arXiv subjects

Sankar Das Sarma

Publications and source records attributed to Sankar Das Sarma.

At least 73 records · Page 4Linked to original sources

arXiv2024

Long-lived Topological Flatband Excitons in Semiconductor Moiré Heterostructures: a Bosonic Kane-Mele Model Platform

Moiré superlattices based on two-dimensional transition metal dichalcogenides (TMDs) have emerged as a highly versatile and fruitful platform for exploring correlated topological electronic phases. One of the most remarkable examples is the recently discovered fractional quantum anomalous Hall effect (FQAHE) under zero magnetic field. Here we propose a minimal structure that hosts long-lived excitons -- a ubiquitous bosonic excitation in TMD semiconductors -- with narrow topological bosonic bands. The nontrivial exciton topology originates from hybridization of moiré interlayer excitons, and is tunable by controlling twist angle and electric field. At small twist angle, the lowest exciton bands are isolated from higher energy bands and provide a solid-state realization of the bosonic Kane-Mele model with topological flatbands, which could potentially support the bosonic version of FQAHE.

cond-mat.mes-hall↗

arXiv2024

Negative intercept of the apparent zero-temperature extrapolated linear-in-$T$ metallic resistivity

We consider the well-known phonon scattering induced high-temperature linear-in-$T$ metallic resistivity, showing that a naive extrapolation of the effective linearity from high temperatures to $T=0$ leads to an apparent zero-temperature negative resistivity. The precise magnitude of this extrapolated $T=0$ negative resistivity depends on the temperature regime from where the extrapolation is carried out, and approaches the correct physical result of zero resistivity at $T=0$ only if the extrapolation starts from $T\gg T_D$, where $T_D$ is the Debye temperature. We establish a theoretical relationship between the negative intercept and the slope of the linear-in-$T$ resistivity as a function of the temperature $T$ from where the extrapolation is carried out. Experimental implications of our finding are discussed for the much-discussed Planckian behavior of the transport scattering rate.

cond-mat.mes-hall↗

arXiv2024

Pseudospin density wave instability in two-dimensional electron bilayers

We investigate the instability of layer pseudospin paramagnetic (PSP) state to the formation of pseudospin density wave (PSDW) in two-dimensional (2D) electron bilayers, analogous to the formation of Overhauser spin density wave (SDW) in a single-layer 2D electron gas (2DEG) with spin 1/2. Our comprehensive study on phase diagrams, based on the self-consistent Hartree-Fock (HF) theory, reveals that the PSDW has a lower energy than both PSP and pseudospin ferromagnetic (PSF) states near the PSP-PSF phase transition boundary. When the two layers are populated by the same number of electrons, the PSDW momentum $Q_c \sim 2k_F$ near the PSP-PSDW boundary, where $k_F= (2πn)^{1/2}$ is the Fermi momentum characterized by the density in one of the two layers, and $Q_c$ decreases as the system transitions to the PSF regime. Extending the HF study to the case of unequal layer densities, the PSP phase is unstable to PSDW for small density imbalances, with momentum $Q_c \sim k_{F,t} + k_{F,b}$, where $k_{F,t}$ and $k_{F,b}$ are Fermi momenta of top and bottom layers, respectively. In PSDW regime, the ground state stability, defined by the energy difference between PSDW and the second lowest-energy state, is one order of magnitude lower than that in PSF regime, and decreases with increasing layer separation $d$. Furthermore, incorporating RPA static screening with the Hubbard-type local field correction leads to disappearance of both SDW and PSDW phases, and pushes the phase boundaries of paramagnetic to ferromagnetic transitions to larger $r_s$ values. Our study on PSDW in 2D electron bilayers is equally applicable to 2D hole bilayers. The idea of pursuing PSDW is, in general, relevant across various 2D bilayer systems, not limited to the parabolic model that we investigate in this paper, and provides a new possibility of exploring novel coherent phases.

