arXiv subjects

Leonid I. Glazman

Publications and source records attributed to Leonid I. Glazman.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous Frequency Shift in Low-Loss Superconducting Granular Aluminum Resonators

Superconducting high-kinetic inductance materials like granular aluminum (grAl) are a versatile part of the circuit quantum electrodynamics (cQED) toolbox, provided their losses at microwave frequencies are low enough. To further advance the use of grAl in quantum devices, it is indispensable to identify the dominant loss mechanisms. The standard approach pairs electromagnetic simulations of resonator geometry with measured temperature and power dependence of resonance frequency and loss rate. Most materials follow the phenomenological theory of two-level systems (TLSs) at low temperatures, resulting in an initial decrease of the resonance frequency with increasing temperature. In our work, we observe an opposite behavior at the lowest temperatures: The resonance frequency initially increases with both temperature and readout power, contradicting the standard TLS model predictions. We are able to explain a part of the data by an alternative mechanism associated with the effect of superconducting quasiparticles in a granular system with spatially-nonuniform gap. Yet, the observed change in the resonance frequency with temperature is not matched by a proportional change in the loss rate. We also observe anomalous positive frequency responses following high-energy events, characterized by several time scales. While anomalous behavior has been reported previously in grAl, the disagreement with standard models is particularly visible in our devices thanks to their exceptionally low loss rates.

quant-ph↗

arXiv2026

Unified topological phase diagram of quantum Hall and superconducting vortex-lattice states

We present the global topological phase diagram of a two-dimensional electron gas placed in a quantizing magnetic field and proximitized by a superconducting vortex lattice. Our theory allows for arbitrary ratios of the pairing amplitude, magnetic field, and chemical potential. By analyzing the Bogoliubov--de Gennes Hamiltonian, we show that the resulting phase diagram is highly nontrivial, featuring a plethora of topological superconducting phases with chiral edge modes of quasiparticles. Landau-level mixing plays an essential role in our theory: even in the weak-pairing limit, it generically splits the integer quantum Hall transition lines into a sequence of transitions with larger Chern number jumps of both signs protected by the symmetries of the superconducting vortex lattice. Interestingly, we find that weak pairing induces trivial or topological superconductivity when chemical potential is tuned to a Landau level energy, depending on the Landau level index.

cond-mat.mes-hall↗

arXiv2026

Twisted Bilayer Graphene Lifetimes At Integer Fillings: An Analytic Result

Twisted bilayer graphene near integer fillings hosts correlated single-particle excitations whose dispersion and linewidth are increasingly accessible experimentally. We study these excitations using the topological heavy-fermion model, which captures both strong correlations and band topology of twisted bilayer graphene. In the decoupled limit, where both the single-particle fc hybridization and the Hund coupling between f and c electrons are absent, the model admits exact solutions in which free Dirac fermions coexist with interacting f electrons that form zero-width Hubbard bands. By treating the fc hybridization and Hund coupling perturbatively around this solvable limit, we obtain analytical results for the single-particle self-energy. From the resulting self-energy, we derive explicit expressions for both dispersion renormalization and scattering rates of both Hubbard-band excitations and low-energy Dirac modes, thereby establishing an analytical framework for understanding correlated excitations in twisted bilayer graphene. We analyze the scattering of the two kinds, Gamma3 and Gamma1,2, of Dirac electrons and find that they arise from different mechanisms. We also briefly investigate the effect of strain. Finally, we compare these analytical expressions with DMFT results for the same model.

