We consider 2+1-dimensional conformal gauge theories coupled to additional degrees of freedom which induce a spatially local but long-range in time $1/{(\tau -{\tau }^{\prime })}^2$ interaction between gauge-neutral local operators. Such theories have been argued to describe the hole-doped cuprates near optimal doping. We focus on a SU(2) gauge theory with $N_h$ flavors of adjoint Higgs fields undergoing a quantum transition between Higgs and confining phases: the $1/{(\tau -{\tau }^{\prime })}^2$ interaction arises from a spectator large Fermi surface of electrons. The large $N_h$ expansion leads to an effective action containing fields which are bilocal in time but local in space. We find a strongly coupled fixed point at order $1/N_h$, with dynamic critical exponent $z>1$. We show that the entropy preserves hyperscaling but nevertheless leads to a linear in temperature specific heat with a coefficient which has a finite enhancement near the quantum critical point.

Density functional theory underlies the most successful and widely used numerical methods for electronic structure prediction of solids. However, it has the fundamental shortcoming that the universal density functional is unknown. In addition, the computational result—energy and charge density distribution of the ground state—is useful for electronic properties of solids mostly when reduced to a band structure interpretation based on the Kohn-Sham approach. Here, we demonstrate how machine learning algorithms can help to free density functional theory from these limitations. We study a theory of spinless fermions on a one-dimensional lattice. The density functional is implicitly represented by a neural network, which predicts, besides the ground-state energy and density distribution, density-density correlation functions. At no point do we require a band structure interpretation. The training data, obtained via exact diagonalization, feeds into a learning scheme inspired by active learning, which minimizes the computational costs for data generation. We show that the network results are of high quantitative accuracy and, despite learning on random potentials, capture both symmetry-breaking and topological phase transitions correctly.

We propose a nanoscale device consisting of a double quantum dot with strong intra- and interdot Coulomb repulsions. In this design, the current can only flow through the lower dot, but is triggered by the gate-controlled occupancy of the upper dot. At low temperatures, our calculations predict the double dot to pass through a narrow Kondo regime, resulting in highly sensitive switching characteristics between three well-defined states—insulating, normal conduction, and resonant conduction.

We investigate the dynamics of chiral edge states in topological polariton systems under laser driving. Using a model system comprised of topolgically trivial excitons and photons with a chiral coupling proposed by Karzig et al. [Phys. Rev. X 5, 031001 (2015)], we investigate the real-time dynamics of a lattice version of this model driven by a laser pulse. By analyzing the time- and momentum-resolved spectral function, measured by time- and angle-resolved photoluminescence in analogy with time- and angle-resolved photoemission spectroscopy in electronic systems, we find that polaritonic states in a ribbon geometry are selectively excited via their resonance with the pump laser photon frequency. This selective excitation mechanism is independent of the necessity of strong laser pumping and polariton condensation. Our work highlights the potential of time-resolved spectroscopy as a complementary tool to real-space imaging for the investigation of topological edge state engineering in devices.

Using conceptually and procedurally consistent density functional theory (DFT) calculations with an advanced meta-generalized gradient approximation (GGA) exchange-correlation functional in ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations, we determine the insulator-metal transition (IMT) of warm dense fluid hydrogen from 50 to 300 GPa to be in better agreement with experiment than previous DFT predictions. The inclusion of nuclear quantum effects via path-integral molecular dynamics (PIMD) sharpens the transition and lowers its temperature relative to the BOMD results. A rapid decrease in the molecular character of the system, as observed via the ionic pair correlation function, coincides with an abrupt conductivity increase, confirming a metallic transition due to molecular hydrogen dissociation that is coincident with abrupt band-gap closure. Comparison of the PIMD and BOMD results clearly demonstrates an isotope effect on the IMT. Exploitation of differing methodologies for using the orbital-dependent and deorbitalized versions of the meta-GGA enables us to quantify exchange-correlation approximation effects. Distinct from stochastic simulations, these results do not depend upon any clever but uncontrolled combination of ground-state and finite-T methodologies and should provide a reliable benchmark for further studies.

