Pinning and creep determine the current-voltage characteristics of a type-II superconductor and thereby its potential for technological applications. The recent development of strong pinning theory provides us with a tool to assess a superconductor's electric properties in a quantitative way. Motivated by the observation of typical excess-current characteristics and field scaling of critical currents, here we analyze current-voltage characteristics measured on 2H-${\mathrm{NbSe}}_2$ and $a$-MoGe type-II superconductors within the setting provided by strong pinning theory. The experimentally observed shift and rounding of the voltage onset is consistent with the predictions of strong pinning in the presence of thermal fluctuations. We find the underlying parameters determining pinning and creep and discuss their consistency.

FeGe in the B20 phase is an experimentally well-studied prototypical chiral magnet exhibiting helical spirals, skyrmion lattices, and individual skyrmions with a robust length of 70 nm. While the helical spiral ground state can be verified by first-principles calculations based on density functional theory, this feature size could not be reproduced even approximately. To develop a coherent picture of the discrepancy between experiment and theory, we investigate in this work the magnetic properties of FeGe from first principles using different electronic-structure methods. We study atomistic as well as micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya interactions, and discuss their subtle dependence on computational, structural, and correlation parameters. In particular, we quantify how these magnetic properties are affected by changes of the lattice parameter, different atomic arrangements, exchange and correlation effects, finite Fermi-function broadening, and momentum-space sampling. In addition, we use the obtained atomistic parameters to determine the corresponding Curie temperature, which agrees well with experiments. Our results indicate that the well-known and well-accepted relation between the micromagnetic parameters and the period of the helical structure is not valid for FeGe. This calls for new experiments exploring the relation by measuring independently the spin stiffness, the spiralization, and the period of the helical spin spiral.

Continuous spectral hole burning and spin-level population oscillations are studied in an inhomogeneously broadened diamond-based spin ensemble composed of substitutional nitrogen and nitrogen-vacancy centers created through neutron irradiation and annealing. The burnt spectral features highlight a detuning-dependent homogeneous hole linewidth that is up to three orders of magnitude narrower than the total inhomogeneous ensemble linewidth. Continuous population oscillations are observed to quickly decay beyond a pump and probe detuning of 5 Hz, and are numerically modelled using a five-level system of coupled rate equations. Fourier analysis of these oscillations highlight discrete ${}^{13}\mathrm{C}$ hyperfine interactions, with energies within the inhomogeneous ensemble linewidth, as well as suspected nuclear $3/2$-spin coupled signatures likely related to the ${}^7\mathrm{Li}$ byproduct of neutron irradiation.

We compute the scrambling rate at the antiferromagnetic (AFM) quantum critical point, using the fixed point theory found by Schlief, Lunts, and Lee [Phys. Rev. X 7, 021010 (2017)]. At this strongly coupled fixed point, there is an emergent control parameter $w\ll 1$ that is a ratio of natural parameters of the theory. The strong coupling is unequally felt by the two degrees of freedom: the bosonic AFM collective mode is heavily dressed by interactions with the electrons, while the electron is only marginally renormalized. We find that the scrambling rates act as a measure of the “degree of integrability” of each sector of the theory: the Lyapunov exponent for the boson ${\lambda }_L^{\left(B\right)}\sim \mathcal{O}\left(\sqrt{w}\right)k_{B}T/\hslash$ is significantly larger than the fermion one ${\lambda }_L^{\left(F\right)}\sim \mathcal{O}\left(w^2\right)k_{B}T/\hslash$, where $T$ is the temperature. Although the interaction strength in the theory is of order unity, the larger Lyapunov exponent is still parametrically smaller than the universal upper bound of ${\lambda }_L=2\pi k_{B}T/\hslash$. We also compute the spatial spread of chaos by the boson operator, whose low-energy propagator is highly nonlocal. We find that this nonlocality leads to a scrambled region that grows exponentially fast at intermediate distances, giving an infinite “butterfly velocity” of the chaos front, a result that has also been found in lattice models with long-range interactions.