cond-mat.str-el↗

arXiv2026

Lifetime and spectral function of topological heavy fermions

Twisted bilayer graphene provides a paradigmatic platform for exploring the interplay between electronic topology and strong correlations. Within the topological heavy fermion model [Song and Bernevig, Phys. Rev. Lett. 129, 047601 (2022)], topology and electron interactions are brought together by including a weak hybridization between the bands of itinerant $c$- and localized $f$-electrons. Hybridization infuses concentrated Berry curvature into the $f$-band, while leaving it flat. These band features have motivated recent proposals of a Mott semimetal phase above the flavor-ordering temperature at charge neutrality. In this work, we develop an analytic theory of the quasiparticle dispersion and lifetime in the Mott semimetal. We reformulate the interacting flat-band Hamiltonian as an on-site Hubbard interaction defined on a set of non-orthogonal orbitals, and compute the electron Green's function using the equation-of-motion method, in close analogy with the Hubbard-III approximation. Unlike the conventional Hubbard model, in our case this approximation is controlled by a well-defined small parameter in the theory. We evaluate the electron self-energy and demonstrate the emergence of well-defined low-energy quasiparticles with the dispersion and relaxation rate proportional to the interaction strength. The quasiparticle spectrum is well-resolved in energy and in momentum down to the very vicinity of the Fermi level. Our results illustrate unconventional spectral properties arising from strong correlations and nontrivial quantum geometry, and have direct relevance for spectroscopic probes such as quantum twisting microscope experiments.

cond-mat.str-el↗

arXiv2026

Recovery dynamics of a gap-engineered transmon after a quasiparticle burst

Ionizing radiation impacts create bursts of quasiparticle density in superconducting qubits. These bursts temporarily degrade qubit coherence which can be detrimental for quantum error correction. Here, we experimentally resolve quasiparticle bursts in 3D gap-engineered transmon qubits by continuously monitoring qubit transitions. Gap engineering allows us to reduce the burst detection rate by a factor of five. This reduction falls four orders of magnitude short of that expected if the quasiparticles were to quickly thermalize to the cryostat temperature. We associate the limited effect of gap engineering with the slow thermalization of the phonons in our chips after the burst.

quant-ph↗

arXiv2026

Nonlocal conductance of a Majorana wire near the topological transition

We develop a theory of the nonlocal conductance $G_{RL}(V)$ for a disordered Majorana wire tuned near the topological transition critical point. Under these conditions, the antisymmetric part of the differential conductance, $[G_{RL}(V) - G_{RL}(-V)] /2$, is the dominant one for a sufficiently long wire. This reflects the charge-neutral nature of the critical modes in the wire. We factorize the conductance into a term describing propagation of the critical modes along the wire, and terms describing the contacts between the wire and the normal leads. Topological transition affects only the former term. At the critical point, the localization length has a logarithmic singularity at the Fermi level, $l(E) \propto \ln(1 / E)$. This singularity directly manifests in the conductance magnitude, as $\ln |G_{RL}(V) / G_Q| \sim L / l(eV)$ for the wire of length $L \gg l(eV)$. Tuning the wire away from the immediate vicinity of the critical point changes the monotonicity of $l(E)$. This change in monotonicity allows us to define the width of the critical region around the transition point.

cond-mat.mes-hall↗

arXiv2025

Tunneling spectroscopy of two-dimensional superconductors with the quantum twisting microscope

The ongoing discoveries of graphene-based superconductors underscore the quest to understand the structure of new superconducting orders. We develop a theory that facilitates the use of the quantum twisting microscope (QTM) for that purpose. This work investigates momentum-conserving tunneling across a planar junction formed by a normal monolayer graphene tip and a superconducting graphene sample within the QTM setting. We show that the bias dependence of the zero-temperature tunneling conductance exhibits singularities that provide momentum-resolved information about the Bogoliubov quasiparticle spectra, including the superconducting gap. Using a model of superconducting twisted bilayer graphene (TBG), we illustrate that simultaneously tuning the tip doping level and the tip-sample twist angle allows for measuring the momentum-resolved superconducting gap in TBG. Our results indicate that momentum-conserving tunneling spectroscopy with the QTM is a promising method for exploring superconductivity in two-dimensional van der Waals materials.

cond-mat.mes-hall↗

arXiv2025

Effect of quasiparticles on the parameters of a gap-engineered transmon

We evaluate the quasiparticle contribution to the frequency shift and relaxation rates of a transmon with the Josephson junctions connecting superconductors that have unequal energy gaps. The gap difference substantially affects the transmon characteristics. We investigate their dependence on the density and effective temperature of the quasiparticles, and on the nominal (unperturbed by the quasiparticles) transmon frequency. At temperatures low compared to the qubit frequency, the gap difference can induce an anomalous positive frequency shift, resulting in a non-monotonic temperature dependence of the transmon frequency. The qubit relaxation rate exhibits a resonance when the qubit frequency matches the gap difference; the shape of the resonance is strongly temperature-dependent. We propose to use these effects to access the details of the quasiparticle energy distribution.

