arXiv subjects

Rajdeep Sensarma

Publications and source records attributed to Rajdeep Sensarma.

At least 19 recordsLinked to original sources

arXiv2026

Van Hove singularity-driven giant Nernst signal in twisted double bilayer graphene

Twisted graphene layers host van Hove singularities (vHSs), peaks in the electronic density of states, thought to drive exotic phases in moir\'e materials, but their effect on thermal transport has remained unclear. Here we show that vHSs in twisted double bilayer graphene (tDBLG) generate an unusually large Nernst signal-the transverse voltage produced by a longitudinal temperature gradient in a magnetic field. The pronounced Nernst peaks at the vHSs of the conduction and valence bands of tDBLG are tunable by an electric field with a maximum value of $\sim 40$ $\mu V K^{-1} T^{-1}$ at $\sim 1$ $K$, which is comparable to the best-known Nernst materials. Our theoretical calculations show that the large enhancement of the Nernst signal arises from the Lifshitz transitions around the vHSs. These findings establish the Nernst effect as a sensitive probe of Fermi-surface topology in moir\'e materials, and identify a universal thermoelectric signature of van Hove singularities.

cond-mat.mes-hall↗

arXiv2026

Probing the pairing symmetry of moir\'e graphene superconductors

The pairing symmetry of magic-angle moir\'e graphene is a fundamental question that remains unresolved. Combining experimental and theoretical inputs, we constrain the superconducting order parameters that can emerge from the incommensurate Kekul\'e spiral (IKS) normal state on the hole-doped side of $\nu = -2$. Imposing the additional experimental constraint of nodal superconductivity, we are left with the task of distinguishing between singlet or triplet pairing, and of determining the d-vector in the latter case. We propose definitive tests to identify the pairing symmetry based on two classes of experiments using the response of the superconducting state to Zeeman field orientation. The first set of predictions is for spectroscopic and thermodynamic measurements sensitive to low-energy excitations near the nodes. The second set is for phase-sensitive measurements of topologically protected Andreev bound states near boundaries, whose spectroscopy is shown to provide a smoking gun signature of the pairing symmetry.

cond-mat.supr-con↗

arXiv2025

Entanglement Entropy from Correlation Functions of Scalar Fields in and out of Equilibrium

We show that odd order R\'enyi entropies $S^{(2q+1)}$ of a system of interacting scalar fields can be calculated as the free energy of $2q+1$ replicas of the system with additional quadratic inter-replica couplings in the subsystem at the time of measurement of the entropy. These couplings replace boundary field matching conditions. This formalism works both in and out of thermal equilibrium, for closed as well as open quantum systems, and provides a general dictionary between measurable correlation functions and entanglement entropy. $S^{(2q+1)}$ can be analytically continued to calculate the von Neumann entropy $S^{\mathrm{vN}}$. We provide an exact formula relating $S^{(2q+1)}$ and $S^{\mathrm{vN}}$ with correlation functions in a non-interacting theory. For interacting theories, we provide rules for constructing all possible Feynman diagrams for $S^{(2q+1)}$. We show that the boundary matching conditions cannot be completely eliminated while calculating R\'enyi entropies of even order due to presence of zero modes in replica space.

cond-mat.stat-mech↗

arXiv2025

Exploring unconventional superconductivity in PdTe via Point Contact Spectroscopy

Palladium Telluride (PdTe), a non-layered intermetallic crystalline compound, has captured attention for its unique superconducting properties and strong spin-orbit coupling. In this work, we investigate the superconducting state of PdTe using point-contact Andreev reflection (PCAR) spectroscopy. The experimental data are analyzed using the Blonder-Tinkham-Klapwijk (BTK) model for s, p and d wave symmetries. Our results reveal clear evidence of unconventional superconductivity. The superconducting gap showing features consistent with either p-wave or d-wave pairing symmetries but cannot be fitted with s-wave symmetry. The observed anisotropic gap structure and deviations from conventional BCS behaviour highlight the complex nature of the pairing interactions in PdTe. These findings provide strong evidence of unconventional pairing symmetry in this material.

cond-mat.supr-con↗

arXiv2025

Symmetry broken states at high displacement fields in ABA trilayer graphene

In this Letter, we present a comprehensive study of magnetotransport in high-mobility trilayer graphene (TLG) devices under a transverse displacement field, focusing on symmetry-broken Landau levels (LLs) from monolayer-like and bilayer-like bands. A striking displacement-field-induced enhancement of the Land\'e g-factor is observed in the zeroth Landau level of the monolayer-like band, highlighting the role of strong electron-electron interactions. Additionally, we find a rich landscape of LL crossings in the Dirac gully region, accompanied by phase transitions between spin-, gully-, and valley-polarized LLs. These experimental observations are successfully modeled using calculations based on optimized tight-binding parameters. Furthermore, our results reveal significant particle-hole asymmetry in the sequence of LLs in the Dirac gullies, attributed to differing g-factor values for electrons and holes. This asymmetry underscores the limitations of non-interacting models in capturing the complexities of strongly correlated multiband systems. This work provides new insights into the interplay of symmetry-breaking mechanisms and strong correlations in Bernal-stacked trilayer graphene, advancing our understanding of quantum transport phenomena in multiband systems.