Surface acoustic waves (SAWs) have been used to transport single electrons across long distances of several hundreds of microns. They can potentially be instrumental in the implementation of scalable quantum processors and quantum repeaters, by facilitating interaction between distant qubits. While most of the work thus far has focused on SAW devices in doped GaAs/AlGaAs heterostructures, we have developed a method of creating lateral $p\text{−}n$ junctions in an undoped heterostructure containing a quantum well, with the expected advantages of having reduced charge noise and increased spin-coherence lifetimes due to the lack of dopant scattering centers. We present experimental observations of SAW-driven single-electron quantized current in an undoped GaAs/AlGaAs heterostructure, where single electrons were transported between regions of induced electrons. We also demonstrate pumping of electrons by a SAW across the submicron depleted channel between regions of electrons and holes, and observe light emission at such a lateral $p\text{−}n$ junction. Improving the lateral confinement in the junction should make it possible to produce a quantized electron-to-hole current and hence SAW-driven emission of single photons.

We study the eigenstate properties of a nonintegrable spin chain that was recently realized experimentally in a Rydberg-atom quantum simulator. In the experiment, long-lived coherent many-body oscillations were observed only when the system was initialized in a particular product state. This pronounced coherence has been attributed to the presence of special “scarred” eigenstates with nearly equally spaced energies and putative nonergodic properties despite their finite energy density. In this paper we uncover a surprising connection between these scarred eigenstates and low-lying quasiparticle excitations of the spin chain. In particular, we show that these eigenstates can be accurately captured by a set of variational states containing a macroscopic number of magnons with momentum $\pi$. This leads to an interpretation of the scarred eigenstates as finite-energy-density condensates of weakly interacting $\pi$ magnons. One natural consequence of this interpretation is that the scarred eigenstates possess long-range connected correlations in both space and time. We verify numerically the presence of this spatiotemporal long-range order and explain how it is consistent with established no-go theorems precluding its existence in ground states and at thermal equilibrium.

The problem of a driven quantum system coupled to a bath and coherently driven is usually treated using either of two approaches: employing the common secular approximation in the laboratory frame (as usually done in the context of atomic physics) or in the rotating frame (prevailing in, e.g., the treatment of solid-state qubits). These approaches are applicable in different parts of the parameter space and yield different results. We show how to bridge between these two approaches by working in the rotating frame without employing the secular approximation with respect to the driving amplitude. This allows us to uncover unusual behaviors in regimes which were previously inaccessible or inadequately treated. Exceptional features such as the qualitative different evolution of the coherence, population inversion at a lower driving amplitude, and unique structure in the resonance fluorescence spectrum of the system are found. We argue that this generalized approach is essential for analyzing hybrid systems, with components that come from distinctly different regimes which can now be treated simultaneously, giving specific examples from recent experiments on quantum dots coupled to optical cavities, and single-spin electron paramagnetic resonance.

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ [Li et al., Nature (London) 572, 624 (2019)], we study how the correlated Ni $3d_{x^2-y^2}$ electrons in the ${\mathrm{NiO}}_2$ layer interact with the electrons in the Nd layer. We show that three orbitals are necessary to represent the electronic structure around the Fermi level: Ni $3d_{x^2-y^2}$, Nd $5d_{3z^2-r^2}$, and a bonding orbital made from an interstitial $s$ orbital in the Nd layer and the Nd $5d_{xy}$ orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni $3d_{x^2-y^2}$ state and the states in the Nd layer is tiny. We also find that the metallic screening by the Nd layer is not so effective in that it reduces the Hubbard $U$ between the Ni $3d_{x^2-y^2}$ electrons just by 10%–20%. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of $T_{\mathrm{c}}\sim 10$ K. These results indicate that ${\mathrm{NdNiO}}_2$ hosts an almost isolated correlated $3d_{x^2-y^2}$ orbital system. We further study the possibility of realizing a more ideal single-orbital system in the Mott-Hubbard regime. We find that the Fermi pockets formed by the Nd-layer states dramatically shrink when the hybridization between the interstitial $s$ state and Nd $5d_{xy}$ state becomes small. By an extensive materials search, we find that the Fermi pockets almost disappear in ${\mathrm{NaNd}}_2{\mathrm{NiO}}_4$ and ${\mathrm{NaCa}}_2{\mathrm{NiO}}_3$.