cond-mat.supr-con↗

arXiv2025

Full characterization of measurement-induced transitions of a superconducting qubit

Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.

quant-ph↗

arXiv2025

Theory of plasmon spectroscopy with the quantum twisting microscope

We consider plasmon-assisted electron tunneling in a quantum twisting microscope (QTM). The dependence of the differential conductance on the two control parameters of the QTM -- the twist angle and bias -- reveals the plasmon spectrum as well as the strength of plasmon-electron interactions in the sample. We perform microscopic calculations for twisted bilayer graphene (TBG), to predict the plasmon features in the tunneling spectra of TBG close to the magic angle for different screening environments. Our work establishes a general framework for inelastic tunneling spectroscopy of collective electronic excitations using the quantum twisting microscope.

cond-mat.str-el↗

arXiv2025

Spontaneous supercurrents and vortex depinning in two-dimensional arrays of $φ_0$-junctions

Two-dimensional arrays of ballistic Josephson junctions are important as model systems for synthetic quantum materials. Here, we investigate arrays of multiterminal junctions which exhibit a phase difference $φ_0$ at zero current. When applying an in-plane magnetic field we observe nonreciprocal vortex depinning currents. We explain this effect in terms of a ratchet-like pinning potential, which is induced by spontaneous supercurrent loops. Supercurrent loops arise in multiterminal $φ_0$-junction arrays as a consequence of next-nearest neighbor Josephson coupling. Tuning the density of vortices to commensurate values of the frustration parameter results in an enhancement of the ratchet effect. In addition, we find a surprising sign reversal of the ratchet effect near frustration 1/3. Our work calls for the search for novel magnetic structures in artificial crystals in the absence of time-reversal symmetry.

cond-mat.supr-con↗

arXiv2025

Subharmonic spin correlations and spectral pairing in Floquet time crystals

Floquet time crystals are characterized by subharmonic behavior of temporal correlation functions. Studying the paradigmatic time crystal based on the disordered Floquet quantum Ising model, we show that its temporal spin correlations are directly related to spectral characteristics and that this relation provides analytical expressions for the correlation function of finite chains, which compare favorably with numerical simulations. Specifically, we show that the disorder-averaged temporal spin correlations are proportional to the Fourier transform of the splitting distribution of the pairs of eigenvalues of the Floquet operator, which differ by $π$ to exponential accuracy in the chain length. We find that the splittings are well described by a log-normal distribution, implying that the temporal spin correlations are characterized by two parameters. We discuss possible implications for the phase diagram of the Floquet time crystals.

cond-mat.stat-mech↗

arXiv2025

High-frequency readout free from transmon multi-excitation resonances

Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurement is carried out by scattering a microwave tone off the qubit. Conventionally, the frequency of this tone is of the same order as the transmon frequency. The measurement fidelity in this approach is limited by multi-excitation resonances in the transmon spectrum which are activated at high readout power. These resonances excite the qubit outside of the computational basis, violating the desired quantum non-demolition character of the measurement. Here, we find that strongly detuning the readout frequency from that of the transmon exponentially suppresses the strength of spurious multi-excitation resonances. By increasing the readout frequency up to twelve times the transmon frequency, we achieve a quantum non-demolition measurement fidelity of 99.93% with a residual probability of leakage to non-computational states of only 0.02%.

quant-ph↗

arXiv2024

Dirac-point spectroscopy of flat-band systems with the quantum twisting microscope

