We have studied nonequilibrium phase transitions in the vortex lattice in superconducting ${\mathrm{MgB}}_2$, where metastable states are observed in connection with an intrinsically continuous rotation transition. Using small-angle neutron scattering and a stop-motion technique, we investigated the manner in which the metastable vortex lattice returns to the equilibrium state under the influence of an ac magnetic field. This shows a qualitative difference between the supercooled case which undergoes a discontinuous transition and the superheated case where the transition to the equilibrium state is continuous. In both cases, the transition may be described by an activated process, with an activation barrier that increases as the metastable state is suppressed, as previously reported for the supercooled vortex lattice [Louden et al., Phys. Rev. B 99, 060502(R) (2019).] Separate preparations of superheated metastable vortex lattices with different domain populations showed an identical transition toward the equilibrium state. This provides further evidence that the vortex lattice metastability, and the kinetics associated with the transition to the equilibrium state, is governed by nucleation and growth of domains and the associated domain boundaries.

We explore the manifestation of quantum fluctuations across the superconductor-insulator transition (SIT) in three different local measurements: the two-particle density of states $P(\mathbf{r},\omega )$, compressibility $\kappa \left(\mathbf{r}\right)$, and diamagnetic susceptibility $\chi \left(\mathbf{r}\right)$ revealed through their local maps. The map of $\chi \left(\mathbf{r}\right)$ displays growing fluctuations upon increasing temperature as well as upon tuning the quantum tuning parameter $g$, as expected, but remarkably, these fluctuations persist well below $T_c$ as the SIT is approached, indicating the quantum nature of these fluctuations. The map of $\kappa \left(\mathbf{r}\right)$ paints a similar picture when tuned via $g$, but in contrast to $\chi \left(\mathbf{r}\right)$, we find a fundamental difference in its evolution with temperature, providing a complementary local probe to $\chi \left(\mathbf{r}\right)$. $P(\mathbf{r},\omega )$, obtained using maximum entropy analytic continuation of Monte Carlo simulations on 2D quantum Josephson junction arrays, shows strongly diminished zero-energy spectral weight in nearly-insulating islands, correlating with regions of suppressed $\kappa \left(\mathbf{r}\right)$. We discuss the experimental implications of our results for scanning Josephson spectroscopy, compressibility, and scanning SQUID measurements in the context of quantum fluctuations.

The eigenstate thermalization hypothesis (ETH) is a successful theory that provides sufficient criteria for ergodicity in quantum many-body systems. Most studies were carried out for Hamiltonians relevant for ultracold quantum gases and single-component systems of spins, fermions, or bosons. The paradigmatic example for thermalization in solid-state physics are phonons serving as a bath for electrons. This situation is often viewed from an open-quantum-system perspective. Here, we ask whether a minimal microscopic model for electron-phonon coupling is quantum chaotic and whether it obeys ETH, if viewed as a closed quantum system. Using exact diagonalization, we address this question in the framework of the Holstein polaron model. Even though the model describes only a single itinerant electron, whose coupling to dispersionless phonons is the only integrability-breaking term, we find that the spectral statistics and the structure of Hamiltonian eigenstates exhibit essential properties of the corresponding random-matrix ensemble. Moreover, we verify the ETH ansatz both for diagonal and off-diagonal matrix elements of typical phonon and electron observables, and show that the ratio of their variances equals the value predicted from random-matrix theory.

The hexagonal phase of $\mathrm{AgBiS}{\mathrm{e}}_2$ has been discovered as a promising thermoelectric material for room-temperature applications. However, its basic conduction type is still ambiguous, and its current ZT value is pretty low. To improve the thermoelectric performance of $\mathrm{AgBiS}{\mathrm{e}}_2$, we apply band engineering to modify its band structure by introducing defects to increase the band degeneracy. From the calculated intrinsic point defect formation energies of $\mathrm{AgBiS}{\mathrm{e}}_2$ at different growth conditions, we clarify that the conducting behavior of $\mathrm{AgBiS}{\mathrm{e}}_2$ is a $p$-type semiconductor, and the Ag vacancy is the dominated acceptor. Based on scrutinizing the band structure of $\mathrm{AgBiS}{\mathrm{e}}_2$, two kinds of methodologies can be used to modify its band structure to achieve high band degeneracy: (i) shifting the Fermi level into the valence band using intrinsic defects, and (ii) converging several valence-band maxima by introducing extrinsic defects. We find that the intrinsic Ag vacancy is helpful to significantly increase the power factor, leading to a large ZT for Ag vacancy-doped $\mathrm{AgBiS}{\mathrm{e}}_2$: the maximum ZT value is increased to 0.3–0.5 at near room temperature. Based on analyzing the bonding characters and atomic energy levels in the compound, we predict several extrinsic dopants (Cu, Rh, and Pd) that can be used to converge three valence-band maxima. Our work provides methodologies to improve the room-temperature thermoelectric applications of $\mathrm{AgBiS}{\mathrm{e}}_2$ by tuning its band structures using intrinsic or extrinsic defects.