The Bose-Einstein condensation of excitons continues to garner immense attention as a prototypical example for observing emergent properties from many-body quantum effects. In particular, titanium diselenide (${\mathrm{TiSe}}_2$) is a promising candidate for realizing exciton condensation and was experimentally observed only very recently [Kogar et al., Science 358, 1314 (2017)]. Surprisingly, the condensate was experimentally characterized by a soft plasmon mode that only exists near the transition temperature $T_c$ of the charge density wave (CDW). Here, we characterize and analyze the experimental spectra using linear-response time-dependent density functional theory and find that the soft mode can be attributed to interband electronic transitions. At the CDW state below $T_c$, the periodic lattice distortions hamper the spontaneous formation of the exciton by introducing a CDW gap. The band gap raises the soft mode and merges it into the regular plasmon. Our surprising results contradict previous simplistic analytical models commonly used in the scientific literature. In addition, we find that a finite electronic temperature $T_e$ introduces a dissipation channel and prevents the condensation above $T_c$. The combined effect of the CDW and $T_e$ explains the fragile temperature dependence of the exciton condensation. Taken together, our work provides the first ab initio atomic-level framework for rationalizing recent experiments and further manipulating exciton condensates in ${\mathrm{TiSe}}_2$.

We introduce and develop a theory of fusion and statistical processes of gapped excitations in Abelian fracton phases. The key idea is to incorporate lattice translation symmetry via its action on superselection sectors, which results in a fusion theory endowed with information about the nontrivial mobility of fractons and subdimensional excitations. This results in a description of statistical processes in terms of local moves determined by the fusion theory. Our results can be understood as providing a characterization of translation-invariant fracton phases. We obtain simple descriptions of the fusion theory in the X-cube and checkerboard fracton models, as well as for gapped electric and magnetic excitations of some gapless $\mathrm{U}\left(1\right)$ tensor gauge theories. An alternate route to the X-cube model fusion theory is provided by starting with a system of decoupled two-dimensional toric code layers, and giving a description of the $p$-string condensation mechanism within our approach. We discuss examples of statistical processes of fractons and subdimensional excitations in the X-cube and checkerboard models. As an application of the ideas developed, we prove that the X-cube and semionic X-cube models realize distinct translation-invariant fracton phases, even when the translation symmetry is broken corresponding to an arbitrary but finite enlargement of the crystalline unit cell.

We present a general methodology toward the systematic characterization of crystalline topological insulating phases with time-reversal symmetry. In particular, taking the two-dimensional spinful hexagonal lattice as a proof of principle, we study windings of Wilson loop spectra over cuts in the Brillouin zone that are dictated by the underlying lattice symmetries. Our approach finds a prominent use in elucidating and quantifying the recently proposed “topological quantum chemistry” concept. Namely, we prove that the split of an elementary band representation (EBR) by a band gap must lead to a topological phase. For this we first show that in addition to the Fu-Kane-Mele ${\mathbb{Z}}_2$ classification, there is $C_2\mathcal{T}$-symmetry-protected $\mathbb{Z}$ classification of two-band subspaces that is obstructed by the other crystalline symmetries, i.e., forbidding the trivial phase. This accounts for all nontrivial Wilson loop windings of split EBRs that are independent of the parametrization of the flow of Wilson loops. Then, by systematically embedding all combinatorial four-band phases into six-band phases, we find a refined topological feature of split EBRs. Namely, we show that while Wilson loop winding of split EBRs can unwind when embedded in higher-dimensional band space, two-band subspaces that remain separated by a band gap from the other bands conserve their Wilson loop winding, hence revealing that split EBRs are at least “stably trivial,” i.e., necessarily nontrivial in the nonstable (few-band) limit but possibly trivial in the stable (many-band) limit. This clarifies the nature of fragile topology that has appeared very recently. We then argue that in the many-band limit, the stable Wilson loop winding is only determined by the Fu-Kane-Mele ${\mathbb{Z}}_2$ invariant implying that further stable topological phases must belong to the class of higher-order topological insulators.

