We describe the chiral Kondo chain model based on the symplectic Kondo effect and demonstrate that it has a quantum critical ground state populated by non-Abelian anyons. We show that the fusion channel of two arbitrary anyons can be detected by locally coupling the two anyons to an extra single channel of chiral current and measuring the corresponding conductance at a finite frequency. Based on such measurements, we propose that the chiral Kondo chain model with symplectic symmetry can be used for the implementation of measurement-only topological quantum computations, and it possesses several distinct features favorable for such applications. The sources and effects of errors in the proposed system are analyzed and possible material realizations are discussed.

We show that the polarizations of sidebands emitted from bulk gallium arsenide driven by a strong terahertz (THz) field while probed with a weak near-infrared laser can be viewed as interferograms from a Michelson-like interferometer for Bloch waves. The fringe contrast of an interferogram is a measure of the nonequilibrium dephasing of Bloch waves in this strongly driven system. An interferogram's oscillation frequency is determined by, among other factors, the THz field strength $F_{\mathrm{THz}}$ and the reduced masses of electron-hole pairs. A simple analytic model brings all measured and calculated spectra into agreement with adjustment of only a single dephasing constant, and predicts the scaling of interferograms with $F_{\mathrm{THz}}$.

Point defects in hexagonal boron nitride (hBN) are often discussed as single-photon emitters for quantum technologies. Understanding the dependence of electronic and optical properties on the geometry might help to identify the atomic structure of the defects and is also crucial in order to make these emitters applicable. Here, we study three defects in a monolayer of hBN, namely, ${\mathrm{C}}_{\text{B}}{\mathrm{V}}_{\text{N}}$, ${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$, and ${\mathrm{C}}_{\text{B}}{\mathrm{O}}_{\text{N}}$, from an ab initio approach. We use (constrained) density functional theory to obtain optimal geometries of the electronic ground state and the first excited state and then refine quasiparticle energies and optical excitation energies using a $GW$ and Bethe-Salpeter equation (BSE) based approach. All three defect systems host transitions between deep-lying defect states. We find the lowest defect exciton of ${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$ at $\sim 4$ eV and of the other two defects at $\sim 2$ eV with significant Stokes shifts of 0.15 and $0.79\mathrm{eV}$, respectively. Finally, we investigate the effects of the Tamm-Dancoff approximation and show that it can have a significant influence on hBN defect excitons calculated from BSE.

The Dirac degeneracy of band structures, along with the valley states after breaking, holds promising potential for robust signal processing. However, the conventional graphene structure only harbors a single pair of Dirac points, limiting its capacity for information storage and manipulation, and hindering its application in small-sized on-chip devices. In this study, we experimentally investigate S-graphene acoustic crystals that host two pairs of Dirac points and valleys. By examining the interplay between these multivalley pairs, we observe double-kink edge states that display $W$-type and $M$-type dispersions self-guiding at zigzag- and bearded boundaries in a single sample, respectively, eliminating the requirement of a cladding layer in conventional valley Hall effect. Additionally, benefiting from the increased degree of freedom, a superdirectional yet adjustable topological antenna is demonstrated by selectively exciting a specific valley branch. This research expands the degree of freedom for valleytronics and has the potential to stimulate the exploration of multiple spin-valley couplings and the design of unique functional devices.

We develop a machine learning model, which predicts structural relaxation from amorphous supercooled liquid structures. The trained networks are able to predict dynamic heterogeneity across a broad range of temperatures and time scales with excellent accuracy and transferability. We use the network transferability to predict dynamic heterogeneity down to the experimental glass transition temperature $T_g$, where structural relaxation cannot be analyzed using molecular dynamics simulations. The results indicate that the strength, the geometry, and the characteristic length scale of the dynamic heterogeneity evolve much more slowly near $T_g$ compared to their evolution at higher temperatures. Our results show that machine learning techniques can provide physical insights on the nature of the glass transition that cannot be gained using conventional simulation techniques.

We have studied the ground state properties of Hubbard model on long six-leg square cylinders with doped hole concentration $5.55\%\le \delta \le 12.5\%$ using density-matrix renormalization group. Our state-of-the-art DMRG study with a large number of states convincingly shows that the nature of the ground state is remarkably sensitive to the presence of next-nearest-neighbor electron hopping $t^{\prime }$. In the positive $t^{\prime }$ side, we find a robust $d$-wave superconducting (SC) phase characterized by coexisting quasi-long-range SC and charge density wave (CDW) correlations. Without $t^{\prime }$ the ground state forms an insulating stripe phase with long-range CDW order. In stark contrast to four-leg cylinders, our results show that the lightly doped Hubbard model on six-leg cylinders remains insulating in the negative $t^{\prime }$ side where the SC correlations decay exponentially with short correlation lengths. In the larger negative $t^{\prime }$ side, the doped holes form a holon Wigner crystal with one doped hole per emergent unit cell and short-range spin-spin correlations.