cond-mat.mes-hall↗

arXiv2025

Andreev versus Tunneling Spectroscopy of Unconventional Flat Band Superconductors

STM experiments in the tunneling and Andreev regimes on graphene-based moire superconductors (SC) show two distinct energy scales whose origin is mysterious. We express the conductance of a normal-SC interface in terms of Green's functions, which allows us to sharpen the issues in two ways. First, we show that the two distinct energy scales cannot be understood in terms of a pseudogap in tunneling and a superconducting gap in the Andreev spectra. Second, the large Fermi velocity vF mismatch between the STM tip and the at band SC renormalizes a transparent interface towards the tunneling regime, and the ballistic Andreev regime cannot be realized in moire SCs. We also discuss self energy corrections to vF that determines the conductance. Finally, we offer a resolution to these problems by modeling the Andreev experiment as a circular metallic disc embedded in an unconventional SC. We show that with strong vF mismatch the low bias conductance is dominated by Andreev bound states induced by the tip at the interface with the unconventional SC. The ABS give rise to the low energy scale seen in Andreev experiments, smaller than the SC gap in tunneling spectroscopy.

cond-mat.supr-con↗

arXiv2024

Controlling particle-hole symmetry of fractional quantum hall states in trilayer graphene

We present a detailed experimental study of the particle-hole symmetry (PHS) of the fractional quantum Hall (FQH) states about half filling in a multiband system. Specifically, we focus on the lowest Landau level of the monolayer-like band of Bernal stacked trilayer graphene (TLG). In pristine TLG, the excitation energy gaps, Land\'e g-factor, effective mass, and disorder broadening of the odd-denominator FQH states are identical to their hole-conjugate counterpart. This precise PH symmetry stems from the lattice mirror symmetry that precludes Landau-level mixing. Introducing a non-zero displacement field $D$ disrupts this mirror symmetry, facilitating the hybridization between the monolayer-like and bilayer-like Landau levels. This inter-band coupling enhances the Landau level mixing factor $\eta$ and activates three-body interactions -- both of which explicitly break the PHS of FQHs. As a result, conventional FQHs are completely destabilized, offering a route to engineer symmetry breaking of FQHs in a controlled way. We establish that the PHS breaking in TLG is of extrinsic origin and is fundamentally distinct from the intrinsic, interaction-driven symmetry breaking observed in the lowest Landau levels of single-layer and bilayer graphene.

cond-mat.mes-hall↗

arXiv2023

Non-equilibrium dynamics of bosons with dipole symmetry: Large-$N$ Keldysh approach

We study the quench and the ramp dynamics of interacting $N$-component charged bosons with dipole symmetry using Schwinger-Keldysh field theory in the large $N$ limit. The equilibrium phase diagram of these bosons shows two phases in the large $N$ limit. The first is a normal phase where both the global $U(N)$ and the dipole symmetries are conserved and the second is a delocalized condensed phase where both the symmetries are broken. In contrast, our explicit computation of the steady state after an instantaneous quantum quench from the condensed phase shows that an additional, novel, delocalized normal phase, where the global $U(N)$ symmetry is conserved but the dipole symmetry is broken, can exist for a range of quench parameters. A study of ramp dynamics of the model shows that the above-mentioned steady state exists only above a critical ramp rate which we estimate.

cond-mat.quant-gas↗

arXiv2023

Signature of Criticality in Angular Momentum Resolved Entanglement of Scalar Fields in $d>1$

The scaling of entanglement entropy with subsystem size fails to distinguish between gapped and gapless ground state of a scalar field theory in $d>1$ dimensions. We show that the scaling of angular momentum resolved entanglement entropy $S_\ell$ with the subsystem radius $R$ can clearly distinguish between these states. For a massless theory with momentum cut-off $Λ$, $S_\ell \sim \ln [ΛR/\ell]$ for $ΛR \gg \ell$, while $S_\ell \sim R^0$ for the massive theory. In contrast, for a free Fermi gas with Fermi wave vector $k_F$, $S_\ell \sim \ln [k_F R]$ for $k_F R \gg \ell$. We show how this leads to an ``area-log'' scaling of total entanglement entropy of Fermions, while the extra factor of $\ell$ leads to a leading area law even for massless Bosons.