Ab initio calculations have been successfully used for evaluating lattice dynamical properties of solids within the (quasi)harmonic approximation (i.e., assuming noninteracting phonons with infinite lifetimes), but it remains difficult to treat anharmonicity in all but the simplest compounds. We detail a systematic information-theory-based approach to deriving ab initio anharmonic force constants: Compressive sensing lattice dynamics (CSLD). The non-negligible terms that are necessary to reproduce the first-principles calculated interatomic forces are automatically selected by minimizing the ${\ell }_1$ norm (sum of absolute values) of the scaled force constants. By using efficient sampling of the configuration space using a modest number of atomic configurations with quasirandom displacements, CSLD is well suited for deriving accurate anharmonic potentials for complex multicomponent compounds with large unit cells. We demonstrate the power and generality of CSLD by calculating the phonon lifetimes and thermal transport properties of type-I Si clathrates.

Rare-earth garnets are an exciting playground for studying the exotic magnetic properties of the frustrated hyperkagome lattice. Here we present a comprehensive study of the single ion and collective magnetic properties of the garnet ${\mathrm{Er}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$. Using inelastic neutron scattering, we find a crystal-field ground-state doublet for ${\mathrm{Er}}^{3+}$ with strong Ising anisotropy along local [100] axes. Magnetic susceptibility and heat-capacity measurements provide evidence for long-range magnetic ordering with $T_N=0.8 \mathrm{K}$, and no evidence for residual entropy is found when cooling through the ordering transition. Neutron powder diffraction reveals that the ground-state spin configuration corresponds to the six-sublattice, Ising antiferromagnetic state (${\mathrm{\Gamma }}_3$) common to many of the rare-earth garnets. However, we also found that $\mu \mathrm{SR}$ appears to be insensitive to the ordering transition in this material, in which a low-temperature relaxation plateau was observed with no evidence of spontaneous muon precession. The combined muon and neutron results may be indicative of a dynamical ground state with a relatively long correlation time. Despite this potential complication, our work indicates that ${\mathrm{Er}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ is an excellent model system for studying the complex metamagnetism expected for a multiaxis antiferromagnet.

Materials with coupled or competing order parameters display highly tunable ground states, where subtle perturbations reveal distinct electronic and magnetic phases. These states generally are underpinned by complex crystal structures, but the role of structural complexity in these phases often is unclear. We use group-theoretic methods and first-principles calculations to analyze a set of coupled structural distortions that underlie the polar charge and orbitally ordered antiferromagnetic ground state of $A$-site ordered ${\mathrm{SmBaMn}}_2{\mathrm{O}}_6$. We show that these distortions play a key role in establishing the ground state and stabilizing a network of domain wall vortices. Furthermore, we show that the crystal structure provides a knob to control competing electronic and magnetic phases at structural domain walls and in epitaxially strained thin films. These results provide new understanding of the complex physics realized across multiple length scales in ${\mathrm{SmBaMn}}_2{\mathrm{O}}_6$ and demonstrate a framework for systematic exploration of correlated and structurally complex materials.

We use femtosecond resonant soft x-ray scattering to measure the ultrafast optical melting of charge-order correlations in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$. By analyzing both the energy-resolved and the energy-integrated order-parameter dynamics, we find evidence of a short-lived nonequilibrium state, the features of which are compatible with a sliding charge-density wave coherently set in motion by the pump. This transient state exhibits shifts in both the quasielastic line energy and its wave vector, as expected from a classical Doppler effect. The wave-vector change is indeed found to directly follow the pump propagation direction. These results demonstrate the existence of sliding charge-order behavior in an unconventional charge-density wave system and underscore the power of ultrafast optical excitation as a tool to coherently manipulate electronic condensates.