Group-IV color centers in diamond (${\mathrm{SiV}}^{–}, {\mathrm{GeV}}^{–}$, and ${\mathrm{SnV}}^{–}$) have emerged as leading solid-state spin-photon interfaces for quantum information processing applications. However, these qubits require cryogenic temperatures to achieve high fidelity operation due to interactions with the thermal phonon environment. In this paper, we (i) derive a detailed model of the decoherence from first-order acoustic phonon processes acting on the spin-orbit fine structure of these color centers, (ii) demonstrate agreement of the model's predicted coherence times with previous measurements, and (iii) identify regimes to suppress phonon-mediated decoherence by changing magnetic field and strain bias to allow higher temperature operation. This methodology enables prediction of decoherence processes in other color centers and solid-state qubit systems coupled to a thermal bath via a parasitic two-level system. By experiment-anchored decoherence models, we facilitate optimizing qubit coherence for specific applications and devices.

The interplay between the quantum effects from low-dimensionality and the spin-orbit coupling leads to exotic ground states with unusual excitations. We report the structural, magnetic, heat capacity, and electronic structure studies of ${\mathrm{Bi}}_2{\mathrm{YbO}}_4\mathrm{Cl}$, which constitutes a structurally perfect 2D square lattice with rare-earth magnetic ${\mathrm{Yb}}^{3+}$ ions. The magnetization and heat capacity data analysis confirms that the ${\mathrm{Yb}}^{3+}$ ion hosts the spin-orbit driven ${\mathit{J}}_{\text{eff}}=\frac{1}{2}$ state at low temperatures. From the fit to the Curie-Weiss law on the magnetic susceptibility data in the low-temperature region, the observed Curie-Weiss temperature is about −1 K, implying an antiferromagnetic (AFM) coupling between the ${\mathrm{Yb}}^{3+}$ moments. The heat capacity data show the presence of a broad maximum at 0.3 K and the absence of any sharp magnetic anomaly down to 0.09 K, indicating the onset of short-range correlations. Our first-principles calculations based on density functional theory provide further insight into the role of the microscopic parameters. In particular, it points out the crucial role of spin-orbit coupling in driving both the ${\mathit{J}}_{\text{eff}}=\frac{1}{2}$ state as well as the antiferromagnetic interaction between the nearest-neighbor ${\mathrm{Yb}}^{3+}$ moments that is consistent with experimental results. The total energy calculations suggest an easy-axis (out-of-plane) anisotropy of the spins.

Manipulation of decoupled Majorana zero modes (MZMs) could enable topologically protected quantum computing. However, the practical realization of the large number of perfectly decoupled MZMs needed to perform nontrivial quantum computation has proven to be challenging so far. Fortunately, even a small number of imperfect MZMs can be used to qualitatively extend the behavior of standard superconducting qubits, allowing for new approaches for noise suppression, qubit manipulation, and readout. Such hybrid devices take advantage of the interplay of Cooper pair tunneling, coherent single-electron tunneling, and Majorana hybridization. Here we provide a qualitative understanding of this system, give analytical results for its ground state energy spanning the full parameter range, and describe potential sensing applications enabled by the interplay between Majorana and Cooper pair tunneling.

Using a high-accuracy variational Monte Carlo approach based on group-convolutional neural networks, we obtain the symmetry-resolved low-energy spectrum of the spin-$1/2$ Heisenberg model on several highly symmetric fullerene geometries, including the famous ${\mathrm{C}}_{60}$ buckminsterfullerene. We argue that as the degree of frustration is lowered in large fullerenes, they display characteristic features of incipient magnetic ordering: Correlation functions show high-intensity Bragg peaks consistent with Néel-like ordering, while the low-energy spectrum is organized into a tower of states. Competition with frustration, however, turns the simple Néel order into a noncoplanar one. Remarkably, we find and predict chiral incipient ordering in a large number of fullerene structures.

Experiments in the high-temperature superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ have revealed both high $T_c$ and strange metal behaviors over a wide pressure range. While first-principles density functional theory has predicted weakly correlated bands that cannot explain these key observations, we report here strongly correlated electronic band structure calculations that reveal the dual nature of Ni-$d$ electrons with almost localized $d_{z^2}$ orbitals due to on-site Coulomb repulsion and flat bands of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ quasiparticles near the Fermi energy. We find that the quasiparticle effective masses are greatly enhanced by Hund's rule coupling and their lifetimes are inversely proportional to temperature, which explains the experimentally observed strange metal behavior in the normal state. Our calculations also reveal strong antiferromagnetic spin correlations of Ni-$d$ electrons, which may provide the pairing force of quasiparticles for high-temperature superconductivity. The presence of flat bands and the interplay of orbital-selective Mott, Hund, and Kondo physics lay the basis for a two-component theory of its pairing mechanism and make ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ a unique platform for exploring rich emergent quantum many-body phenomena in the future.

