arXiv subjects

Leonid Levitov

Publications and source records attributed to Leonid Levitov.

At least 37 records · Page 2Linked to original sources

arXiv2023

Superconductivity near spin and valley orders in graphene multilayers: a systematic study

Spin excitations that soften near the onset of magnetic order have long been known to act as `paramagnon' pairing glue that can drive spin-triplet superconductivity. Recent findings of superconductivity in graphene bilayers and trilayers, occurring in the proximity of different itinerant ordered phases polarized in isospin (spin and valley), have motivated us to conduct a comprehensive investigation of an isospin extension of the paramagnon pairing mechanism in the vicinity of spin/isospin orders. In each case, we identify a soft mode, associated with the order parameter fluctuations, that mediates pairing interaction. We develop an approach that relates the soft mode described through summation of the contributions most strongly divergent at the onset of spin/valley isospin orders. This interaction is not always attractive, but if it is, it gives rise to an enhancement of superconducting $T_c$ in an appropriate pairing channel. In the cases when the pairing interaction is attractive, it leads to the formation of a superconducting state which can be either spin-triplet and valley-singlet or vice versa, depending on the specific isospin order type. These findings demonstrate that the occurrence of superconductivity in the vicinity of an itinerant magnetic phase is a generic phenomenon, closely mirroring experimental observations.

cond-mat.supr-con↗

arXiv2023

Collective excitations in chiral Stoner magnets

We argue that spin and valley-polarized metallic phases recently observed in graphene bilayers and trilayers support chiral edge modes that allow spin waves to propagate ballistically along system boundaries without backscattering. The chiral edge behavior originates from the interplay between the momentum-space Berry curvature in Dirac bands and the geometric phase of a spin texture in position space. The edge modes are weakly confined to the edge, featuring dispersion which is robust and insensitive to the detailed profile of magnetization at the edge. This unique character of edge modes reduces their overlap with edge disorder and enhances the mode lifetime. The mode propagation direction reverses upon reversing valley polarization, an effect that provides a clear testable signature of geometric interactions in isospin-polarized Dirac bands.

cond-mat.mes-hall↗

arXiv2023

Collinear scattering and long-lived excitations in two-dimensional electron fluids

For a long time, it has been thought that 2D Fermi gases could support long-lived excitations, thanks to the collinear quasiparticle scattering controlled by phase space constraints at a 2D Fermi surface. We present a direct calculation that reveals such excitations. The excitation lifetimes are found to exceed the fundamental bound set by Landau Fermi-liquid theory by a factor as large as $(T_F/T)^α$ with $α\approx 2$. These excitations represent Fermi-surface modulations of an odd parity, one per each odd angular momentum. To explain this surprising behavior, we employ a connection between the linearized quantum kinetic equation and the dynamics of a fictitious quantum particle moving in a 1D reflectionless ${\rm sech^2}$ potential. In this framework, we identify the long-lived excitations in Fermi gases as zero modes that arise from supersymmetry.

cond-mat.mes-hall↗

arXiv2023

Isospin and momentum polarized orders in bilayer graphene

Electron bands in the untwisted bilayer graphene flatten out in a transverse electric field, offering a promising platform for correlated electron physics. We predict that the spin/valley isospin magnetism, resembling that seen in moiré bands, coexists with momentum-polarized phases occurring via a ``flocking transition'' in momentum space. This transition results in the electron momentum distribution being spontaneously displaced relative to the $K$ and $K'$ valley centers. These phases feature unusual observables such as persistent currents in the ground state. Momentum-polarized carriers ``sample'' the Berry curvature of the conduction band, resulting in a unique behavior of the anomalous Hall conductivity and other effects that do not occur in previously studied systems.

cond-mat.mes-hall↗

arXiv2022

Spin-triplet superconductivity at the onset of isospin order in biased bilayer graphene

