arXiv subjects

Sankar Das Sarma

Publications and source records attributed to Sankar Das Sarma.

At least 19 recordsLinked to original sources

arXiv2026

Scaling of qubit coherence in quantum dot based Majorana chains: Rabi and Ramsey oscillations of Majorana qubits formed by four and six quantum dots

We present a theoretical study of the Rabi and Ramsey coherence time of Majorana-based qubits formed by double $N$-site quantum-dot artificial Kitaev chains, following the recent experimental realization arXiv:2607.09511. We generalize the qubit to two chains of arbitrary length and compare the Rabi and Ramsey oscillations of the four-dot and six-dot qubits, simulated with realistic disorder for three sets of parameters: small and large superconducting gaps, and the experimental parameters. We find that, in the pristine limit, the Rabi oscillation is independent of the chain length, whereas the Ramsey oscillation of the six-dot qubit suffers from larger leakage, because the third site suppresses the unwanted energy splitting away from the sweet spot but at the same time makes the deliberate splitting difficult to achieve, which requires a larger detuning that could excite the bulk. In reality with disorder, neither coherence time improves universally with the chain length. The Rabi coherence is set by the competition of three dephasing channels, and the additional dots pay off only if the fluctuation of the interchain tunneling is suppressed; the Ramsey coherence is set by the fluctuation of the energy splitting during the $σ_z$ rotation, which for the four-dot qubit is fixed by the sweet-spot disorder alone and does not depend on the mean splitting, whereas for the six-dot qubit the detuning that generates the splitting adds a fluctuation growing with the splitting itself, so the additional dots pay off only for small splittings. This implies that a longer chain is thus a better quantum memory but not necessarily a better qubit under manipulation. Although topological protection is expected in a longer Kitaev chain qubit, observing it in the current Rabi and Ramsey measurement protocols remains an experimental challenge.

cond-mat.mes-hall↗

arXiv2026

Magnetic Breakdown and Anomalous Quantum Oscillation in Rhombohedral Tetralayer Graphene

We investigate magnetic breakdown near Van Hove singularities (VHSs) in the electron-doped rhombohedral tetralayer graphene, where chiral superconductivity has recently been reported. Using the noninteracting band structure and Kubo formula, we identify anomalous Shubnikov-de Haas effects: Ring-like structures in the Landau fan and anomalous high-frequency peaks in the frequency spectra. These anomalous quantum oscillations can be understood by the reconstruction from magnetic breakdown among three nearby Fermi pockets separated by VHSs. Remarkably, these qualitative anomalous features persist into a stronger-VHS regime, where the semiclassical picture breaks down. The temperature and (weak) disorder dependence of the oscillations are also investigated. Our results establish that the magnetic-breakdown-induced anomalous quantum oscillation provides a general distinctive probe for the underlying Fermi-surface geometry associated with VHSs and may explain the recent quantum oscillation experiment in rhombohedral tetralayer graphene [arXiv:2606.05356].

cond-mat.mes-hall↗

arXiv2026

An asymptotically solvable model of many-body critical phases: mobility edges, scars, and inverted scars

While the prethermal regime of random many-body localized (MBL) systems is dominated by accidental many-body resonances, another class of resonances, originating from the underlying potential structure, is expected in large-size deterministic systems. It is known that this class of resonances can lead to single-particle critical phases that are neither localized nor extended, but the consequences in interacting systems remain unclear. In this work, we construct an asymptotically solvable model of a one-dimensional nearest-neighbor interacting spin chain, whose spatial structure induces a hierarchy of mirror-like many-body resonances. We derive two phases in the thermodynamic limit, characterized by the satisfaction and violation of a version of the weak eigenstate thermalization hypothesis (ETH). While these two phases are similar to the usual MBL and ETH phases, there exist rare eigenstates that behave like the opposite phase, interpreted as many-body scars and inverted scars. Surprisingly, the two phases can be separated by a finite-temperature phase transition, corresponding to a thermodynamic many-body mobility edge, which was often believed to be impossible. Our results also suggest the existence of delocalized rare regions in an otherwise-localized interacting Aubry-André model, even if there are no low-disorder regions like those in random systems. This challenges the common belief that there is no avalanche instability in quasiperiodic MBL.

cond-mat.dis-nn↗

arXiv2026

Conventional and practical metallic superconductivity arising from repulsive Coulomb coupling