Quantum nondemolition (QND) measurements are a precious resource for quantum information processing. Repetitive QND measurements can boost the fidelity of qubit preparation and measurement, even when the underlying single-shot measurements are of low fidelity. However, this fidelity boost is limited by the degree in which the physical system allows for a truly QND process—slight deviations from ideal QND measurement result in bit flip errors (“quantum jumps”) if the measurement is repeated too often. Here, we develop a theoretical framework to understand and quantify the resulting error arising from deviation from perfect QND measurement in model spin qubit systems. We first develop our model on the ubiquitous example of exchange-coupled electron spins qubits tunnel-coupled to a charge reservoir. We then extend it to electron-nuclear spin systems, to illustrate the crucial similarities and differences between the two limits. Applied to the well-understood platform of a donor nuclear spin in silicon, the model shows excellent agreement with experiments. For added generality, we conclude the work by considering the effect of anisotropic spin couplings.

Recent experiments have observed Cooper pair splitting in quantum dots coupled to superconductors, and efficient schemes for controlling and timing the splitting process are now called for. Here, we propose and analyze an adiabatic Cooper pair splitter that can produce a regular flow of spin-entangled electrons in response to a time-dependent and periodic gate voltage. The splitting process is controlled by moving adiabatically back and forth along an avoided crossing between the empty state and the singlet state of two quantum dots that are coupled to a superconductor, followed by the emission of the split Cooper pairs into two normal-state drains. The scheme does not rely on fine-tuned resonance conditions and is therefore robust against experimental imperfections in the driving signal. We identify a range of driving frequencies, where the output currents are quantized and proportional to the driving frequency combined with suppressed low-frequency noise. We also discuss the main sources of cycle-missing events and evaluate the statistics of electrons emitted within a period of the drive as well as the distribution of waiting times between them. Realistic parameter estimates indicate that the Cooper pair splitter can be operated in the gigahertz regime.

The Einstein–de Haas (EdH) effect is a fundamental, mechanical consequence of any temporal change of magnetism in an object. EdH torque results from conserving the object's total angular momentum: The angular momenta of all the specimen's magnetic moments, together with its mechanical angular momentum. Although the EdH effect is usually small and difficult to observe, it increases in magnitude with detection frequency. We explore the frequency dependence of EdH torque for a thin film permalloy microstructure by employing a ladder of flexural beam modes (with five distinct resonance frequencies spanning from 3 to 208 MHz) within a nanocavity optomechanical torque sensor via magnetic hysteresis curves measured at mechanical resonances. At low dc fields, the gyrotropic resonance of a magnetic vortex spin texture overlaps the 208 MHz mechanical mode. The massive EdH mechanical torques arising from this coresonance yield a fingerprint of vortex core pinning and depinning in the sample. The experimental results are discussed in relation to mechanical torques predicted from both macrospin (at high dc magnetic field) and finite-difference solutions to the Landau-Lifshitz-Gilbert (LLG) equation. A global fit of the LLG solutions to the frequency-dependent data reveals a statistically significant discrepancy between the experimentally observed and simulated torque phase behaviours at spin-texture transitions that can be reduced through the addition of a time constant to the conversion between magnetic cross-product torque and mechanical torque, constrained by experiment to be in the range of 0.5–4 ns.

We study de Haas-van Alphen oscillations in a marginal Fermi liquid resulting from a three-dimensional metal tuned to a quantum-critical point (QCP). We show that the conventional approach based on extensions of the Lifshitz-Kosevich formula for the oscillation amplitudes becomes inapplicable when the correlation length exceeds the cyclotron radius. This breakdown is due to (i) nonanalytic finite-temperature contributions to the fermion self-energy, (ii) an enhancement of the oscillatory part of the self-energy by quantum fluctuations, and (iii) nontrivial dynamical scaling laws associated with the quantum critical point. We properly incorporate these effects within the Luttinger-Ward-Eliashberg framework for the thermodynamic potential by treating the fermionic and bosonic contributions on equal footing. As a result, we obtain the modified expressions for the oscillations of entropy and magnetization that remain valid in the non-Fermi-liquid regime.