Generative modeling with machine learning has provided a new perspective on the data-driven task of reconstructing quantum states from a set of qubit measurements. As increasingly large experimental quantum devices are built in laboratories, the question of how these machine learning techniques scale with the number of qubits is becoming crucial. We empirically study the scaling of restricted Boltzmann machines (RBMs) applied to reconstruct ground-state wave functions of the one-dimensional transverse-field Ising model from projective measurement data. We define a learning criterion via a threshold on the relative error in the energy estimator of the machine. With this criterion, we observe that the number of RBM weight parameters required for accurate representation of the ground state in the worst case – near criticality – scales quadratically with the number of qubits. By pruning small parameters of the trained model, we find that the number of weights can be significantly reduced while still retaining an accurate reconstruction. This provides evidence that overparametrization of the RBM is required to facilitate the learning process.

We introduce a massively parallel replica-exchange grand-canonical sampling algorithm to simulate materials at realistic conditions, in particular surfaces and clusters in reactive atmospheres. Its purpose is to determine in an automated fashion equilibrium phase diagrams for a given potential-energy surface and for any observable sampled in the grand-canonical ensemble. The approach enables an unbiased sampling of the phase space and is embarrassingly parallel. It is demonstrated for a model of the Lennard-Jones system describing a surface in contact with a gas phase. Furthermore, the algorithm is applied to ${\mathrm{Si}}_M$ clusters ($M=2,4$) in contact with an ${\mathrm{H}}_2$ atmosphere, with all interactions described at the ab initio level, i.e., via density-functional theory, with the Perdew-Burke-Ernzerhof gradient-corrected exchange-correlation functional. We identify the most thermodynamically stable phases at finite $T,p({\mathrm{H}}_2$) conditions.

Proximity-induced magnetization (PIM) has broad implications across interface-driven spintronics applications employing spin currents. We directly determine the scaling between PIM in Pt and the temperature-dependent interface magnetization in an adjacent ferromagnet (FM) using depth-resolved magnetometry. The magnetization due to PIM does not follow the generally expected linear scaling with the FM interface magnetization, as a function of temperature. Instead, it vanishes while the FM interface magnetization remains. The effective magnetic susceptibilities of heavy-metal (HM) layers are shown to give rise to the previously unexplained asymmetric PIM found in HM/FM/HM trilayers.

Due to the complexity of the space of quantum many-body states, the computation of expectation values by statistical sampling is, in general, a hard task. Neural network representations of such quantum states, which can be physically implemented by neuromorphic hardware, could enable efficient sampling. A scheme is proposed that leverages this capability to speed up sampling from so-called neural quantum states encoded by a restricted Boltzmann machine. Due to the complex network parameters, a direct hardware implementation is not feasible. We overcome this problem by considering a phase-reweighting scheme for sampling expectation values of observables. Applying our method to a set of paradigmatic entangled quantum states we find that, in general, the phase-reweighted sampling is subject to a form of sign problem, which renders the sampling computationally costly. The use of neuromorphic chips could allow reducing computation times and thereby extend the range of tractable system sizes.

We report theoretical and experimental evidence that ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ in magnetic fields greater than 1.6 T applied along the $c$ axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wave vectors $\mathbf{k}=(0,0,\pm 0.03)\times 2\pi /c$ at the Fermi level when the Eu spins are fully aligned along the $c$ axis. Shubnikov–de Haas oscillations measured in fields parallel to $c$ reveal a cyclotron effective mass of $m_c^{*}=0.08m_e$ and a Fermi surface of extremal area $A_{\mathrm{ext}}=0.24{\mathrm{nm}}^{-2}$, corresponding to 0.1% of the area of the Brillouin zone. The small values of $m_c^{*}$ and $A_{\mathrm{ext}}$ are consistent with quasiparticles near a Weyl node. The identification of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.