In the recent work of Vlaic et al. [Nat. Commun. 8, 14549 (2017)], it has been shown that Pb nanocrystals grown on the electron accumulation layer at the (110) surface of InAs are in the regime of Coulomb blockade. This enabled a scanning tunneling spectroscopy study of the superconducting parity effect across the Anderson limit. The nature of the tunnel barrier between the nanocrystals and the substrate has been attributed to a quantum constriction of the electronic wave function at the interface due to the large Fermi wavelength of the electron accumulation layer in InAs. In this paper, we detail and review the arguments leading to this conclusion. Furthermore, we show that, thanks to this highly clean tunnel barrier, this system is remarkably suited for the study of discrete electronic levels induced by quantum confinement effects in the Pb nanocrystals. We identified three distinct regimes of quantum confinement. For the largest nanocrystals, quantum confinement effects appear through the formation of quantum well states regularly organized in energy and in space. For the smallest nanocrystals, only atomiclike electronic levels separated by a large energy scale are observed. Finally, in the intermediate size regime, discrete electronic levels associated to electronic wave functions with a random spatial structure are observed, as expected from random matrix theory.

The nonequilibrium Green's functions (NEGF) approach is a versatile theoretical tool, which allows to describe the electronic structure, spectroscopy, and dynamics of strongly correlated systems. The applicability of this method is, however, limited by its considerable computational cost. Due to the treatment of the full two-time dependence of the NEGF, the underlying equations of motion involve a long-lasting non-Markovian memory kernel that results in at least an $N_t^3$ scaling in the number of time points $N_t$. The system's memory time is, however, reduced in the presence of a thermalizing bath. In particular, dynamical mean field theory (DMFT), one of the most successful approaches to strongly correlated lattice systems, maps extended systems to an effective impurity coupled to a bath. In this work, we systematically investigate how the memory time can be truncated in nonequilibrium DMFT simulations of the Hubbard and Hubbard-Holstein models. We show that suitable truncation schemes, which substantially reduce the computational cost, result in excellent approximations to the full time evolution. This approach enables the propagation to longer times, making fundamental processes such as prethermalization and the final stages of thermalization accessible to nonequilibrium DMFT.

Controlling energy flows in solids through switchable electron-lattice cooling can grant access to a range of interesting and potentially useful energy transport phenomena. Here we discuss a tunable electron-lattice cooling mechanism arising in graphene due to phonon emission mediated by resonant scattering on defects in a crystal lattice, which displays an interesting analogy to the Purcell effect in optics. In that, the electron-phonon cooling rate is enhanced due to hot carrier trapping at resonant defects. Resonant dependence of this process on carrier energy translates into gate-tunable cooling rates, exhibiting strong enhancement of cooling that occurs when the carrier energy is aligned with the electron resonance of the defect.

Hexagonal BN ($h$-BN) is attracting a lot of attention for two-dimensional electronics and as a host for single-photon emitters. We study the properties of native defects and impurities in $h$-BN using density functional theory with a hybrid functional. Native vacancy and antisite defects have high formation energies, and are unlikely to form under thermodynamic equilibrium for typical growth conditions. Self-interstitials can have low formation energies when the Fermi level is near the band edges, and may form as charge compensating centers; however, their low migration barriers render them highly mobile, and they are unlikely to be present as isolated defects. The defect chemistry of $h$-BN is most likely dominated by defects involving carbon, oxygen, and hydrogen impurities. Substitutional carbon and oxygen, as well as interstitial hydrogen and boron vacancy–hydrogen complexes, are low-energy defects in $h$-BN. Based on our results, we can rule out several proposed sources for defect-related luminescence in $h$-BN. In particular, we find that the frequently observed 4.1 eV emission cannot be associated with recombination at ${\mathrm{C}}_{\mathrm{N}}$, as has been commonly assumed. We suggest alternative assignments for the origins of this emission, with ${\mathrm{C}}_{\mathrm{B}}$ as a candidate. We also discuss possible defect origins for the recently observed single-photon emission in $h$-BN, identifying interstitials or their complexes as plausible centers.

In as-grown bulk crystals of ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ with $x\lesssim 0.3$, excess Fe ($y>0$) is inevitable and correlates with a suppression of superconductivity. At the same time, there remains the question as to whether the character of the antiferromagnetic correlations associated with the enhanced anion height above the Fe planes in Te-rich samples is compatible with superconductivity. To test this, we have annealed as-grown crystals with $x=0.1$ and 0.2 in Te vapor, effectively reducing the excess Fe and inducing bulk superconductivity. Inelastic neutron scattering measurements reveal low-energy magnetic excitations consistent with short-range correlations of the double-stripe type; nevertheless, cooling into the superconducting state results in a spin gap and a spin resonance, with the extra signal in the resonance being short range with a mixed single-stripe/double-stripe character, which is different than other iron-based superconductors. The mixed magnetic character of these superconducting samples does not appear to be trivially explainable by inhomogeneity.