The quest for unconventional superconductivity governed by Coulomb repulsion between electrons rather than phonon attraction received new momentum with the advent of moiré graphene. Initially, delineating the phonon and Coulomb-repulsion-based pairing mechanisms has proven to be a challenging task, however the situation has changed after recent discovery of superconductivity in non-twisted graphene bilayers and trilayers. Superconductivity occurring at the phase boundaries of spin and valley polarized orders calls for non-phonon scenarios, yet the specific pairing mechanisms remain to be understood. Here we analyze a striking example -- superconductivity in graphene bilayers occurring at the onset of valley-polarized order induced by a magnetic field. We describe an attraction-from-repulsion mechanism for pairing mediated by a quantum-critical mode, which fully explains the observed phenomenology. While it is usually notoriously difficult to infer the pairing mechanism from the observed superconducting phases, this case presents a rare exception, allowing for a fairly unambiguous identification of the origin of the pairing glue. A combination of factors such as the location of superconducting phase at the onset of isospin-polarized phase, a threshold in a magnetic field, above which superconductivity occurs, and its resilience at high magnetic fields paints a clear picture of a triplet superconductivity driven by quantum-critical fluctuations.

cond-mat.supr-con↗

arXiv2022

Quantum critical behavior in magic-angle twisted bilayer graphene

The flat bands of magic-angle twisted bilayer graphene (MATBG) host strongly-correlated electronic phases such as correlated insulators, superconductors and a strange-metal state. The latter state, believed to be key for understanding the electronic properties of MATBG, is obscured by various phase transitions and thus could not be unequivocally differentiated from a metal undergoing frequent electron-phonon collisions. Here, we report transport measurements in superconducting MATBG in which the correlated insulator states are suppressed by screening. The uninterrupted metallic ground state shows resistivity that is linear in temperature over three decades and spans a broad range of doping including those where a correlation-driven Fermi surface reconstruction occurs. This strange-metal behavior is distinguished by Planckian scattering rates and a linear magnetoresistivity. In contrast, near charge neutrality or a fully-filled flat band, as well as for devices twisted away from the magic angle, we observe the archetypal Fermi liquid behavior. Our measurements demonstrate the existence of a quantum critical phase whose fluctuations dominate the metallic ground state throughout a continuum of doping. Further, we observe a transition to the strange metal upon suppression of the superconducting order, suggesting a relationship between quantum fluctuations and superconductivity in MATBG.

cond-mat.str-el↗

arXiv2022

Wide-range $T^2$ resistivity and umklapp scattering in moiré graphene

We argue that the unusually strong electron-electron interactions in the narrow bands in moiré superlattices originate from compact Wannier orbitals. Enhanced overlaps of electronic wavefunctions, enabled by such orbitals, result in a strong el-el superlattice umklapp scattering. We identify the umklapp scattering processes as a source of the strong temperature-dependent resistivity observed in these systems. In a simple model, the umklapp scattering predicts a $T$-dependent resistivity that grows as $T^2$ and is getting bigger as the Wannier orbital radius decreases. We quantify the enhancement in el-el scattering by the Kadowaki-Woods (KW) ratio, a quantity that is sensitive to umklapp scattering but, helpfully, insensitive to the effects due to the high density of electronic states. Our analysis predicts anomalously large KW ratio values that clearly indicate the importance of the umklapp el-el processes and their impact on the $T$-dependent resistivity.

cond-mat.mes-hall↗

arXiv2021

Superconductivity in the vicinity of an isospin-polarized state in a cubic Dirac band

We present a theory of superconducting pairing originating from soft critical fluctuations near isospin-polarized states in rhombohedral trilayer graphene. Using a symmetry-based approach, we determine possible isospin order types and derive the effective electron-electron interactions mediated by isospin fluctuations. Superconductitivty arising due to these interactions has symmetry and order parameter structure that depend in a unique way on the "mother" isospin order. This model naturally leads to a superconducting phase adjacent to isospin-ordering phase transition, which mimics the behavior observed in experiment. The symmetry of the paired state predicted for the isospin order type inferred in experiments matches the observations. These findings support a scenario of superconductivity originating from electron-electron interactions.