Realizing and probing topological superfluids is a key goal for fundamental science, with exciting technological promises. Here, we show that chiral $p_x+i{p}_y$ pairing in a two-dimensional topological superfluid can be detected through circular dichroism, namely, as a difference in the excitation rates induced by a clockwise and counterclockwise circular drive. For weak pairing, this difference is to a very good approximation determined by the Chern number of the superfluid, whereas there is a nontopological contribution scaling as the superfluid gap squared that becomes significant for stronger pairing. This gives rise to a competition between the experimentally driven goal to maximize the critical temperature of the superfluid, and observing a signal given by the underlying topology. Using a combination of strong-coupling Eliashberg and Berezinskii-Kosterlitz-Thouless theory, we analyze this tension for an atomic Bose-Fermi gas, which represents a promising platform for realizing a chiral superfluid. We identify a wide range of system parameters where both the critical temperature is high and the topological contribution to the dichroic signal is dominant.

Experimental evidence for a transient enhancement of the superconducting critical temperature of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ in the presence of an intense THz or infrared pump pulse was ascribed to nonlinear phononics effects. Here, I introduce a simple phenomenological Ginzburg-Landau model of this phenomenon, to explore further consequences and possible experimental tests of this interpretation. This treatment predicts that, upon cooling below $T_c$ in the absence of pumping, (a) an abrupt softening of a Raman-active mode frequency and (b) a spontaneous lattice distortion, growing linearly with $(T_c-T)$ should occur. Numerical estimates for ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ indicate that the frequency softening could likely be observable, whereas the lattice distortion may be too small. A comparison with Raman experiments for the relevant phonon modes in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ does not lend support to the nonlinear phononics interpretation. On the other hand, however, a very large (up to $18\%$) phonon frequency softening just below $T_c$, qualitatively similar to the predictions of the present model for its size and abrupt temperature dependence, was observed over 20 years ago in ${\text{HgBa}}_2{\text{Ca}}_3{\text{Cu}}_4{\text{O}}_{10+x}$; its explanation in terms of Josephson plasmons has been controversial. Light-induced superconductivity still needs to be investigated in this material and it may be of interest to explore if it is present and possibly connected, via the mechanism discussed here, to the observed anomalous phonon behavior.

We have performed magnetotransport measurements on electrically induced two-dimensional hole gases in undoped GaSb/AlSb quantum wells. The mobilities of the holes are sufficient to observe Shubnikov–de Haas oscillations for a few teslas of perpendicular magnetic field. We extracted the effective masses of holes in the valence bands from temperature-dependent Shubnikov–de Haas oscillations. The effective masses, in the unit of the free-electron mass, strongly depend on the width of the quantum wells and are 0.14–0.16 for the spin-degenerated subbands in an 8-nm-thick quantum well and 0.44–0.52 for one of the spin-split subbands in a 25-nm-thick quantum well. Furthermore, by fitting the weak antilocalization correction to the classical magnetoresistance at low magnetic fields, we obtained the phase coherence length of the system. The phase coherence length increases with hole density, reaching maxima of around 1100 and 600 nm for the 8- and 25-nm-thick quantum wells, respectively. These achievements build upon our previous results on GaSb quantum wells and further our understanding of their properties. They therefore lay the groundwork for realizing spin-based electronics based on the strong spin-orbit interaction in this promising system.

Coherently coupled two-component Bose-Einstein condensates (BEC) exhibit vortex confinement resembling quark confinement in quantum chromodynamics (QCD). Fractionally quantized vortices winding only in one of two components are attached by solitons, and they cannot stably exist alone. Possible stable states are “hadrons” either of mesonic type, i.e., molecules made of a vortex and antivortex in the same component connected by a soliton, or of baryonic type, i.e., molecules made of two vortices winding in two different components connected by a soliton. Mesonic molecules move straight with a constant velocity while baryonic molecules rotate. We numerically simulate collision dynamics of mesonic and baryonic molecules and find that the molecules swap partners in collisions in general like chemical and nuclear reactions, as well as summarizing all collisions as vortex reactions, and describe those by Feynman diagrams. We find a selection rule for final states after collisions of vortex molecules, analogous to that for collisions of hadrons in QCD.