cond-mat.mes-hall↗

arXiv2024

Disordered Majorana nanowires: Studying disorder without any disorder

The interplay of disorder and short finite wire length is the crucial physics hindering progress in the semiconductor-superconductor nanowire platform for realizing non-Abelian Majorana zero modes (MZM). Disorder effectively segments the nanowire into isolated patches of quantum dots (QD) which act as subgap Andreev bound states often mimicking MZMs. In this work, we propose and develop a new theoretical approach to model disorder, effectively a spatially varying effective mass model, which does not rely on incorporating unknown microscopic details of disorder into the Hamiltonian. This model effectively segments the wire into multiple QDs, characterized by highly enhanced effective mass at impurity sites leading to the segmentation of the wire into effective random QDs. We find that this model can reproduce disorder physics, providing a crystal clear way to understand the effects of disorder by comparing the mean free path to the superconducting coherence length. In addition, this model allows precise control over the disorder regime, enabling us to evaluate the reliability of topological invariants (TI) in predicting MZMs. We find that TIs alone may yield a significant false positive rate as indicators for topology in the actual wire with increasing disorder strength. Therefore, we propose new indicators to characterize the spatial distribution of the zero-energy state, emphasizing the key necessity for isolated MZMs localized at wire ends. Employing this set of new indicators for stringent characterizations, we explore their experimental relevance to the measured differential conductance spectra. Our findings highlight the critical role of isolated localized states, beyond the TI, in identifying topological MZMs. We believe that this approach is a powerful tool for studying realistic Majorana nanowires where disorder and short wire length obfuscate the underlying topological physics.

cond-mat.mes-hall↗

arXiv2024

Eliashberg theory for dynamical screening in bilayer exciton condensation

We study the effect of dynamical screening of interactions on the transition temperatures ($T_c$) of exciton condensation in a symmetric bilayer of quadratically dispersing electrons and holes by solving the linearized Eliashberg equations for the anomalous interlayer Green's functions. We find that $T_c$ is finite for the range of density and layer separations studied, decaying exponentially with interlayer separation. $T_c$ is suppressed well below that predicted by a Hartree Fock mean field theory with unscreened Coulomb interaction, but is above the estimates from the statically screened Coulomb interaction. Furthermore, using a diagrammatic framework, we show that the system is always an exciton condensate at zero temperature but $T_c$ is exponentially small for large interlayer separation.

cond-mat.str-el↗

arXiv2024

Integer and fractional quantum anomalous Hall effects in pentalayer graphene

We critically analyze the recently reported observation of integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects at zero applied magnetic field in pentalayer graphene. Our quantitative activation and variable range hopping transport analysis of the experimental data reveals that the observed IQAHE and FQAHE at different fillings all have similar excitation gaps of the order of $5-10$ K. In addition, we also find that the observed FQAHE manifests a large hidden background contact series resistance >10 k$Ω$ of unknown origin whereas this contact resistance is much smaller ~500 $Ω$ in the observed IQAHE. Both of these findings are surprising as well as inconsistent with the well-established phenomenology of the corresponding high-field integer and fractional quantum Hall effects in 2D semiconductor systems.

cond-mat.mes-hall↗

arXiv2024

Topological flat bands, valley polarization, and interband superconductivity in magic-angle twisted bilayer graphene with proximitized spin-orbit couplings

We study theoretically the magic-angle twisted bilayer graphene with proximity-induced Ising and Rashba spin-orbit couplings on the top layer. Topological flat bands (with three distinct phases) are generically realized by the spin-orbit couplings. Using a mean field analysis, we find that (partial) valley polarization prevails for a wide range of doping, suppressing the usual superconductivity with a pairing between time-reversal partners. Remarkably, we uncover that observable unconventional intervalley interband phonon-mediated superconductivity (with the highest $T_c\approx 1.2$K) can coexist with strong valley imbalance due to the approximate Fermi surface nesting between two flat bands not related by time-reversal symmetry, and the dominant pairing is an intersublattice Ising pairing, corresponding to a mixture of $p$- and $d$-waves. In contrast, the intrasublattice Ising phonon-mediated superconductivity with $s$- and $f$-wave mixing emerges in the absence of valley imbalance. Our work reveals an unprecedented route of realizing unconventional superconductivity.

