arXiv subjects

Jung-Jung Su

Publications and source records attributed to Jung-Jung Su.

17 recordsLinked to original sources

arXiv2025

Experimental signature of layer skyrmions and implications for band topology in twisted WSe2 bilayers

Transition metal dichalcogenide (TMD) twisted homobilayers have been established as an ideal platform for studying strong correlation phenomena, as exemplified by the recent discovery of fractional Chern insulator (FCI) states in twisted MoTe2 and Chern insulators (CI) and unconventional superconductivity in twisted WSe2 (tWSe2). In these systems, nontrivial topology in the strongly layer-hybridized regime can arise from a spatial patterning of interlayer tunneling amplitudes and layer-dependent potentials that yields a lattice of layer skyrmions. Here we report on experimental signatures of skyrmion textures in the layer degree of freedom of Rhombohedral-stacked (R-stacked) tWSe2 homobilayers. This observation is based on scanning tunneling spectroscopy that separately resolves the Gamma-valley and K-valley moiré electronic states. We show that Gamma-valley states are subjected to a moiré potential with an amplitude of ~ 120 meV. At ~150 meV above the Gamma-valley, the K-valley states are subjected to a weaker moiré potential of ~30 meV. Most significantly, we reveal opposite layer polarizations of the K-valley at the MX and XM sites within the moiré unit cell, confirming the theoretically predicted layer skyrmion texture. The dI/dV mappings allow the parameters that enter the continuum model of moiré bands in twisted TMD bilayers to be determined experimentally, further establishing a direct correlation between the shape of the LDOS profile in real space and the topology of topmost moiré band.

cond-mat.mes-hall↗

arXiv2022

Excitonic Josephson effect induced by interlayer tunneling current: robust evidence for exciton condensation

The Josephson effect can be regarded as a striking manifestation of exciton condensation. It has been suggested to tune the condensate phase of bilayer excitons by applying interlayer tunneling current. A poorly-understood phenomenon observed by Huang {\it et al} [Phys. Rev. Lett. $\bf 109$, 156802 (2012)] demonstrates that the critical values of the interlayer tunneling current at either edge can be controlled by passing a second interlayer tunneling current at the other edge. We successfully attribute this novel coupling to excitonic Josephson effect induced by tunneling-current generated relative phases and indicate that Huang's experiment is very robust evidence for exciton condensation. We furthermore make a new proposal: there exists a critical Josephson current beyond which Josephson coupling collapses and external currents prefer to convert into edge-state currents to meet Huang's observation at the second tunneling current above $\pm$16nA -- a sudden decoupling of two edges accompanied by a large interedge voltage. Also, we make a feasible suggestion to detect Josephson current by measuring induced magnetic field of a ring-shaped excitonic Josephson junction. This work not only solves the long-standing uncertainty regarding the identification of exciton condensation but also opens a new direction to explore exciton condensation.

cond-mat.supr-con↗

arXiv2020

Voltage-Controlled Magnetic Reversal in Orbital Chern Insulators

Chern insulator ferromagnets are characterized by a quantized anomalous Hall effect, and have so far been identified experimentally in magnetically-doped topological insulator (MTI) thin films and in bilayer graphene moir{é} superlattices. We classify Chern insulator ferromagnets as either spin or orbital, depending on whether the orbital magnetization results from spontaneous spin-polarization combined with spin-orbit interactions, as in the MTI case, or directly from spontaneous orbital currents, as in the moir{é} superlattice case. We argue that in a given magnetic state, characterized for example by the sign of the anomalous Hall effect, the magnetization of an orbital Chern insulator will often have opposite signs for weak $n$ and weak $p$ electrostatic or chemical doping. This property enables pure electrical switching of a magnetic state in the presence of a fixed magnetic field.

cond-mat.str-el↗

arXiv2020

Role of the compensating current in the weak Josphson coupling regime: An extended study on excitonic Josephson junctions

Huang's experiment [Phys. Rev. Lett. $\bf 109$, 156802 (2012)] found, in the quantum Hall bilayer of the Corbino geometry, the interlayer tunneling currents at two edges are coupled to each other and one of two tunneling currents is referred to as the compensating current of the other. Our another work[arXiv:2006.15329] has explained this exotic coupling phenomenon as a result of excitonic Josephson effect induced by interlayer tunneling current. In this paper, we study the same setup -- excitonic Josephson junction -- but in the weak Josephson coupling regime, which occurs for large junction length. Interestingly, we find the compensating current drives the other edge to undergo a nonequilibrium phase transition from a superfluid to resistive state, which is signaled by an abrupt jump of the critical tunneling current. We also identify the critical exponent and furthermore offer more experimental prediction.

cond-mat.supr-con↗

arXiv2016

Spatially-indirect Exciton Condensate Phases in Double Bilayer Graphene

We present a theory of spatially indirect exciton condensate states in systems composed of a pair of electrically isolated Bernal graphene bilayers. The ground state phase diagram in a two-dimensional displacement-field/inter-bilayer-bias space includes layer-polarized semiconductors, spin-density-wave states, exciton condensates, and states with mixed excitonic and spin order. We find that two different condensate states, distinguished by a chirality index, are stable under different electrical control conditions.