Using a leading algorithmic implementation of the functional renormalization group (fRG) for interacting fermions on two-dimensional lattices, we provide a detailed analysis of its quantitative reliability for the Hubbard model. In particular, we show that the recently introduced multiloop extension of the fRG flow equations for the self-energy and two-particle vertex allows for a precise match with the parquet approximation also for two-dimensional lattice problems. The refinement with respect to previous fRG-based computation schemes relies on an accurate treatment of the frequency and momentum dependences of the two-particle vertex, which combines a proper inclusion of the high-frequency asymptotics with the so-called truncated unity fRG for the momentum dependence. The adoption of the latter scheme requires, as an essential step, a consistent modification of the flow equation of the self-energy. We quantitatively compare our fRG results for the self-energy and momentum-dependent susceptibilities and the corresponding solution of the parquet approximation to determinant quantum Monte Carlo data, demonstrating that the fRG is remarkably accurate up to moderate interaction strengths. The presented methodological improvements illustrate how fRG flows can be brought to a quantitative level for two-dimensional problems, providing a solid basis for the application to more general systems.

Liquid crystals formed by acute-angle bent core (ABC) molecules with a 1,7 naphthalene central core show an intriguing phase behavior with the nematic phase accompanied by poorly understood additional phases. In this work, we characterize the physical properties of an ABC material, such as birefringence, dielectric permittivities, elastic constants, and surface alignment and present x-ray diffraction and transmission electron microscopy studies of their ordering. The ABC molecular shape resembling the letter $\lambda$ yields a very small splay elastic constant in the uniaxial nematic phase and results in the formation of a tetragonal positionally ordered columnar phase consisting of alternating polar and apolar molecular columns with a uniform uniaxial director that can be bent but not splayed.

Kondo insulators are strongly correlated systems in which a clean insulating gap emerges only at very low temperature due to many-body effects involving localized $f$ electrons. However, certain Kondo insulators, like $\mathrm{Sm}{\mathrm{B}}_6$ and ${\mathrm{Ce}}_3\mathrm{Bi}{}_4\mathrm{Pt}{}_3$, display metallic behaviors at extremely low temperature and have defied current understanding. Recent advances in topological effects in materials have raised attention on the protected surface states in these “topological Kondo insulators” as a potential resolution to some of the puzzling behaviors. Here we resolve these puzzles via a different route, by showing that the emergent Kondo insulating scale is extremely vulnerable against a moderate degree of disorder, such that the gap is filled with a small number of states. Therefore, the real samples are probably never truly insulating and this in turn compromises the essential building block of topological considerations. Our results suggest strongly that systems like the Slater insulators would be a more promising direction to extend the realm of topology to strongly correlated systems.

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect formation via the one-dimensional transverse-field Ising model on two different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system. We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.

Non-Fermi liquids are strange metals whose physical properties deviate qualitatively from those of conventional metals due to strong quantum fluctuations. In this paper, we report transport measurements on the $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ superconductor, which has a quantum critical point of a nematic order without accompanying antiferromagnetism. We find that in addition to a linear-in-temperature resistivity ${\rho }_{xx}\propto T$, which is close to the Planckian limit, the Hall angle varies as $cot{\theta }_{\mathrm{H}}\propto T^2$ and the low-field magnetoresistance is well scaled as $\mathrm{\Delta }{\rho }_{xx}/{\rho }_{xx}\propto {tan}^2{\theta }_{\mathrm{H}}$ in the vicinity of the nematic quantum critical point. This set of anomalous charge transport properties show striking resemblance with those reported in cuprate, iron-pnictide, and heavy fermion superconductors, demonstrating that the critical fluctuations of a nematic order with $\mathit{q}\approx 0$ can also lead to a breakdown of the Fermi liquid description.

We study quantum criticality of the magnetic-field-induced charge density wave (CDW) order in correlated spinless Dirac fermions on the $\pi$-flux square lattice at zero temperature as a prototypical example of the magnetic catalysis, by using the infinite density matrix renormalization group. It is found that the CDW order parameter $M\left(B\right)$ exhibits an anomalous magnetic field $\left(B\right)$ scaling behavior characteristic of the $(2+1)$-dimensional chiral Ising universality class near the quantum critical point, which leads to a strong enhancement of $M\left(B\right)$ compared with a mean-field result. We also establish a global phase diagram in the interaction-magnetic field plane for the fermionic quantum criticality.

We analyze various microscopic properties of the nematic fractional quantum Hall effect (FQHN) in the thermodynamic limit, and present necessary conditions required of the microscopic Hamiltonians for the nematic fractional quantum Hall effect to be robust. Analytical expressions for the degenerate ground state manifold, ground state energies, and gapless nematic modes are given in compact forms with the input interaction and the corresponding ground state structure factors. We relate the long wavelength limit of the neutral excitations to the guiding center metric deformation, and show explicitly the family of trial wave functions for the nematic modes with spatially varying nematic order near the quantum critical point. For short range interactions, the dynamics of the FQHN is completely determined by the long wavelength part of the ground state structure factor. The special case of the FQHN at $\nu =1/3$ is discussed with theoretical insights from the Haffnian parent Hamiltonian, leading to a number of rigorous statements and experimental implications.