cond-mat.supr-con↗

arXiv2024

Bound state-continuum resonance transition in a shallow quantum well

We show a transition from a bound state to a continuum resonance in a shallow quantum well (QW) by electrostatic gating to bend the conduction band edge. This bound state-continuum resonance (BSCR) transition is particularly relevant in topological quantum computing platforms where shallow InAs QWs are used. We predict the observed capacitance jump and the parallel metal-insulator transition accompanied the BSCR transition. An experimental puzzle in shallow InAs QWs is the mobility drop at an electron density smaller than expected for the bound-state second subband occupation. We explain this puzzle as a result of intersubband scattering involving a level-broadened continuum resonance, mediated by screened Coulomb impurities.

cond-mat.mes-hall↗

arXiv2024

Interacting quasiperiodic spin chains in the prethermal regime

Recent progress in the study of many-body localization (MBL) in strongly disordered interacting spin chains has emphasized the importance of distinguishing finite time prethermal behavior from long time and large volume asymptotics. We re-examine a reported non-ergodic extended (NEE) regime in the interacting quasiperiodic Ganeshan-Pixley-Das Sarma model from this perspective, and propose that this regime is a prethermal feature. Indeed, we argue that the NEE regime may be identified through a change in the functional form of spin-spin autocorrelation functions, demonstrating that the NEE regime is distinguishable within intermediate-time dynamics. This is in contrast with existing conjectures relating the NEE regime to the presence of an asymptotic mobility edge in the single-particle spectrum. Thus, we propose a mechanism for the formation of an NEE regime which does not rely on asymptotic properties of the spin chain. Namely, we propose that the NEE regime emerges due to regularly spaced deep wells in the disorder potential. The highly detuned sites suppress spin transport across the system, effectively cutting the chain, and producing a separation of time scales between the spreading of different operators. To support this proposal, we show that the NEE phenomenology also occurs in random models with deep wells but with no mobility edges, and does not occur in quasiperiodic models with mobility edges but with no deep wells. Our results support the broad conclusion that there is not a sharp distinction between the dynamics of quasiperiodically and randomly disordered systems in the prethermal regime. More specifically, we find that generic interacting quasiperiodic models do not have stable intermediate dynamical phases arising from their single-particle mobility edges, and that NEE phenomenology in such models is transient.

cond-mat.dis-nn↗

arXiv2024

Engineering the Kitaev spin liquid in a quantum dot system

The Kitaev model on a honeycomb lattice may provide a robust topological quantum memory platform, but finding a material that realizes the unique spin liquid phase remains a considerable challenge. We demonstrate that an effective Kitaev Hamiltonian can arise from a half-filled Fermi-Hubbard Hamiltonian where each site can experience a magnetic field in a different direction. As such, we provide a method for realizing the Kitaev spin liquid on a single hexagonal plaquette made up of twelve quantum dots. Despite the small system size, there are clear signatures of the Kitaev spin-liquid ground state, and there is a range of parameters where these signatures are predicted, allowing a potential platform where Kitaev spin-liquid physics can be explored experimentally in quantum dot plaquettes.

cond-mat.mes-hall↗

arXiv2024

Role of many phonon modes on the high-temperature linear-in-$T$ electronic resistivity