We propose that the electronic structure of twisted bilayer graphene (TBG) can be understood as Dirac fermions coupled with opposite pseudomagnetic fields generated by the moiré pattern. The two low-energy flat bands from each monolayer valley originate from the two zeroth pseudo Landau levels of Dirac fermions under such opposite effective magnetic fields, which have opposite sublattice polarizations and carry opposite Chern numbers $\pm 1$, giving rise to helical edge states in the gaps below and above the low-energy bulk bands near the first magic angle. We argue that small Coulomb interactions would split the eightfold degeneracy (including valley and physical spin) of these zeroth pseudo Landau levels, and may lead to insulating phases with nonvanishing Chern numbers at integer fillings. Besides, we show that all the high-energy bands below or above the flat bands are also topologically nontrivial in the sense that for each valley the sum of their Berry phases is quantized as $\pm \pi$. Such quantized Berry phases give rise to nearly flat edge states, which are dependent on truncations on the moiré length scale. Our paper provides a complete and clear picture for the electronic structure and topological properties of TBG, and has significant implications on the nature of the correlated insulating phase observed in experiments.

The neutrally charged silicon vacancy in diamond is a promising system for quantum technologies that combines high-efficiency optical spin initialization with long spin lifetimes ($T_2\approx 1$ ms at 4 K) and up to 90% of optical emission into its 946-nm zero-phonon line. However, the electronic structure of ${\mathrm{SiV}}^0$ is poorly understood, making further exploitation difficult. Performing photoluminescence spectroscopy of ${\mathrm{SiV}}^0$ under uniaxial stress, we find the previous excited electronic structure of a single ${}^{3}A_{1u}$ state is incorrect, and identify instead a coupled ${}^{3}E_u\text{−}{}^{3}A_{2u}$ system, the lower state of which has forbidden optical emission at zero stress and efficiently decreases the total emission of the defect. We propose a solution employing finite strain to define a spin-photon interface scheme using ${\mathrm{SiV}}^0$ .

We use tunable, vacuum ultraviolet laser based angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations to study the electronic properties of Dirac semimetal candidate cubic ${\mathrm{PtBi}}_2$. In addition to bulk electronic states we also find surface states in ${\mathrm{PtBi}}_2$, which is expected as ${\mathrm{PtBi}}_2$ was theoretically predicated to be a candidate Dirac semimetal. The surface states are also well reproduced from DFT band calculations. Interestingly, the topological surface states form Fermi contours rather than double Fermi arcs that were observed in ${\mathrm{Na}}_3\mathrm{Bi}$. The surface bands forming the Fermi contours merge with bulk bands in proximity to the Dirac point projections, as expected. Our data confirm the existence of Dirac states in ${\mathrm{PtBi}}_2$ and reveal the fragility of the Fermi arcs in Dirac semimetals. Because the Fermi arcs are not topologically protected in general, they can be deformed into Fermi contours, as proposed by M. Kargarian et al. [Proc. Natl. Acad. Sci. USA 113, 8648 (2016)]. Our results demonstrate the validity of this theory in ${\mathrm{PtBi}}_2$.

Since the discovery of superconductivity and correlated insulators at fractional electron fillings in twisted bilayer graphene, most theoretical efforts have been focused on describing this system in terms of an effective extended Hubbard model. However, it was recognized that an exact tight-binding model on the moiré superlattice which captures all the subtleties of the bands can be exceedingly complicated. Here, we pursue an alternative framework of coupled wires to describe the system based on the observation that the lattice relaxation effect is strong at small twist angles, which substantially enlarges the AB and BA stacking domains. Under an out-of-plane electric field which can have multiple origins, the low-energy physics of the system is dominated by interconnected wires with (approximately) gapless one-dimensional (1D) conducting quantum valley Hall domain wall states. We demonstrate that the Coulomb interaction likely renders the wires a $U{\left(2\right)}_2$ $(1+1)\text{D}$ conformal field theory (CFT) with a tunable Luttinger parameter for the charge $U\left(1\right)$ sector. Spin-triplet and spin-singlet Cooper pair operators both have quasi-long-range order in this CFT. The junction between the wires at the AA stacking islands can lead to either a two-dimensional superconductor, or an insulator.