A concrete question is discussed: Can there be conventional s-wave superconductivity in regular 3D (or 2D) metals, i.e., electrons in a jellium background, interacting via the standard Coulomb coupling? We are interested in 'practical' SC that can in principle be observed in experiments, so the $T=0$ ground state being SC is not of interest, or for that matter a $T_c$ which is exponentially small and therefore 'impractical' is also not of interest in the current work. We discuss both 2D and 3D cases, focusing mostly on the 3D case. We find that almost any theory based on the BCS-Migdal-Eliashberg paradigm, with some form of screened Coulomb coupling replacing the electron-phonon coupling in the BCS or Eliashberg theory, would uncritically predict absurdly high $T_c\sim100$ K for s-wave SC in all metals (including the alkali metals, which are well-described by the jellium model) arising from the unavoidable fact that the Fermi, plasmon, and Coulomb potential energy scales are all $>10^4$ K. Therefore, we conclude, based on reduction ad absurdum, that the violation of the venerable Migdal theorem in this problem is sufficiently disruptive that no significance can be attached to numerous existing theoretical publications in the literature claiming plasmon-induced (or other similar Coulomb coupling-induced) practical SC. Using a careful analysis of the Eliashberg gap equations we find that the $T_c$ of the 3D (or 2D) electron gas can be reduced well below $\sim1$ K depending on choices of frequency cut-off parameters that are introduced to satisfy Migdall's theorem but are apriori unknown. The only believable result is the one discovered 60 years ago by Kohn and Luttinger predicting non-s-wave SC arising from Friedel oscillations with exponentially low $T_c$. We provide several theoretical approaches using both BCS and Eliashberg theories and different screening models to make our point.

cond-mat.supr-con↗

arXiv2026

Stripe-tuned superconductivity in single-flavor metals with nontrivial quantum geometry

We study how the interplay between nontrivial quantum geometry and an applied stripe potential affects superconductivity in a two-dimensional single-flavor metal. Assuming a weak contact attractive interaction and focusing on the lowest subband in the presence of a strong stripe potential, we analytically derive two possible pairing states in the quasi-one-dimensional limit. In addition to the conventional longitudinal $p_y$-wave order (with the stripes along the $y$ direction), we find that an exotic transverse $p_x$-wave order can be stabilized. The competition between these two orders is controlled by the electron density of each stripe and the Berry-curvature-dressed interaction. Notably, the transverse $p_x$ wave order develops a nodal line at $k_x=0$, while the longitudinal $p_y$ order is fully gapped. We discuss the possible experimental probes distinguishing these orders. Our results establish a way of controlling the pairing symmetry through a stripe potential, predicting superconductivity with nontrivial quantum geometry.

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arXiv2026

2D Transport in an in-plane magnetic field

A parallel in-plane magnetic field could, in principle, distinguish between two competing physical scenarios for the experimentally observed density-tuned 2D metal-insulator transition (where decreasing the carrier density leads to a crossover from an effective metal to an effective insulator): Wigner crystallization or Anderson localization. Since the main scattering mechanism in 2D doped semiconductors arises from screened random charged impurities and screening in turn depends on the electronic density of states, the in-plane magnetic field could distinguish between the two by decreasing screening through spin polarization and this enhances the effective critical density for Anderson localization compared with Wigner crystallization. We give the general theory and provide results for the quantitative magnitudes of the spin polarization effect on the transition density by focusing on two recent experiments [Z. Ge, et al, arXiv:2510.12009, T. Han, et al, arXiv:2604.00113], noting that the critical density may actually decrease if the dominant scattering is by short-ranged defects instead of long-ranged charged impurities. The difference between the two cases arises from whether spin polarization dominates screening (enhanced critical density) or the Fermi surface (suppressed critical density).

cond-mat.mes-hall↗

arXiv2026

Avoiding Dilution: Using Diffusion and Vision Transformers to resolve Majorana Features in Nanowires at High Temperature

