism, on a Fermi liquid are well established. A comprehensive understanding of such a competition in a metal whose Fermi surface has a non-trivial topology is yet to be achieved. Here, we address this question in a prototypical system: the 2D Rashba semimetal. We show that dominant superconductivity interplays with Rashba spin orbit interactions (SOI) in forming a novel intrinsic anomalous Hall effect (AHE) with gapless edge states of Bogoliubov-de Gennes (BdG) quasiparticles. As in the case of itinerant ferromagnets, the intrinsic AHE arises from Berry curvature effects in the band structure. This phenomenon is robust even as sub-dominant ferromagnetism dramatically changes the nature of pairing symmetry. An emergent spin Hall phase involving a change in Fermi-surface topology is found to accompany this Lifshitz quantum phase transition. We demonstrate the coexistence of the original and novel AHE in the presence of weak disorder. We offer a comparison of our results with experiments on the two dimensional electron gas at oxide hetero-interfaces as well as make some testable predictions.

cond-mat.supr-con↗

arXiv2014

Charge-Density-Wave Order in 2H-NbSe$_{2}$

Competition between collective states like charge density wave and superconductivity is played out in some of the transition metal dichalcogenides unencumbered by the spin degrees of freedom. Although 2H-NbSe$_2$ has received much less attention than some of the other members of the family (like 1T-TiSe$_2$ and 2H-TaSe$_2$), it shows superconductivity at 7.2 K and incommensurate charge ordering at 33 K. Recent experiments, notably Angle Resolved Photoemission spectroscopy, have cast serious doubts on the mechanism of Fermi surface nesting via electron-phonon interaction. The normal state has been found to be a poor, incoherent metal and remarkably, the coherence increases in the broken symmetry state. From a preformed excitonic liquid scenario, we show that there exists a natural understanding of the experimental data on 2H-NbSe$_2$ based on electron-electron interaction. The collective instabilities, in this scenario, are viewed as a condensation of an incoherent excitonic liquid already present at high temperature.

cond-mat.str-el↗

arXiv2013

Phase segregation of superconductivity and ferromagnetism at LaAlO$_3$/SrTiO$_3$ interface

The highly conductive two-dimensional electron gas formed at the interface between insulating SrTiO$_3$ and LaAlO$_3$ shows low-temperature superconductivity coexisting with inhomogeneous ferromagnetism. The Rashba spin-orbit interaction with in-plane Zeeman field of the system favors $p_x \pm i p_y$-wave superconductivity at finite momentum. Owing to the intrinsic disorder at the interface, the role of spatial inhomogeneity on the superconducting and ferromagnetic states becomes important. We find that for strong disorder, the system breaks up into mutually excluded regions of superconductivity and ferromagnetism. This inhomogeneity-driven electronic phase separation accounts for the unusual coexistence of superconductivity and ferromagnetism observed at the interface.

cond-mat.supr-con↗

arXiv2013

Oxygen-vacancy clustering and pseudogap behaviour at LaAlO3/SrTiO3 interface

The 2DEG at the LaAlO3/SrTiO3 interface promises to add a new dimension to emerging electronic devices due to its high tunability. Defects in the form of Oxygen vacancies in titanate surfaces and interfaces, on the other hand, play a key role in the emergence of novel phases. Based on an effective model, we study the influence of Oxygen vacancies on superconductivity and ferromagnetism at LaAlO3/SrTiO3 interface. Using the Bogoliubov-de Gennes formulation in conjunction with Monte-Carlo simulation, we find a clustering of the Oxygen vacancies at the interface that favours formation of coexisting ferromagnetic puddles spatially separated from superconductivity. We also find a carrier freeze-out at low temperatures, observed experimentally in wide-variety of samples. Sufficiently large amount of Oxygen vacancy leads to pseudo-gap like behaviour in the superconducting state.

cond-mat.supr-con↗