We show that a quadratic system of pseudofermions, with tunable fractionalized statistics, can host a rich phase diagram on a one-dimensional chain with nearest- and next-nearest-neighbor hopping. Using a combination of numerical and analytical techniques, we show that by varying the statistical angle and the ratio of the hopping, the system stabilizes two Tomonaga-Luttinger liquids (TLL) with central charges $c=1$ and 2, respectively, along with the inversion symmetry broken bond-ordered (BO) insulating phase. Interestingly, the two quantum phase transitions in the system, (1) between the two TLLs and (2) the $c=1$ TLL and BO phase, can be engendered by solely tuning the statistics of the pseudofermions. Our analysis shows that both these transitions are continuous and novel with the former lacking a local order-parameter based description and the latter of Berezinskii-Kosterlitz-Thouless type. These phases and phase transitions can be of direct experimental relevance in the context of recent studies of fermionic cold atoms.

We study the detection of electrons undergoing coherent transfer via adiabatic passage (CTAP) in a triple quantum-dot system with a quantum point contact sensing the charge of the middle dot. In the ideal scenario, the protocol amounts to perfect change transfer between the external dots with vanishing occupation of the central dot at all times, rendering the measurement and its backaction moot. Nevertheless, even with minor corrections to the protocol, a small population builds up in the central dot. We study the measurement backaction using a Bayesian formalism simulation of an instantaneous detection at the time of maximal occupancy of the dot. We show that the interplay between the measurement backaction and the nonadiabatic dynamics induces a change in the success probability of the protocol, which quantitatively agrees with a continuous detection treatment. We introduce a correlated measurement signal to certify the nonoccupancy of the central dot for a successful CTAP protocol, which, in the weak-measurement limit, confirms a vanishing occupation of the central dot. Our proposed correlated signal purports the proper experimental method by which to confirm CTAP.

Fluid-saturated porous metamaterials described following Biot's theory support two longitudinal elastic waves. The phase velocity and attenuation of these waves depend nonlinearly on porosity and viscosity of the fluid. Furthermore, when two fluid-saturated porous metamaterials are arranged to form a periodic composite, different band gaps are opened for the two longitudinal waves and these couple to form anticrossings in the dispersion relation. The complex band structure of one-dimensional composites is derived and compared with numerical transmission through a finite sample obtained by the finite element method. It is found that the anticrossings disappear rapidly as viscosity increases, while attenuation band gaps become dominated by the fastest of the two longitudinal waves. Increasing porosity further leads to wider and lower-frequency band gaps. These results are relevant to practical applications of fluid-saturated porous metamaterials, e.g., to engineered soils.

Systematic studies of magnetic ordering, magnetic-field-induced transitions, electronic structure, and optical properties of the frustrated spinel ${\mathrm{GeCo}}_2{\mathrm{O}}_4$ (GCO) are reported. Our results reveal that GCO orders antiferromagnetically (AFM) at $T_{\mathrm{N}}=20.4$ K but with significant short-range ferromagnetic (FM) order up to $T$ $\sim 5T_{\mathrm{N}}$. The paramagnetic susceptibility ($\chi$) fits the modified Curie–Weiss law, $\chi ={\chi }_{\mathrm{o}}$ + $C$/($T-$ $\theta$), with $\theta =+51$ K for 100 K < $T<800$ K. The fit to high-temperature-series expansion of $\chi (T$) yields J${}_1$/${\mathrm{k}}_{\mathrm{B}}=14.7$ K as the dominant FM exchange coupling for the pyrochlore lattice of ${\mathrm{Co}}^{2+}$ spins consisting of alternate planes of Kagomé (KGM) and Triangular (TRI) spins lying perpendicular to [111] direction. From the analysis of the M-H plots at 2 K and published results, three critical fields are identified: $H_d\sim 11$ kOe due to AFM domains, $H_{C1}\approx$ 44 kOe related to spin-flips and FM ordering of the TRI spins, and $H_{C2}\approx 97$ kOe related to FM ordering of the KGM spins. For $H$ > $H_{C2}$, GCO is a forced ferromagnet with some canting of the spins. Magnetic field dependence of $T_{\mathrm{N}}$ follows the relation $T_{\mathrm{N}}\left(H\right)=T_{\mathrm{N}}$(0)–$D_1{H}^2$ valid for antiferromagnets with $D_1=6.63\times {10}^{-10}$ K/${\mathrm{Oe}}^2$. This magnitude of T${}_{\mathrm{N}}$(H) along with the temperature dependence of H${}_d$, H${}_{C1}$, and H${}_{C2}$ are used to construct the H-T phase diagram. From the magnitudes of the Curie constant ($C$) and the saturation magnetization at 2 K it is shown that ${\mathrm{Co}}^{2+}$ ions in GCO have the ground state with effective spin $S$ = 1/2. High resolution x-ray photoelectron spectra of 2p and 3d orbitals of Co and Ge confirm the divalent and tetravalent electronic states of Co and Ge, respectively, in GCO. The energy band gap ($E_g=3.28$ eV) evaluated using DFT+U calculations is in good agreement with the experimental results ($E_g=3.16$ eV) obtained from the diffuse reflectance spectroscopy.