cond-mat.supr-con↗

arXiv2021

Synchronizing Bloch-oscillating free carriers in moiré flat bands

Achieving Bloch oscillations of free carriers under a direct current, a long-sought-after collective many-body behavior, has been challenging due to stringent constraints on the band properties. We argue that the flat bands in moiré graphene fulfill the basic requirements for observing Bloch oscillations, offering an appealing alternative to the stacked quantum wells used in previous work aiming to access this regime. Bloch-oscillating moiré superlattices emit a comb-like spectrum of incommensurate frequencies, a property of interest for converting direct currents into high-frequency currents and developing broad-band amplifiers in THz domain. The oscillations can be synchronized through coupling to an oscillator mode in a photonic or plasmonic resonator. Phase-coherent collective oscillations in the resonant regime provide a realization of current-pumped THz lasing.

cond-mat.mes-hall↗

arXiv2021

Activating superconductivity in a repulsive system by high-energy degrees of freedom

We discuss superconducting pairing in a narrow conduction band sandwiched between unoccupied and occupied bands, an arrangement that enables an unconventional pairing mechanism governed by Coulomb repulsion. Pairing interaction originates from repulsion-assisted scattering between far-out pair states in the higher-energy bands and those at the Fermi level. Optimizing the bandstructure design and carrier density in order to bring plasma frequency below the bandgap renders the repulsion unscreened for the processes with a large frequency transfer. This allows the pairing to fully benefit from the pristine Coulomb repulsion strength. The repulsion-induced attraction is particularly strong in two dimensions and is assisted by a low density of carriers and the resulting low plasma frequency values. We assess the possible connection of this mechanism to superconductivity in magic-angle twisted bilayer graphene where the bandstructure features wide dispersive upper and lower minibands. We use a simple model to illustrate the importance of the far-out pairs in these bands and predict testable signatures of this superconductivity mechanism.

cond-mat.supr-con↗

arXiv2021

Universality in Phyllotaxis: a Mechanical Theory

One of humanity's earliest mathematical inquiries might have involved the geometric patterns in plants. The arrangement of leaves on a branch, seeds in a sunflower, and spines on a cactus exhibit repeated spirals, which appear with an intriguing regularity providing a simple demonstration of mathematically complex patterns. Surprisingly, the numbers of these spirals are pairs of Fibonacci numbers consecutive in the series 1, 2, 3, 5, 8, 13, 21, 34, 55... obeying a simple rule 1+2=3, 2+3=5, 5+8=13 and so on. This article describes how physics helps to clarify the origin of this fascinating behavior by linking it to the properties of deformable lattices growing and undergoing structural rearrangements under stress.

nlin.PS↗

arXiv2020

Untying the insulating and superconducting orders in magic-angle graphene

The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene prompts fascinating questions about the relationship of these orders. Independent control of the microscopic mechanisms governing these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing its separation from a metallic screening layer. We observe quenching of correlated insula-tors in devices with screening layer separations that are smaller than a typical Wannier orbital size of 15nm, and with the twist angles slightly deviating from the magic value 1.10 plus(minus) 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in the devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 T, giving rise to quantized Hall states with a Chern number of 2. Our study suggests reexamination of the often-assumed mother-child relation between the insulating and superconducting phases in moire graphene, and illustrates a new approach to directly probe microscopic mechanisms of superconductivity in strongly-correlated systems.

cond-mat.supr-con↗

arXiv2020

Superscreening by a Retroreflected Hole Backflow in Tomographic Electron Fluids

Electron hydrodynamics gives rise to surprising correlated behaviors in which electrons "cooperate" to quench dissipation and reduce the electric fields needed to sustain the flow. Such collective "free" flows are usually expected at the hydrodynamic lengthscales exceeding the electron-electron scattering mean free path $\ell_{\rm ee}$. Here we predict that in two-dimensional electron gases the collective free flows actually occur at the distances much smaller than $\ell_{\rm ee}$, in a nominally ballistic regime. The sub-$\ell_{\rm ee}$ free flows arise due to retroreflected holes originating from head-on electron electron collisions, which retrace the paths of impinging electrons and cancel out their potential. An exact solution, obtained in Corbino geometry, predicts potential strongly screened by the hole backflow. Screened potential is described by a fractional power law $r^{-5/3}$ over a wide range of $r$ values, from macroscales down to deep sub-$\ell_{\rm ee}$ scales, and a distinct non-Fermi-liquid temperature dependence.