cond-mat.stat-mech↗

arXiv2023

Building Entanglement Entropy out of Correlation Functions for Interacting Fermions

We provide a prescription to construct Rényi and von Neumann entropy of a system of interacting fermions from a knowledge of its correlation functions. We show that Rényi entanglement entropy of interacting fermions in arbitrary dimensions can be represented by a Schwinger Keldysh free energy on replicated manifolds with a current between the replicas. The current is local in real space and is present only in the subsystem which is not integrated out. This allows us to construct a diagrammatic representation of entanglement entropy in terms of connected correlators in the standard field theory with no replicas. This construction is agnostic to how the correlators are calculated, and one can use calculated, simulated or measured values of the correlators in this formula. Using this diagrammatic representation, one can decompose entanglement into contributions which depend on the one-particle correlator, two particle correlator and so on. We provide analytic formula for the one-particle contribution and a diagrammatic construction for higher order contributions. We show how this construction can be extended for von-Neumann entropy through analytic continuation. For a practical implementation of a quantum state, where one usually has information only about few-particle correlators, this provides an approximate way of calculating entanglement commensurate with the limited knowledge about the underlying quantum state.

cond-mat.stat-mech↗

arXiv2022

Fragile electronic superconductivity in Bi Single crystal

It was presumed that semimetal Bismuth (Bi) would not show superconductivity (SC) even at ultra-low temperatures ($<$10 mK) due to its very low carrier density ($\approx 3\times10^{17}$cm$^{-3}$). Recently, we have established bulk superconductivity in ultra-pure (99.9999\%) Bi single crystal at $\mathrm{T_C = 0.53}$ mK with an extrapolated upper critical field $\mathrm{H_C(0) = 5.2μ}$T measured along the [$0001$] (trigonal) -crystallographic direction. At very low concentrations of the charge carriers, we are dealing with fragile Cooper pairs with an estimated large coherence length $\mathrm{ξ_{GL}(0)\approx 96 μ}$m. We also stated that one needs to go beyond the conventional electron-phonon coupling (BCS-like) mechanism to understand the SC state in Bi. Bi is a compensated semi-metal with electrons and holes as charge carriers. In order to find the charge carriers responsible for the SC, we report the temperature dependence of the anisotropic critical field along the [$01\bar 10$] (bisectrix)-crystallographic direction and compared it with the earlier data from measurements along the trigonal. Our theoretical analysis of the anisotropy of critical fields suggests that the light electrons in the three pockets of Bi bands are responsible for the SC and indicates that Bi is an extremely weak type-II (close to type-I) superconductor. Finally, we review the current theories proposed to explain the SC in Bi.

cond-mat.supr-con↗

arXiv2022

Excitonic Metal and Non-Fermi Liquid Behaviour in Twisted Double Bilayer Graphene near Charge Neutrality

Twisted double bilayer graphene is a compensated semi-metal near the charge neutrality point with the presence of small electron and hole pockets in its band structure. We show that strong Coulomb attraction between the electrons and holes can lead to the formation of indirect excitons. Condensation of these excitons at low temperature creates an excitonic metal with charge density wave order for an appropriate range of interaction strength. This has interesting implications for low-temperature transport in the system as a function of carrier density and temperature. The reorganization of the single particle excitations and their density of states in the excitonic metal can lead to peaks in resistivity as a function of carrier density, recently seen in experiments at low temperatures. The fluctuations of the Landau damped order parameter in the quantum critical metal lead to non-Fermi liquid behaviour, which can explain the sublinear $T^{2/3}$ dependence of the resistance near the charge neutrality point.

cond-mat.str-el↗

arXiv2022

Evidence of a compensated semimetal with electronic correlations at the CNP of twisted double bilayer graphene

Recently, magic-angle twisted bilayer graphene (MATBLG) has shown the emergence of various interaction-driven novel quantum phases at the commensurate fillings of the moir'e superlattice, while the charge neutrality point (CNP) remains mostly a vanilla insulator. Here, we show an emerging phase of nearly compensated semimetallicity at the CNP of twisted double bilayer graphene (TDBLG), a close cousin of MATBLG, with signatures of electronic correlation. Using electrical and thermal transport, we find almost two orders of magnitude enhancement of the thermopower in magnetic fields much smaller than the extreme quantum limit, accompanied by a large magnetoresistance($\sim 2500\%$) at CNP. This provides indisputable experimental evidence that TDBLG near CNP is a compensated semimetal. Moreover, at low temperatures, we observe an unusual sublinear temperature dependence of resistance. A recent theory predicts the formation of an excitonic metal near CNP, where small electron and hole pockets coexist. We understand the sublinear temperature dependence in terms of critical fluctuations in this theory.