We theoretically consider the possibility that phonons may be playing a role in the observed linear-in-$T$ resistivity in cuprates by focusing on the obvious question: How can phonon scattering be consistent with a linear-in-$T$ resistivity with a constant slope given that cuprates have many phonon modes with different energies and electron-phonon couplings (e.g. 21 phonon modes for LSCO)? We show using an arbitrarily large number of independent phonon modes that, within a model Boltzmann transport theory, the emergent high-$T$ linear-in-$T$ resistivity manifests an approximately constant slope independent of the number of phonon modes except in some fine-tuned narrow temperature regimes. We also comment on the quantitative magnitude of the linear-in-$T$ resistivity in cuprates pointing out the constraints on the effective electron-phonon coupling necessary to produce the observed resistivity.

cond-mat.mes-hall↗

arXiv2024

Machine Learning the Disorder Landscape of Majorana Nanowires

We develop a practical machine learning approach to determine the disorder landscape of Majorana nanowires by using training of the conductance matrix and inverting the conductance data in order to obtain the disorder details in the system. The inversion carried out through machine learning using different disorder parametrizations turns out to be unique in the sense that any input tunnel conductance as a function of chemical potential and Zeeman energy can indeed be inverted to provide the correct disorder landscape. Our work opens up a qualitatively new direction of directly determining the topological invariant and the Majorana wave-function structure corresponding to a transport profile of a device using simulations that quantitatively match the specific conductance profile. In addition, this also opens up the possibility for optimizing Majorana systems by figuring out the (generally unknown) underlying disorder only through the conductance data. An accurate estimate of the applicable spin-orbit coupling in the system can also be obtained within the same scheme.

cond-mat.mes-hall↗

arXiv2024

Energetic comparison of exciton gas versus electron-hole plasma in a bilayer two-dimensional electron-hole system

We study the zero-temperature phase diagram of a symmetric electron-hole bilayer system by comparing the ground state energies of two distinct limiting cases, characterized by an electron-hole plasma or an exciton gas, respectively. For the electron-hole plasma, the random phase approximation is used; for the exciton gas, we consider three different approximations: the unscreened Coulomb interaction, the statically screened one, and the dynamically screened one under the plasmon-pole approximation. Our results suggest that the exciton gas is stable at small layer separation. However, static screening in general suppresses the formation of excitons, and dynamic screening gives different results depending on the representative energy scale we used in the plasmon-pole approximation. We conclude that energetic considerations alone are very sensitive to the approximation schemes, and the phase diagram of the system may depend crucially on exactly how the electron-hole attraction is treated in the theory. For very small and very large densities, however, all our approximations show the exciton gas to have lower energy than the plasma.

cond-mat.mes-hall↗

arXiv2024

Assessing quantum dot SWAP gate fidelity using tensor network methods

Advanced tensor network numerical methods are used to explore the fidelity of repeated SWAP operations on a system comprising 20-100 quantum dot spin qubits in the presence of valley leakage and electrostatic crosstalk. The fidelity of SWAP gates is largely unaffected by Zeeman splitting and valley splitting, except when these parameters come into resonance. The fidelity remains independent of the overall valley phase for valley eigenstates, while for generic valley states, some minor corrections arise. We analyze the fidelity scaling for long qubit chains without valley effects, where crosstalk represents the only error source.

cond-mat.mes-hall↗

arXiv2024

Electronic transport, metal-insulator transition, and Wigner crystallization in transition metal dichalcogenide monolayers

Two recent electronic transport experiments from Columbia University and Harvard University have reported record high mobility and low channel densities in transition metal dichalcogenide (TMD) WSe$_2$ monolayers [J. Pack, et al., arXiv:2310.19782; A. Y. Joe, et al., Phys. Rev. Lett. 132, 056303 (2024)]. A two-dimensional (2D) metal-insulator transition (MIT) is demonstrated in the Columbia sample at low densities, a regime where the formation of a Wigner crystal (WC) is theoretically anticipated in the absence of disorder. We employ the finite-temperature Boltzmann theory to understand the low-temperature transport properties of monolayer TMDs, taking into account realistic disorder scattering. We analyze the experimental results, focusing on the 2D MIT behavior and the influence of temperature and density on mobility and resistivity in the metallic phase. We provide a discussion of the nontrivial carrier density dependence of our transport results. Our analysis elucidates the linear-in-$T$ resistivity in the metallic phase, attributing it to Friedel oscillations associated with screened charged impurities. Furthermore, we explore whether Coulomb disorder could lead to the MIT through either a quantum Anderson localization transition or a classical percolation transition. Our theoretical estimates of the disorder-induced MIT critical densities, although smaller, are within a factor of ~2 of the experimental critical density. We examine the exceptionally high melting temperature ~10 K of WCs observed experimentally in the MoSe$_2$ systems at low density, an order of magnitude larger than the pristine melting temperature. This suggests that the observed 2D low-density MIT behavior is likely a result of the complex interplay between disorder effects and interaction-driven WC physics, offering a comprehensive understanding of the low-temperature transport phenomena in TMD monolayers.