cond-mat.mes-hall↗

arXiv2020

Dipole-active collective excitations in moiré flat bands

Collective plasma excitations in moiré flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional two-dimensional plasmon modes, dispersing as $\sqrt{k}$ at low frequencies and plunging into particle-hole continuum at higher frequencies, the moiré plasmons pierce through the flat-band continuum and acquire a strong over-the-band character. Due to the complex structure of the moiré superlattice unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in the superlattice Brillouin zone. Using a toy Hubbard model for the correlated insulating order in a flat band, we predict that these high-frequency modes become strongly dipole-active upon the system undergoing charge ordering, with the low-frequency modes gapped out within the correlated insulator gap. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical measurements, providing a convenient diagnostic of the correlated states.

cond-mat.mes-hall↗

arXiv2020

Tunable electron-phonon interactions in long-period superlattices

The efficiency of optical emitters can be dramatically enhanced by reducing the effective mode volume (the Purcell effect). Here we predict an analogous enhancement for electron-phonon (el-ph) scattering, achieved by compressing the electronic Wannier orbitals. Reshaping of Wannier orbitals is a prominent effect in graphene moiré superlattices (SLs) where the orbitals are tunable by the twist angle. A reduction of the orbital effective volume leads to an enhancement in the effective el-ph coupling strength, yielding the values considerably bigger than those known for pristine monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values predicted from the enhanced spectral density of electronic excitations. The enhanced phonon emission and scattering rates are manifest in the observables such as electron-lattice cooling and the linear-$T$ resistivity, both of which are directly tunable by the moiré twist angle.

cond-mat.mes-hall↗

arXiv2020

Three-Body Bound States of Quantum Particles: Higher Stability Through Braiding

Cold atoms embedded in a degenerate Fermi system interact via a fermionic analog of the Casimir force, which is an attraction of a -1/r form at distances shorter than the Fermi wavelength. Interestingly, the hydrogenic two-body bound states do not form in this regime because the interaction strength is too weak under realistic conditions, and yet the three-body bound states can have a considerably higher degree of stability. As a result, the trimer bound states can form even when the dimer states are unstable. A quasiclassical analysis of quantum states supported by periodic orbits singles out the "figure-eight" orbits, predicting bound states that are more stable than the ones originating from circular orbits. The discrete energies of these states form families of resonances with a distinct structure, enabling a direct observation of signatures of figure-eightbraiding dynamics.

cond-mat.quant-gas↗

arXiv2019

Plasmonic Drag in a Flowing Fermi Liquid

Collective modes in two-dimensional electron fluids show an interesting response to a background carrier flow. Surface plasmons propagating on top of a flowing Fermi liquid acquire a non-reciprocal character manifest in a $\pm k$ asymmetry of mode dispersion. The nonreciprocity arises due to Fermi surface polarization by the flow. The flow-induced interactions between quasiparticles make collective modes of the system uniquely sensitive to subtle "motional" Fermi-liquid effects. The flow-induced Doppler-type frequency shift of plasmon resonances, arising due to electron interactions, can strongly deviate from the classical value. This opens a possibility to directly probe motional Fermi-liquid effects in plasmonic near-field imaging experiments.

cond-mat.mes-hall↗

arXiv2019

Angular Superdiffusion and Directional Memory in Two-Dimensional Electron Fluids

We demonstrate that 2D Fermi liquids can support peculiar excitations that are not subject to Landau's $T^2$ dissipation. The long-lived excitations relax through correlated angular dynamics involving "lock-step" angular displacements along the Fermi surface occurring in collinear two-particle collisions, a surprising behavior that is unique to 2D systems. We develop a microscopic picture of the non-Brownian random walk, describing the angular dynamics as anomalous diffusion ("superdiffusion") on the Fermi surface. Strongly-correlated dynamics with directional memory, mediated by novel undamped excitations, dominates at moderately long times, pushing the onset of conventional hydrodynamics to abnormally large timescales. This exotic behavior can be directly probed by momentum-resolved tunneling techniques.

cond-mat.mes-hall↗