cond-mat.mes-hall↗

arXiv2022

Non-equilibrium scalar field dynamics starting from Fock states: Absence of thermalization in one dimensional phonons coupled to fermions

We propose a new method to study non-equilibrium dynamics of scalar fields starting from non-Gaussian initial conditions using Keldysh field theory. We use it to study dynamics of phonons coupled to non-interacting bosonic and fermionic baths, starting from initial Fock states. We find that in one dimension long wavelength phonons coupled to fermionic baths do not thermalize both at low and high bath-temperatures. At low temperature, constraints from energy-momentum conservation lead to a narrow bandwidth of particle-hole excitations and the phonons effectively do not see this bath. On the other hand, the strong band-edge divergence of the particle-hole density of states leads to an undamped polariton-like mode of the dressed phonons above the band edge of the particle-hole excitations. These undamped modes contribute to the lack of thermalization of long wavelength phonons at high temperatures. In higher dimensions, these constraints and the divergence of density of states are weakened and lead to thermalization at all wavelengths.

cond-mat.mes-hall↗

arXiv2022

Subgap two-particle spectral weight in disordered $s$-wave superconductors: Insights from mode coupling approach

We study the two-particle spectral functions and collective modes of weakly disordered superconductors using a disordered attractive Hubbard model on square lattice. We show that the disorder induced scattering between collective modes leads to a finite subgap spectral weight in the long wavelength limit. In general, the spectral weight is distributed between the phase and the Higgs channels, but as we move towards half-filling the Higgs contribution dominates. The inclusion of the density fluctuations lowers the frequency at which this mode occurs, and results in the phase channel gaining a larger contribution to this subgap mode. Near half-filling, the proximity of the system to the charge density wave (CDW) instability leads to strong fluctuations of the effective disorder at the commensurate wave-vector ($[π,π]$). We develop an analytical mode coupling approach where the pure Goldstone mode in the long wavelength limit couples to the collective mode at $[π,π]$. This provides insight into the location and distribution of the two-particle spectral weights between the Higgs and the phase channels.

cond-mat.supr-con↗

arXiv2021

Non-equilibrium Dynamics of Renyi Entropy for Bosonic Many-Particle Systems

We propose a new field theoretic method for calculating Renyi entropy of a sub-system of many interacting Bosons without using replica methods. This method is applicable to dynamics of both open and closed quantum systems starting from arbitrary initial conditions. Our method identifies the Wigner characteristic of a reduced density matrix with the partition function of the whole system with a set of linear sources turned on only in the subsystem and uses this to calculate the subsystem's Renyi entropy. We use this method to study evolution of Renyi entropy in a non-interacting open quantum system starting from an initial Fock state. We find a relation between the initial state and final density matrix which determines whether the entropy shows non-monotonic behaviour in time. For non-Markovian dynamics, we show that the entropy approaches its steady state value as a power law with exponents governed by non-analyticities of the bath. We illustrate that this field-theoretic method can be used to study large bosonic open quantum systems.

cond-mat.stat-mech↗

arXiv2021

Many-body localized to ergodic transitions in a system with correlated disorder

We study the transition from a many-body localized phase to an ergodic phase in spin chain with correlated random magnetic fields. Using multiple statistical measures like gap statistics and extremal entanglement spectrum distributions, we find the phase diagram in the disorder-correlation plane, where the transition happens at progressively larger values of the correlation with increasing values of disorder. We then show that one can use the average of sample variance of magnetic fields as a single parameter which encodes the effects of the correlated disorder. The distributions and averages of various statistics collapse into a single curve as a function of this parameter. This also allows us to analytically calculate the phase diagram in the disorder-correlation plane.

cond-mat.dis-nn↗

arXiv2021

Thermal effects on collective modes in disordered $s$-wave superconductors

We investigate the effect of thermal fluctuations on the two-particle spectral function for a disordered $s$-wave superconductor in two dimensions, focusing on the evolution of the collective amplitude and phase modes. We find three main effects of thermal fluctuations: (a) the phase mode is softened with increasing temperature reflecting the decrease of superfluid stiffness; (b) remarkably, the non-dispersive collective amplitude modes at finite energy near ${\bf q}=[0,0]$ and ${\bf q}=[π,π]$ survive even in presence of thermal fluctuations in the disordered superconductor; and (c) the scattering of the thermally excited fermionic quasiparticles leads to low energy incoherent spectral weight that forms a strongly momentum-dependent background halo around the phase and amplitude collective modes and broadens them. Due to momentum and energy conservation constraints, this halo has a boundary which disperses linearly at low momenta and shows a strong dip near the $[π,π]$ point in the Brillouin zone.

cond-mat.supr-con↗