Recent years have seen strong progress in quantum simulation of gauge-theory dynamics using ultracold-atom experiments. A principal challenge in these efforts is the certification of gauge invariance, which has recently been realized [Yang et al., arXiv:2003.08945]. One major but poorly investigated experimental source of gauge-invariance violation is an imperfect preparation of the initial state. Using the time-dependent density-matrix renormalization group, we analyze the robustness of gauge-invariant dynamics against potential preparation defects in the above ultracold-atom implementation of a $\mathrm{U}\left(1\right)$ gauge theory. We find defects related to an erroneous initialization of matter fields to be innocuous, as the associated gauge-invariance violation remains strongly localized throughout the time evolution. A defect due to faulty initialization of the gauge field leads to a mild proliferation of the associated violation. Furthermore, we characterize the influence of immobile and mobile defects by monitoring the spread of entanglement entropy. Overall, our results indicate that the aforementioned experimental realization exhibits a high level of fidelity in the gauge invariance of its dynamics at all evolution times. Our work provides strong evidence that ultracold-atom setups can serve as an extremely reliable framework for the quantum simulation of gauge-theory dynamics.

Leakage of Cooper pairs through heterointerfaces leads to the superconducting proximity effect, which has been utilized extensively in building functional quantum devices and inducing novel superconductivity. While an atomically sharp interface in real-space is known to be crucial for effective Cooper pair proximity transfer, the influence of electronic valleys in the momentum space has not been sufficiently investigated. Here, we report the observation of valley dependent superconducting proximity effect in a heterostructure with twisted overlapping. The heterostructure is realized by growth of multidomain Bi(111) films on a single-crystal $\mathrm{Nb}{\mathrm{Se}}_2$ substrate with molecular beam epitaxy. With spectroscopic imaging scanning tunneling spectroscopy, we identified different types of atomic overlapping in the Bi films, and measured drastic changes of proximity-induced superconducting gap sizes on the differently oriented Bi domains. Based on our theoretical model calculation, this phenomenon can be interpreted as valley dependent superconducting proximity coupling between the Bi film and $\mathrm{Nb}{\mathrm{Se}}_2$. We also investigated the lateral proximity effect between two adjacent Bi domains, which determines a significant reduction of the mean free path of electrons, associated with interfacial scattering. Our study expands the scope of tunable physical properties with the valley degree of freedom.

Twisted graphene multilayers have been demonstrated to yield a versatile playground to engineer controllable electronic states. Here, by combining first-principles calculations and low-energy models, we demonstrate that twisted graphene trilayers provide a tunable system where Van Hove singularities can be controlled electrically. In particular, it is shown that besides the band flattening, bulk valley currents appear, which can be quenched by local chemical dopants. We finally show that in the presence of electronic interactions, a nonuniform superfluid density emerges whose nonuniformity gives rise to spectroscopic signatures in dispersive higher-energy bands. Our results put forward twisted trilayers as a tunable van der Waals heterostructure displaying electrically controllable flat bands and bulk valley currents.

For the recently discovered cuprate superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$, we propose a lattice structure which resembles the model considered by Lieb to represent the vastly oxygen-deficient material. We first investigate the stability of the Lieb-lattice structure and then construct a multiorbital Hubbard model based on first-principles calculation. By applying the fluctuation-exchange approximation to the model and solving the linearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings closely compete with each other and, more interestingly, that the intraorbital and interorbital pairings coexist. We further show that if the energy of the $d_{3z^2-r^2}$ band is raised to make it “incipient” with the lower edge of the band close to the Fermi level within a realistic band filling regime, $s\pm$-wave superconductivity is strongly enhanced. We reveal an intriguing relation between the Lieb model and the two-orbital model for the usual ${\mathrm{K}}_2{\mathrm{NiF}}_4$ structure where a close competition between $s\text{−}$ and $d\text{−}\mathrm{wave}$ pairings is known to occur. The enhanced superconductivity in the present model is further shown to be related to an enhancement found previously in the bilayer Hubbard model with an incipient band.