cond-mat.mes-hall↗

arXiv2024

Symmetry breaking in zero-field two-dimensional electron bilayers

We theoretically consider bilayers of two dimensional (2D) electron gases as in semiconductor quantum wells, and investigate possible spontaneous symmetry breaking transitions at low carrier densities driven by interlayer Coulomb interactions. We use a self-consistent technique implementing mean field truncations of the interacting four-fermion terms, and find a U(1) layer symmetry breaking transition at low carrier densities where the individual layer identities are lost leading to an effective pseudospin XY ferromagnet in the 2D plane. Our results validate earlier theoretical works using simpler restricted Hartree-Fock techniques, and establish the pseudospin XY ferromagnet as a possible low-density symmetry broken phase of 2D bilayers.

cond-mat.mes-hall↗

arXiv2024

Disorder effects on the so-called Andreev band in Majorana nanowires

We comment on a recent publication Phys. Rev. Lett. 130, 207001 (2023), pointing out that the periodic model for the superconducting gap and/or the spin splitting used by the authors is artificial and does not apply to any real systems. In addition, we show that the resulting Andreev band introduced by this artificial and unrealistic periodicity is suppressed by the potential disorder invariably present in all experimental systems. The results of this model are therefore contrived and do not apply to any experimental system.

cond-mat.mes-hall↗

arXiv2024

Interaction and coherence in 2D bilayers

In bilayer systems, the additional layer pseudospin enables the emergence of interlayer coherence (IC), which is a direct consequence of the interlayer Coulomb interaction. This study presents a comprehensive HF investigation of IC in 2D bilayers, uncovering ground-state behaviors and temperature-dependent phase transitions that are distinct from single-layer 2DEG. This IC signals a spontaneous breaking of the U(1) symmetry in pseudospin. We explore the zero-temperature phase diagrams as a function of the electron density and interlayer separation within the HF formalism. We also calculate the critical temperature ($T_c$) of the interlayer coherence onset by self-consistently solving the HF gap-like equation. We contrast this IC phase in e-e bilayers with the closely related excitonic superfluid phase in e-h bilayers. Though both e-e and e-h bilayers spontaneously break the pseudospin U(1) symmetry, e-h bilayers produce BCS-BEC crossover intrinsic to the excitons acting as effective bosons or Cooper pairs, whereas the symmetry-broken phase in e-e bilayers is akin to the XY or easy-plane pseudospin ferromagnetism. Using the same system parameters and a similar theoretical framework, we find that $T_c$ of the interlayer coherent phase in e-e bilayers is about one-third of that in exciton condensates, suggesting a weaker IC in e-e bilayers. In addition, we examine the effect of a weak interlayer tunneling on the IC order parameter, drawing parallels with the influence of an effective in-plane magnetic field on the XY pseudospin ferromagnetism. Our findings provide a comparative theoretical framework that bridges the gap between the IC physics in e-e and e-h bilayers, contributing to a unified understanding of phase transitions in low-dimensional electron/hole systems and establishing in particular the same universality class for interlayer phase coherence in both e-e and e-h bilayers.

cond-mat.str-el↗