Identifying Majorana zero modes in semiconductor--superconductor nanowires requires ultra-low temperature transport measurements in dilution refrigerators, making device screening slow and resource-intensive. Here, we investigate whether high-temperature conductance data can be used to infer low-temperature Majorana nanowire properties before committing devices to dilution-refrigerator characterization. We generate paired high- and low-temperature conductance simulations for disordered Majorana nanowires and train neural networks to perform two related tasks. First, we use a Shifted Window U-Net Transformer diffusion-inspired architecture to reconstruct low-temperature conductance from thermally broadened high-temperature measurements, achieving high-fidelity recovery with $R^2 \approx {0.95}$ for local conductance and $R^2 \approx {0.91}$ for nonlocal conductance. Second, we train a Video Vision Transformer-based network to predict the low-temperature topological visibility directly from high-temperature conductance, obtaining $R^2 \approx {0.80}$. These results demonstrate that machine-learning models can recover and infer low-temperature Majorana features from experimentally easier high-temperature data, providing a practical route for rejecting poor devices early thus avoiding slow and resource-intensive dilution refrigeration for non-promising devices. This high-temperature screening approach could substantially accelerate the experimental feedback loop for Majorana nanowire device development.

cond-mat.mes-hall↗

arXiv2026

Symmetric localization of $ν_{\text{tot}}=4/3$ fractional topological insulator edges

Motivated by the recent twisted MoTe$_2$ experiment [arXiv:2601.18508], we develop a disordered interacting edge theory of a fractional topological insulator at $ν_{\text{tot}}=4/3$, consisting of two time-reversal-conjugated $ν=2/3$ fractional quantum Hall states. For an $S_z$-conserving edge, we uncover three distinct phases with two possible conductance values per edge in the long-edge limit: $\frac{2}{3}\frac{e^2}{h}$ and $\frac{4}{3}\frac{e^2}{h}$. In the presence of $S_z$-changing perturbations (e.g., Rashba spin-orbit coupling), an interaction-induced insulating edge state can emerge without breaking time-reversal or charge-conservation symmetry, corresponding to the absence of a topologically protected edge state. We show an exact mapping (with a special choice of parameters) to a noninteracting fermionic theory exhibiting Anderson localization, and the weak-coupling phase diagrams are also constructed, showing that symmetric localization can emerge regardless of other $S_z$-conserving perturbations. Our results showcase an explicit, experimentally relevant example that the edge-state two-terminal transport can yield false-negative results in identifying the $ν_{\text{tot}}=4/3$ fractional topological insulators.

cond-mat.str-el↗

arXiv2026

Evolution from an acoustic-plasmon-mediated superconductivity to an acoustic-phonon-mediated superconductivity in bilayers

Motivated by recent developments in van der Waals heterostructures, we revisit the acoustic plasmon mechanism of superconductivity in bilayer systems composed of a light layer (LL) and heavy layer (HL) by employing Eliashberg theory. The exchange of virtual plasmons in the HL can lead to a retarded in time attractive interaction between electrons of LL that we model through the screened interaction in the bilayer system within the random phase approximation. We explore the evolution from acoustic plasmon mediated superconductivity to phonon mediated superconductivity by studying the evolution of $T_c$ as the HL mass is increased by a few orders of magnitude compared with the electronic mass in LL. The lower HL mass corresponds to the bilayer acoustic plasmon, while the latter regime is closer to the Born-Oppenheimer regime of acoustic phonon mediated strongly retarded pairing. The heavy HL mass limit is known to obey Migdal's theorem by virtue of the small ratio of the two individual layer masses. We study the nonadiabatic effects for the arbitrary mass ratio with no small parameter systematically by using a frequency cut off in the Eliashberg theory, providing $T_c$ as a function of this cut off.

cond-mat.supr-con↗

arXiv2026

Superconductivity from phonon-mediated retardation in a single-flavor metal

We study phonon-mediated pairings in a single-flavor metal with a tunable Berry curvature. In the absence of Berry curvature, we discover an unexpected possibility: $p$-wave superconductivity emerging purely from the retardation effect, while the static BCS approximation fails to predict its existence. The gap function exhibits sign-change behavior in frequency (owing to the dynamical structure of the phonon-mediated interaction in the $p$-wave channel), and $T_c$ obeys a BCS-like scaling. We further show that the Berry curvature stabilizes the chiral $p$-wave superconductivity and can induce transitions to higher-angular-momentum pairings. Our results establish that the phonon-mediated mechanism is a viable pairing candidate in single-flavor systems, such as the quarter-metal superconductivity observed in rhombohedral graphene multilayers.

cond-mat.supr-con↗

arXiv2026

Coherently synchronized oscillations in many-body localization

We find an unexpected phenomenon of coherently synchronized oscillations in a mirror-symmetric many-body localized system. A synchronization transition of the spin oscillations is found by changing the spin-spin interactions. To understand this phenomenon, an effective Ising model based on local integrals of motion is proposed. We find that the synchronization transition can be understood as a paramagnetic-to-ferromagnetic Ising transition. Based on the Ising model, we theoretically estimate the synchronized frequencies and the synchronization transition points, which agree well with numerical results.