We address the key open problem of a higher-dimensional generalization of the Sachdev-Ye-Kitaev (SYK) model. We construct a model on a lattice of SYK dots with nonrandom intersite hopping. The crucial feature of the resulting band dispersion is the presence of a Lifshitz point where two bands touch with a tunable power-law divergent density of states (DOS). For a certain regime of the power-law exponent, we obtain a class of interaction-dominated non-Fermi-liquid (NFL) states, which exhibits exciting features such as a zero-temperature scaling symmetry, an emergent (approximate) time reparameterization invariance, a power-law entropy-temperature relationship, and a fermion dimension that depends continuously on the DOS exponent. Notably, we further demonstrate that these NFL states are fast scramblers with a Lyapunov exponent ${\lambda }_L\propto T$, although they do not saturate the upper bound of chaos, rendering them truly unique.

We present a theory of a surface barrier for isolated solitons to enter chiral magnets, in an analogous way to that of the Bean-Livingston barrier in the type II superconductors. With this theory, we discuss hysteresis observed in magnetoresistance (MR) measurements of monoaxial chiral helimagnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$. We argue that a large jump in the decreasing field process of MR of micrometer-sized samples is due to the disappearance of the surface barrier for the chiral soliton at a field $H=H_{\mathrm{b}}$. This argument is justified through agreement between the experimental results at 10 K and our theoretical result $H_{\mathrm{b}}/H_{\mathrm{c}}=4/{\pi }^2\sim 0.4$ (with thermodynamic critical field $H_{\mathrm{c}}$) based on the sine-Gordon model.

Recently, C. M. Pépin et al. [Science 357, 382 (2017)] reported the formation of several new iron polyhydrides ${\mathrm{FeH}}_x$ at pressures in the megabar range and spotted ${\mathrm{FeH}}_5$, which forms above 130 GPa, as a potential high-$T_{\text{c}}$ superconductor because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A. Majumdar et al. [Phys. Rev. B 96, 201107 (2017)] and A. G. Kvashnin et al. [J. Phys. Chem. C 122, 4731 (2018)] based on ab initio Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither ${\mathrm{FeH}}_5$ nor its precursor, ${\mathrm{FeH}}_3$, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the ${\mathrm{FeH}}_3$ structure type by adding more electrons to partially fill one of the Fe-H hybrid bands (as, e.g., in ${\mathrm{NiH}}_3$). Critical temperatures, however, will remain low because the $d$-metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds.

We study theoretically the low-energy hole states in Si, Ge, and Ge/Si core/shell nanowires (NWs). The NW core in our model has a rectangular cross section, the results for a square cross section are presented in detail. In the case of Ge and Ge/Si core/shell NWs, we obtain very good agreement with previous theoretical results for cylindrically symmetric NWs. In particular, the NWs allow for an unusually strong and electrically controllable spin-orbit interaction (SOI) of Rashba type. We find that the dominant contribution to the SOI is the “direct Rashba spin-orbit interaction” (DRSOI), which is an important mechanism for systems with heavy-hole–light-hole mixing. Our results for Si NWs depend significantly on the orientation of the crystallographic axes. The numerically observed dependence on the growth direction is consistent with analytical results from a simple model, and we identify a setup where the DRSOI enables spin-orbit energies of the order of millielectronvolt in Si NWs. Furthermore, we analyze the dependence of the SOI on the electric field and the cross section of the Ge or Si core. A helical gap in the spectrum can be opened with a magnetic field. For this gap, we obtain the largest $g$ factors for magnetic fields applied perpendicular to the NW and parallel to the electric field.

In the presence of interactions, a closed, homogeneous (disorder-free) many-body system is believed to generically heat up to an “infinite temperature” ensemble when subjected to a periodic drive: in the spirit of the ergodicity hypothesis underpinning statistical mechanics, this happens as no energy or other conservation law prevents this. Here we present an interacting Ising chain driven by a field of time-dependent strength, where such heating begins only below a threshold value of the drive amplitude, above which the system exhibits nonergodic behavior. The onset appears at strong, but not fast driving. This in particular puts it beyond the scope of high-frequency expansions. The onset location shifts, but it is robustly present, across wide variations of the model Hamiltonian such as driving frequency and protocol, as well as the initial state. The portion of nonergodic states in the Floquet spectrum, while thermodynamically subdominant, has a finite entropy. We find that the magnetization as an emergent conserved quantity underpinning the freezing; indeed, the freezing effect is readily observed, as initially magnetized states remain partially frozen up to infinite time. This result, which resembles the Kolmogorov-Arnold-Moser theorem for classical dynamical systems, could be a valuable ingredient for extending Floquet engineering to the interacting realm.