Motivated by the recent development of the quantum twisting microscope, we formulate a theory of elastic momentum-resolved tunneling across a planar tunnel junction between a monolayer graphene layer situated on a tip and a twisting graphene-based sample. We elucidate features in the dependence of the tunnel current on bias and twist angle, which reflect the sample band structure and allow the tip to probe the momentum-and energy-resolved single-particle excitations of the sample. While the strongest features originate from the Fermi edge of the tip, we argue that features associated with the tip Dirac points provide a more immediate and precise map of the sample band structure. We specifically compute the low-temperature tunneling spectrum of magic angle twisted bilayer graphene (MATBG) rotated relative to the tip by nearly commensurate angles, highlighting the potential of Dirac-point spectroscopy to measure single-particle spectral functions of flat bands along specific lines in reciprocal space. Furthermore, our analysis of tunneling matrix elements suggests a method to extract the ratio of the intra-and inter-sublattice tunneling parameters $w_0/w_1$ of MATBG from the differential tunneling conductance. Finally, we discuss signatures of $C_{3z}$ symmetry breaking in the tunneling spectrum using strained MATBG as an example. Our work establishes a general theoretical framework for Dirac-point spectroscopy of flat-band systems using the quantum twisting microscope.

cond-mat.mes-hall↗

arXiv2024

Theory of phonon spectroscopy with the quantum twisting microscope

We develop a theory of probing phonon modes of van-der-Waals materials using the quantum twisting microscope. While elastic tunneling dominates the tunneling current at small twist angles, the momentum mismatch between the K-points of tip and sample at large twist angles can only be bridged by inelastic scattering. This allows for probing phonon dispersions along certain lines in reciprocal space by measuring the tunneling current as a function of twist angle and bias voltage. We illustrate this modality of the quantum twisting microscope by developing a systematic theory for graphene-graphene junctions. We show that beyond phonon dispersions, the tunneling current also encodes the strength of electron-phonon couplings. Extracting the coupling strengths for individual phonon modes requires careful consideration of various inelastic tunneling processes. These processes are associated with the intralayer and interlayer electron-phonon couplings and appear at different orders in a perturbative calculation of the tunneling current. We find that the dominant process depends on the particular phonon mode under consideration. Our results inform the quest to understand the origin of superconductivity in twisted bilayer graphene and provide a case study for quantum-twisting-microscope investigations of collective modes.

cond-mat.mes-hall↗

arXiv2024

Unconventional discontinuous transitions in isospin systems

We show that two-dimensional fermions with dispersion $k^2$ or $k^4$ undergo a first-order Stoner transition to a fully spin-polarized state despite that the spin susceptibility diverges at the critical point. We extend our analysis to systems with dispersion $k^{2α}$ and spin and valley isospin and show that there is a cascade of instabilities into fractional-metal states with some electron bands fully depleted; narrow intermediate ranges of partially-depleted bands exist for $α<1$ or $α> 2$. The susceptibility becomes large near each transition. We discuss applications to biased bilayer and tri-layer graphene and moiré systems.

cond-mat.str-el↗

arXiv2024

Unconventional discontinuous transitions in a 2D system with spin and valley degrees of freedom

We analyze the transition into the most favorable ordered state for a system of 2D fermions with spin and valley degrees of freedom. We show that for a range of rotationally invariant dispersions, the ordering transition is highly unconventional: the associated susceptibility diverges (or almost diverges) at the transition, yet immediately below it the system jumps discontinuously into a fully polarized state. We analyze the dispersion of the longitudinal and transverse collective modes in different parameter regimes above and below the transition. Additionally, we consider ordering in a system with full $\mathrm{SU(4)}$ symmetry and show that there is a cascade of discontinuous transitions into a set of states, which includes a quarter-metal, a half-metal and a three-quarter metal. We compare our results with the data for biased bilayer and tri-layer graphene.

cond-mat.str-el↗

arXiv2024

Thermal transport across a Josephson junction in a dissipative environment

At zero temperature, a Josephson junction coupled to an ohmic environment displays a quantum phase transition between superconducting and insulating phases, depending whether the resistance of the environment is below or above the resistance quantum. At finite temperature, this so-called Schmid transition turns into a crossover. We determine the conditions under which the temperature dependence of the thermal conductance, which characterizes heat flow from a hot to cold resistor across the Josephson junction, displays universal scaling characteristic of the Schmid transition. We also discuss conditions for heat rectification to happen in the circuit. Our work can serve as a guide for identifying signatures of the Schmid transition in heat transport experiments.

cond-mat.supr-con↗