cond-mat.dis-nn↗

arXiv2026

Towards a microscopic model for an electronic quantum charge liquid

We provide a route to constructing an electronic quantum charge liquid (QCL), a state made up of fermions at fractional filling of a lattice that does not break translation. Starting with spinless fermions at filling $ν=3/2$ we pair them to get bosons at filling $ν=3/4$ per unit cell. The tetramer model, a generalization of the dimer model, on the square lattice is evaluated as a candidate bosonic QCL at filling $ν= 3/4$. It is shown that these models exhibit a local $\mathbb{Z}_4$ symmetry. Upon numerical study of a family of tetramer wavefunctions it is found that while one is gapless due to $\mathrm{U}(1)^3$ symmetry at least one other can be definitively shown to be gapped. The gapped nature of this state, along with its $\mathbb{Z}_4$ symmetry, leads us to propose that it is an example of the elusive bosonic QCL displaying the minimal $\mathbb{Z}_4$ topological order. We conclude by discussing possible extensions to other lattice geometries, electronic QCLs, and to Rydberg atoms.

cond-mat.str-el↗

arXiv2026

Interplay of disorder and interaction in quantum Hall systems: from fractional quantum Hall liquids to Wigner crystals and amorphous solids

We investigate the interplay of disorder and interaction in two-dimensional electron systems in a strong magnetic field, focusing on the transition between Wigner crystals and fractional quantum Hall liquids. We first study classical Wigner crystals with charged impurities, revealing an evolution from a coherent crystal to local crystalline domains with short-range order and eventually to an amorphous state as impurity concentration increases. We then analyze noninteracting quantum electron crystals created by periodic potentials, showing that their structure factor exhibits both peaks and rings, distinct from classical Wigner crystals. Finally, we explore fractional quantum Hall liquids with random short-range disorder and quenched charged impurities, demonstrating that the ground state can evolve from an incompressible liquid to a localized ordered state and eventually to an amorphous state as disorder strength increases. In general, we find that random charged impurities lead to longer-range crystalline ordering than the short-range random disorder. Our findings highlight the rich interplay between disorder and interaction in quantum Hall systems and provide insights into experimental observations of these phenomena. By qualitative comparison with a recent STM experiment [Nature \textbf{628}, 287 (2024)], we conclude that the 2D system crosses over from an incompressible homogeneous fractional quantum Hall liquid to a generic locally ordered solid and eventually to a disordered amorphous solid at large disorder.

cond-mat.mes-hall↗

arXiv2026

Rashba spin-orbit coupling and artificially engineered topological superconductors

One of the most important physical effects in condensed matter physics is the Rashba spin-orbit coupling (RSOC), introduced in seminal works by Emmanuel Rashba. In this article, we discuss, describe, and review (providing critical perspectives on) the crucial role of RSOC in the currently active research area of topological quantum computation. Most, if not all, of the current experimental topological quantum computing platforms use the idea of Majorana zero modes as the qubit ingredient because of their non-Abelian anyonic property of having an intrinsic quantum degeneracy, which enables nonlocal encoding protected by a topological energy gap. It turns out that RSOC is a crucial ingredient in producing a low-dimensional topological superconductor in the laboratory, and such topological superconductors naturally have isolated localized midgap Majorana zero modes. In addition, increasing the RSOC strength enhances the topological gap, thus enhancing the topological immunity of the qubits to decoherence. Thus, Rashba's classic work on SOC may lead not only to the realization of localized non-Abelian anyons, but also fault tolerant quantum computation.

cond-mat.mes-hall↗

arXiv2026

Large Scale Optimization of Disordered Hubbard Models through Tensor and Neural Networks