cond-mat.mtrl-sci↗

arXiv2016

Microscopic Theory of Equilibrium Polariton Condensates

We present a microscopic theory of the equilibrium polariton condensate state of a semiconductor quantum well in a planar optical cavity. The theory accounts for the adjustment of matter excitations to the presence of a coherent photon field, predicts effective polariton-polariton interaction strengths that are weaker and condensate exciton fractions that are smaller than in the commonly employed exciton-photon model, and yields effective Rabi coupling strengths that depend on the detuning of the cavity photon energy relative to the bare exciton energy. The dressed quasiparticle bands that appear naturally in the theory provide a mechanism for electrical manipulation of polariton condensates.

cond-mat.mes-hall↗

arXiv2013

Plastic response by dislocation glide in solid helium under dc strain rate loading

We develop a model for the gliding of dislocations and plasticity in solid He-4. This model takes into account the Peierls barrier, multiplication and interaction of dislocations, as well as classical thermally and mechanically activated processes leading to dislocation glide. We specifically examine the dc stress-strain curve and how it is affected by temperature, strain rate, and dislocation density. As a function of temperature and shear strain, we observe plastic deformation and discuss how this may be related to the experimental observation of elastic anomalies in solid hcp He-4 that have been discussed in connection with the possibility of supersolidity or giant plasticity. Our theory gives several predictions for the dc stress strain curves, for example, the yield point and the change in the work-hardening rate and plastic dissipation peak, that can be compared directly to constant strain rate experiments and thus provide bounds on model parameters.

cond-mat.mtrl-sci↗

arXiv2013

Dislocation induced anomalous softening of solid helium

The classical motion of gliding dislocation lines in slip planes of crystalline solid helium leads to plastic deformation even at temperatures far below the Debye temperature and can affect elastic properties. In this work we argue that the gliding of dislocations and plasticity may be the origin of many observed elastic anomalies in solid He-4, which have been argued to be connected to supersolidity. We present a dislocation motion model that describes the stress-strain $τ$-$ε$ curves and work hardening rate $dτ/dε$ of a shear experiment performed at constant strain rate $\dotε$ in solid helium. The calculated $dτ/dε$ exhibits strong softening with increasing temperature due to the motion of dislocations, which mimics anomalous softening of the elastic shear modulus $μ$. In the same temperature region the motion of dislocations causes dissipation with a prominent peak.

cond-mat.mtrl-sci↗

arXiv2011

The role of glass dynamics in the anomaly of the dielectric function of solid helium

We propose that acousto-optical coupling of the electric field to strain fields around defects in disordered $^4$He is causing an increase of the dielectric function with decreasing temperature due to the arrested dynamics of defect excitations. A distribution of such low-energy excitations can be described within the framework of a glass susceptibility of a small volume fraction inside solid $^4$He. Upon lowering the temperature the relaxation time $τ(T)$ of defects diverges and an anomaly occurs in the dielectric function $ε(ω, T)$ when $ωτ(T) \sim 1$. Since $ε(ω, T)$ satisfies the Kramers-Kronig relation, we predict an accompanying peak in the imaginary part of $ε(ω, T)$ at the same temperature, where the largest change in the amplitude has been seen at fixed frequency. We also discuss recent measurements of the amplitude of the dynamic dielectric function that indicate a low-temperature anomaly similar to the one seen in the resonance frequency of the torsional oscillator and shear modulus experiments.

cond-mat.mtrl-sci↗

arXiv2010

Shear modulus in viscoelastic solid $^4$He

The complex shear modulus of solid $^4$He exhibits an anomaly in the same temperature region where torsion oscillators show a change in period. We propose that the observed stiffening of the shear modulus with decreasing temperature can be well described by a viscoelastic component that possesses an increasing relaxation time as temperature decreases. Since a glass is a viscoelastic material, the response functions derived for a viscoelastic material are identical to those obtained for a glassy component due to a time delayed restoring back-action. By generalizing the viscoelastic equations for stress and strain to a multiphase system of constituents, composed of patches with different damping and relaxation properties, we predict that the maximum change of the magnitude of the shear modulus and the maximum height of the dissipation peak are independent of an applied external frequency. The same response expressions allow us to calculate the temperature dependence of the shear modulus' amplitude and dissipation. Finally, we demonstrate that a Vogel-Fulcher-Tammann (VFT) relaxation time is in agreement with available experimental data.

cond-mat.stat-mech↗

arXiv2010

The glassy response of double torsion oscillators in solid Helium-4

Single and double torsion oscillators have been used successfully to measure the anomalous change in resonant frequency and accompanying dissipation in solid He-4. We present a glass description of the mechanical anomalies found in torsion oscillator measurements. Our results show that it is not necessary to invoke a supersolid interpretation to explain these mechanical anomalies. Previously, we demonstrated that the back-action of a glassy subsystem present in solid He-4 can account for frequency change and dissipation peak in many single torsion oscillator experiments. Here, we show that the same glassy back-action can explain the experimental results of the composite torsion oscillator developed by the Rutgers group, which measures the response of solid He-4 at the in-phase mode f1=496 Hz and out-of-phase mode f2=1173 Hz.