We measure quasiparticle tunneling across a constriction in the first Landau level. In the limit of weak backscattering, the dependence of the tunneling conductance on temperature and dc-bias is in qualitative disagreement with existing theories. For stronger backscattering, data obtained in the $\nu =1/3$ state can be fitted to weak backscattering theory with the predicted effective fractional charge of $e^{*}=e/3$. The scaling parameter $g$ is however not universal and depends strongly on the gate voltage applied to the constriction. At $\nu =4/3$, a more complex picture emerges. We propose an interpretation in terms of selective tunneling between the multiple modes present at the edge.

We use the Higgs mechanism to investigate connections between higher-rank symmetric $\text{U}\left(1\right)$ gauge theories and gapped fracton phases. We define two classes of rank-2 symmetric $\text{U}\left(1\right)$ gauge theories: the $(m,n)$ scalar and vector charge theories, for integer $m$ and $n$, which respect the symmetry of the square (cubic) lattice in two (three) spatial dimensions. We further provide local lattice rotor models whose low-energy dynamics are described by these theories. We then describe in detail the Higgs phases obtained when the $\text{U}\left(1\right)$ gauge symmetry is spontaneously broken to a discrete subgroup. A subset of the scalar charge theories indeed have X-cube fracton order as their Higgs phase, although we find that this can only occur if the continuum higher-rank gauge theory breaks continuous spatial rotational symmetry. However, not all higher-rank gauge theories have fractonic Higgs phases; other Higgs phases possess conventional topological order. Nevertheless, they yield interesting novel exactly solvable models of conventional topological order, somewhat reminiscent of the color code models in both two and three spatial dimensions. We also investigate phase transitions in these models and find a possible direct phase transition between four copies of ${\mathbb{Z}}_2$ gauge theory in three spatial dimensions and X-cube fracton order.

We theoretically study the topological robustness of the surface physics induced by Weyl Fermi-arc surface states in the presence of short-ranged quenched disorder and surface-bulk hybridization. This is investigated with numerically exact calculations on a lattice model exhibiting Weyl Fermi arcs. We find that the Fermi-arc surface states, in addition to having a finite lifetime from disorder broadening, hybridize with nonperturbative bulk rare states making them no longer bound to the surface (i.e., they lose their purely surface spectral character). Thus, we provide strong numerical evidence that the Weyl Fermi arcs are not topologically protected from disorder. Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder, persisting all the way to the Anderson-localized phase by forming localized current loops that live within the localization length of the surface. Thus, the Weyl semimetal is not topologically robust to the presence of disorder, but the surface chiral velocity is.

Interlayer excitons with direct optical transitions are observed coexisting with intralayer excitons in the same $K$ valleys in bilayer, few-layer, and bulk ${\mathrm{MoSe}}_2$ single crystals by confocal reflection contrast spectroscopy. Quantitative analysis using the Dirac-Bloch equations provides unambiguous state assignment of all the measured resonances. The interlayer excitons in bilayer ${\mathrm{MoSe}}_2$ have a large binding energy of 153 meV and a narrow linewidth of 20 meV. Their spectral weight is comparable to the commonly studied higher-order intralayer excitons. At the same time, the interlayer excitons are characterized by distinct transition energies and permanent dipole moments, providing a promising high temperature and optically accessible platform for dipolar exciton physics.

Epitaxial films of the B20-structure compound ${\mathrm{Fe}}_{1-y}{\mathrm{Co}}_y\mathrm{Ge}$ were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulklike values of one Bohr magneton per Fe atom for FeGe to zero for nonmagnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content $y$ and diverges at $y\sim 0.45$. This indicates a zero crossing of the DMI, which we reproduced in calculations using first-principles methods. We also measured the longitudinal and Hall resistivity of our films as a function of magnetic field, temperature, and Co content $y$. The Hall resistivity is expected to contain contributions from the ordinary, anomalous, and topological Hall effects. Both the anomalous and topological Hall resistivities show peaks around $y\sim 0.5$. Our first-principles calculations show a peak in the topological Hall constant at this value of $y$, related to the strong spin polarization predicted for intermediate values of $y$. Our calculations predict half-metallicity for $y=0.6$, consistent with the experimentally observed linear magnetoresistance at this composition, and potentially related to the other unusual transport properties for intermediate value of $y$. While it is possible to reconcile theory with experiment for the various Hall effects for FeGe, the large topological Hall resistivities for $y\sim 0.5$ are much larger than expected when the very small emergent fields associated with the divergence in the DMI are taken into account.