The magnetic and electronic nature of the $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{SrTiO}}_3$ spinel/perovskite interface is explored by means of x-ray absorption spectroscopy. Polarized x-ray techniques combined with atomic multiplet calculations reveal localized magnetic moments assigned to ${\mathrm{Ti}}^{3+}$ at the interface with equivalent size for in- and out-of-plane magnetic field directions. Although magnetic fingerprints are revealed, the ${\mathrm{Ti}}^{3+}$ magnetism can be explained by a paramagnetic response at low temperature under applied magnetic fields. Modeling the x-ray linear dichroism results in a ${\mathrm{\Delta }}_0\sim 1.9$ eV splitting between the $t_{2g}$ and $e_g$ states for the ${\mathrm{Ti}}^{4+}3d^0$ orbitals. In addition these results indicate that the lowest energy states have the out-of-plane $d_{xz}/d_{yz}$ symmetry. The isotropic magnetic moment behavior and the lowest energy $d_{xz}/d_{yz}$ states are in contrast to the observations for the two-dimensional electron gas at the perovskite/perovskite interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ that exhibits an anisotropic magnetic $d_{xy}$ ground state.

We theoretically investigate the interlayer dipolar and exchange couplings between an array of metallic magnetic nanowires grown on top of an extended ultrathin yttrium iron garnet film. The calculated interlayer dipolar coupling agrees with observed anticrossings [Chen et al., Phys. Rev. Lett. 120, 217202 (2018)], concluding that the interlayer exchange coupling is suppressed by a spacer layer between the nanowires and film for Ni, but not necessarily for Co. The Kittel mode in the nanowire array couples chirally to spin waves in the film, even though Damon-Eshbach surface modes do not exist. The chirality can be suppressed by a strong interlayer exchange coupling.

We have studied the merging of two identical ${}^4\mathrm{He}$ droplets at zero temperature, caused by their van der Waals mutual attraction. During the early stages of the merging, density structures appear which closely match the experimental observations by Vicente et al. [J. Low Temp. Phys. 121, 627 (2000)]. When the droplets are merging, quantized vortex-antivortex ring pairs nucleate at the surface and annihilate inside the merged droplet producing a roton burst. We also observe the nucleation of quantized vortex-antivortex rings that wrap the droplet surface and remain localized on the surface until they eventually decay into short-wavelength surface waves. Analysis of the kinetic energy spectrum discloses the existence of a regime where turbulence caused by vortex interaction and annihilation is characterized by a Kolmogorov power law. This is followed by another regime where roton radiation—produced by vortex-antivortex annihilation—dominates, whose hallmark is a weak, turbulent surface dynamics. We suggest that similar processes might appear in superfluid helium droplets after they capture impurities or if they are produced by hydrodynamic instability of a liquid jet. Experiments on collisions between recently discovered self-bound Bose-Einstein condensates should display a similar phenomenology.

Ferromagnetic topological insulators have emerged as a promising platform for quantum anomalous Hall (QAH) effect with a dissipationless edge transport. However, the observation of QAH effect has so far been restricted to extremely low temperatures. We investigate the microscopic origin of ferromagnetism coupled with topological insulators in vanadium-doped (${\mathrm{Sb},\mathrm{Bi})}_2{\mathrm{Te}}_3$ employing the x-ray magnetic circular dichroism, angle-resolved two-photon photoemission spectroscopy, combined with first-principles calculations. We found a negative spin polarization, and thus an antiparallel magnetic moment at the Te site with respect to that of the vanadium dopants, which plays the key role in the ferromagnetic order. We ascribe it to the hybridization between Te $p$ and V $d$ majority spin states at the Fermi energy ($E_{\mathrm{F}}$), being supported by a Zener-type $p\text{−}d$ exchange interaction scenario. The substitution of Bi at the Sb site suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of Te $p$ states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.