cond-mat.mtrl-sci↗

arXiv2010

Charge density wave in hidden order state of URu$_2$Si$_2$

We argue that the hidden order state in URu$_2$Si$_2$ will induce a charge density wave. The modulation vector of the charge density wave will be twice that of the hidden order state, $Q_{CDW} = 2Q_{HO}$. To illustrate how the charge density wave arises we use a Ginzburg-Landau theory that contains a coupling of the charge density wave amplitude to the square of the HO order parameter $Δ_{HO}$. This simple analysis allows us to predict the intensity and temperature dependence of the charge density wave order parameter in terms of the susceptibilities and coupling constants used in the Ginzburg-Landau analysis.

cond-mat.str-el↗

arXiv2010

A glass anomaly in the shear modulus of solid $^4$He

The shear modulus of solid $^4$He exhibits an anomalous change of order 10%[1, 2] at low temperatures that is qualitatively similar to the much smaller frequency change in torsional oscillator experiments. We propose that in solid $^4$He the stiffening of the shear modulus with decreasing temperature can be described with a glass susceptibility assuming a temperature dependent relaxation time $τ(T)$. The glass susceptibility captures the freezing out of glassy degrees of freedom below a characteristic crossover temperature $T_X$. There the dynamic response of the solid satisfies $ωτ(T_X) \sim 1$, thus leading to an increase in the shear modulus. Within this model we predict that the maximum change of the amplitude of the shear modulus and the height of the dissipation peak are independent of the applied frequency $ω$. Our calculations also show a qualitative difference in behavior of the shear modulus depending on the temperature behavior of the glass relaxation time $τ(T)$. These predictions can be tested by comparing the complex shear modulus with experiments at different frequencies and for different levels of disorder. [1] J. Day and J. Beamish, Nature 150, 853 (2007). [2] J. Day, O. Syshchenko, J. Beamish, Phys. Rev. B 79, 214524(2009).

cond-mat.stat-mech↗

arXiv2010

A glassy contribution to the heat capacity of hcp $^4$He solids

We model the low-temperature specific heat of solid $^4$He in the hexagonal closed packed structure by invoking two-level tunneling states in addition to the usual phonon contribution of a Debye crystal for temperatures far below the Debye temperature, $T < Θ_D/50$. By introducing a cutoff energy in the two-level tunneling density of states, we can describe the excess specific heat observed in solid hcp $^4$He, as well as the low-temperature linear term in the specific heat. Agreement is found with recent measurements of the temperature behavior of both specific heat and pressure. These results suggest the presence of a very small fraction, at the parts-per-million (ppm) level, of two-level tunneling systems in solid $^4$He, irrespective of the existence of supersolidity.

cond-mat.stat-mech↗

arXiv2010

Critical Tunneling Currents in Quantum Hall Superfluids: Pseudospin-Transfer Torque Theory

At total filling factor $ν=1$ quantum Hall bilayers can have an ordered ground state with spontaneous interlayer phase coherence. The ordered state is signaled experimentally by dramatically enhanced interlayer tunnel conductances at low bias voltages; at larger bias voltages inter-layer currents are similar to those of the disordered state. We associate this change in behavior with the existence of a critical current beyond which static inter-layer phase differences cannot be maintained, and examine the dependence of this critical current on sample geometry, phase stiffness, and the coherent tunneling energy density. Our analysis is based in part on analogies between coherent bilayer behavior and spin-transfer torque physics in metallic ferromagnets. Comparison with recent experiments suggests that disorder can dramatically suppress critical currents.

cond-mat.mes-hall↗

arXiv2008

Room-Temperature Superfluidity in Graphene Bilayers

Because graphene is an atomically two-dimensional gapless semiconductor with nearly identical conduction and valence bands, graphene-based bilayers are attractive candidates for high-temperature electron-hole pair condensation. We present estimates which suggest that the Kosterlitz-Thouless temperatures of these two-dimensional counterflow superfluids can approach room temperature.

cond-mat.mes-hall↗

arXiv2008

How to make a bilayer exciton condensate flow

Bose condensation is responsible for many of the most spectacular effects in physics because it can promote quantum behavior from the microscopic to the macroscopic world. Bose condensates can be distinguished by the condensing object; electron-electron Cooper-pairs are responsible for superconductivity, Helium atoms for superfluidity, and ultracold alkali atoms in vapors for coherent matter waves. Electron-hole pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because exciton phase coherence is never perfectly spontaneous. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected, or how its experimental manifestation would be altered by the phase-fixing exciton creation and annhilation processes which are inevitably present. In this article we explain how to induce an exciton supercurrent in separately contacted bilayer condensates, and predict electrical effects which enable unambiguous detection.

cond-mat.mes-hall↗