We theoretically demonstrate a practical method for tuning randomly disordered 2D quantum-dot grids underlying spin qubit platforms using vision-based neural networks trained on tensor-network generated charge-stability data. We show that a simulatable local $3\times 3$ window already contains sufficient information to tune the central dot within a much larger array, thereby validating a sliding-window approach in which one tunes a local region and then translates that window across the lattice to calibrate a larger device. This avoids the computationally intractable necessity for obtaining the ground states for large systems with exponentially large Hilbert space. For the experimentally relevant case where only the on-site disorder is unknown, the neural network predicts the relevant parameters with very high fidelity in the $3\times 3$ setting [$R^2 >0.99$], and after fine tuning on only a small number of larger-device samples, it retains high accuracy for the central dot of a $5\times 5$ plaquette [$R^2\approx 0.98$]. When all the dots parameters are treated as unknown, prediction of the on-site disorder remains robust [$R^2>0.9$ for both $3\times 3$ and $5\times 5$], although the remaining parameters are substantially more difficult to infer from the same charge-stability data. This shows that the most practically important disorder parameter for tuning can still be inferred reliably even in the fully disordered setting for the computationally difficult 5x5 arrays.

cond-mat.mes-hall↗

arXiv2026

Majorana zero modes in semiconductor-superconductor hybrid structures: Defining topology in short and disordered nanowires through Majorana splitting

Majorana zero modes (MZMs) are bound midgap topological excitations at the ends of a 1D topological superconductor, which must come in pairs. If the two MZMs in the pair are sufficiently well-separated by a distance much larger than their individual localization lengths, then the MZMs behave as non-Abelian anyons which can be braided to carry out fault-tolerant topological quantum computation. In this `topological' regime of well-separated MZMs, their overlap is exponentially small, leading to exponentially small Majorana splitting, thus enabling the MZMs to be topologically protected by the superconducting gap. In real experimental samples, however, the existence of disorder and the finite length of the 1D wire considerably complicate the situation, leading to ambiguities in defining `topology' since the Majorana splitting between the two end modes may not necessarily be small in disordered wires of short length. We theoretically study this situation by calculating the splitting in experimentally relevant short disordered wires, and explicitly investigating the applicability of the `exponential protection' constraint as a function of disorder, wire length, and other system parameters in realistic models of nanowires currently being used experimentally. We find that the exponential regime is highly constrained, and is suppressed for disorder somewhat less than the topological superconducting gap. We provide detailed results and discuss the implications of our theory for the currently active experimental search for MZMs in superconductor-semiconductor hybrid platforms. A general consequence of our work is that `topology' in finite disordered wires may not be uniquely defined, necessitating a careful analysis which depends on the context.

cond-mat.mes-hall↗

arXiv2026

Singlet-only always-on gapless exchange (SAGE) spin qubits: Charge noise effects and two-qubit gates

Singlet-only always-on gapless exchange (SAGE) spin qubits are an alternative type of exchange-only (EO) qubits that encode a single qubit in the spins of four electrons located in four tunnel-coupled quantum dots. While conventional EO qubits are susceptible to local magnetic field gradients caused by local nuclear environments and $g$-factor variations, the SAGE qubit subspace is inherently protected from magnetic-gradient-induced Pauli errors by virtue of the singlet-only encoding, which is invariant under magnetic field gradients, and the always-on exchange couplings, which provide energetic leakage protection. However, the always-on operation simultaneously increases the qubit's sensitivity to charge noise. Here, starting from a Hubbard model describing the underlying electronic structure of the coupled quantum dots, we characterize the performance of SAGE qubits in the presence of $1/f$ charge noise that induces fluctuations in both the dot chemical potentials and the interdot tunnel couplings. We calculate SAGE idle coherence times and show that realistic CPMG-like pulse sequences can be used to significantly extend SAGE single-qubit coherence times for experimentally relevant charge noise strengths. We likewise study the fidelity of SAGE two-qubit gates in the presence of charge and magnetic noise and again propose a simple refocusing strategy to mitigate the noise, while increased ramp times of the entangling pulse suppress leakage into noncomputational states.

cond-mat.mes-hall↗

arXiv2026

Impurity-induced thermal crossover in fractional Chern insulators

The recent experimental observation of fractional quantum anomalous Hall (FQAH) states in rhombohedral multilayer graphene has attracted significant attention. One of the most intriguing observations is that the FQAH states at various fractional fillings give way to IQAH states as the temperature is lowered. In this work, we propose a mechanism for the appearance of FQAH states within a finite temperature range in a toy model. The model consists of a flat Chern band and impurities, and we analyze the effects of impurities on the system's behavior at finite temperatures. We believe that the crossover may arise from the competition between the energy penalty for thermal excitations and the increase in entropy. We support our theoretical argument with numerical calculations using exact diagonalization. Our results suggest that impurities may play a crucial role in the crossover from the FQAH to IQAH states in rhombohedral pentalayer graphene.

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