We uncover an infinite family of time-reversal symmetric $3d$ interacting topological insulators of bosons or spins, in time-periodically driven systems, which we term Floquet topological paramagnets (FTPMs). These FTPM phases exhibit intrinsically dynamical properties that could not occur in thermal equilibrium and are governed by an infinite set of ${\mathbb{Z}}_2$-valued topological invariants, one for each prime number. The topological invariants are physically characterized by surface magnetic domain walls that act as unidirectional quantum channels, transferring quantized packets of information during each driving period. We construct exactly solvable models realizing each of these phases, and discuss the anomalous dynamics of their topologically protected surface states. Unlike previous encountered examples of Floquet SPT phases, these $3d$ FTPMs are not captured by group cohomology methods and cannot be obtained from equilibrium classifications simply by treating the discrete time translation as an ordinary symmetry. The simplest such FTPM phase can feature anomalous ${\mathbb{Z}}_2$ (toric code) surface topological order, in which the gauge electric and magnetic excitations are exchanged in each Floquet period, which cannot occur in a pure $2d$ system without breaking time reversal symmetry.

We report on the in-plane anisotropy of the Seebeck and Nernst coefficients as well as of the electrical resistivity determined for a series of strain-detwinned single crystals of $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$. Two underdoped samples ($x=0.024$, 0.045) exhibiting a transition from the tetragonal paramagnetic phase to the orthorhombic spin density wave (SDW) phase (at $T_{\mathrm{tr}}=100$ and 60 K, respectively) show an onset of Nernst anisotropy at temperatures above 200 K, which is significantly higher than $T_{\mathrm{tr}}$. In the optimally doped sample ($x=0.06$) the transport properties also appear to be in-plane anisotropic below $T\approx 120$ K, despite the fact that this particular composition does not show any evidence of long-range magnetic order. However, the anisotropy observed in the optimally doped crystal is rather small, and for the Seebeck and Nernst coefficients the difference between values measured along and across the uniaxial strain has an opposite sign to those observed for underdoped crystals with $x=0.024$ and 0.045. For these two samples, the insensitivity of the Nernst anisotropy to the SDW transition suggests that the origin of nematicity might be something other than magnetic.

Dirac electrons in graphene have a valley degree of freedom that is being explored as a carrier of information. In that context of “valleytronics” one seeks to coherently manipulate the valley index. Here, we show that reflection from a superlattice potential can provide a valley switch: Electrons approaching a pristine-graphene–superlattice-graphene interface near normal incidence are reflected in the opposite valley. We identify the topological origin of this valley switch, by mapping the problem onto that of Andreev reflection from a topological superconductor, with the electron-hole degree of freedom playing the role of the valley index. The valley switch is ideal at a symmetry point of the superlattice potential, but remains close to 100% in a broad parameter range.

We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.

In this work, motivated by the sine-square deformation (SSD) for (1+1)-dimensional quantum critical systems, we study the nonequilibrium quantum dynamics of a conformal field theory (CFT) with SSD, which was recently proposed to have a continuous energy spectrum and continuous Virasoro algebra. In particular, we study the time evolution of entanglement entropy after a quantum quench from a uniform CFT, which is defined on a finite space of length $L$, to a sine-square deformed CFT. We find that there is a crossover time $t^{*}$ that divides the entanglement evolution into two interesting regions. For $t\ll t^{*}$, the entanglement entropy does not evolve in time; for $t\gg t^{*}$, the entanglement entropy grows as $S_A\left(t\right)\simeq \frac{c}{3}logt$, which is independent of the lengths of the subsystem and the total system. This $logt$ growth with no revival indicates that a sine-square deformed CFT effectively has an infinite length, in agreement with previous studies based on energy spectrum analysis. Furthermore, we study the quench dynamics for a CFT with Möbius deformation, which interpolates between a uniform CFT and a sine-square deformed CFT. The entanglement entropy oscillates in time with period $L_{\text{eff}}=Lcosh\left(2\theta \right)$, with $\theta =0$ corresponding to the uniform case and $\theta \to \infty$ corresponding to the SSD limit. Our field theory calculation is confirmed by a numerical study on a (1+1)-dimensional critical fermion chain.