We develop a theory of magnetic breakdown (MB) near high-order saddle points in the dispersions of two-dimensional materials, where two or more semiclassical cyclotron orbits approach each other. MB occurs due to quantum tunneling between several trajectories, which leads to nontrivial scattering amplitudes and phases. We show that for any saddle point this problem can be solved by mapping it to a scattering problem in a 1D tight-binding chain. Moreover, the occurrence of magnetic breakdown on the edges of the Brillouin zone facilitates the delocalization of the bulk Landau level states and the formation of 2D orbit networks. These extended network states compose dispersive mini bands with finite energy broadening. This effect can be observed in transport experiments as a strong enhancement of the longitudinal bulk conductance in a quantum Hall bar. In addition, it may be probed in STM experiments by visualizing bulk current patterns.

We report the large-amplitude coherent soft phonon dynamics dominated by anharmonicity for ferroelectric semiconductor near topological transitions In-doped (Sn,Pb)Te using high-field terahertz spectroscopy. The phonon frequency is dramatically increased through intense terahertz excitation of soft phonons having large dielectric response. The atomic displacement and phonon anharmonicity, which are quantitively retrieved from the field strength dependent response, are the most enhanced near the ferroelectric transition; the displacement is as large as the spontaneous ferroelectric distortion and the anharmonic positional free energy for phonons reaches up to 75% of the total one. The present findings are crucial for nonlinear phononics including terahertz control of emergent topological phases.

Density functional perturbation theory (DFPT) is a well-established method to study responses of molecules and solids, especially responses to atomic displacements or to different perturbing fields (electric, magnetic). Like for density functional theory (DFT), the treatment of metals is delicate, due to the Fermi-Dirac (FD) statistics and electronic bands crossing the Fermi energy. At zero temperature, there is an abrupt transition from occupied states to unoccupied ones, usually addressed with smearing schemes. Also, at finite temperature, fractional occupations are present, and the occupation numbers may vary in response to the perturbation. In this paper, we establish the characteristics of DFPT stemming from the underlying variational principle, in the case of metals. After briefly reviewing variational DFT for metals, the convexity of the entropy function of the occupation number is analyzed, and at finite temperature, the benefit of resmearing the FD broadening with the Methfessel-Paxton one is highlighted. Then the variational expressions for the second-order derivative of the free energy are detailed, exposing the different possible gauge choices. The influence of the inaccuracies in the unperturbed wave functions from the prior DFT calculation is studied. The whole formalism is implemented in the abinit software package.

Topological magnon insulators (TMI) are ordered magnets supporting chiral edge magnon excitations. These edge states are envisioned to serve as topologically protected information channels in low-loss magnonic devices. The standard description of TMI is based on linear spin-wave theory (LSWT), which approximates magnons as free noninteracting particles. However, magnon excitations of TMI are genuinely interacting even at zero temperature, calling into question descriptions based on LSWT alone. Here we perform a detailed nonlinear spin-wave analysis to investigate the stability of chiral edge magnons. We identify three general breakdown mechanisms: (1) The edge magnon couples to itself, generating a finite lifetime that can be large enough to lead to a spectral annihilation of the chiral state. (2) The edge magnon hybridizes with the extended bulk magnons and, as a consequence, delocalizes away from the edge. (3) Due to a bulk-magnon mediated edge-to-edge coupling, the chiral magnons at opposite edges hybridize. We argue that, in general, these breakdown mechanisms may invalidate predictions based on LSWT and violate the notion of topological protection. We discuss strategies how the breakdown of chiral edge magnons can be avoided, e.g., via the application of large magnetic fields. Our results highlight a challenge for the realization of chiral edge states in TMI and in other bosonic topological systems without particle number conservation.

The recent discovery of superconductivity in bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (327-LNO) under pressure stimulated much interest in layered nickelates. However, superconductivity was not found in another bilayer nickelate system, ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_6$ (326-LNO), even under pressure. To understand the similarities and differences between 326-LNO and 327-LNO, using density functional theory and the random phase approximation (RPA), we systematically investigate 326-LNO under pressure. The large crystal-field splitting between the $e_g$ orbitals caused by the missing apical oxygen moves the $d_{3z^2-r^2}$ orbital farther away from the Fermi level, implying that the $d_{3z^2-r^2}$ orbital plays a less important role in 326-LNO than in 327-LNO. This also results in a smaller bandwidth for the $d_{x^2-y^2}$ orbital and a reduced energy gap for the bonding-antibonding splitting of the $d_{3z^2-r^2}$ orbital in 326-LNO, as compared to 327-LNO. Moreover, the in-plane hybridization between the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals is found to be small in 326-LNO, while it is much stronger in 327-LNO. Furthermore, the low-spin ferromagnetic state is found to be the likely ground state in 326-LNO under high pressure. The weak interlayer coupling suggests that $s_{\pm }$-wave pairing is unlikely in 326-LNO. The robust in-plane ferromagnetic coupling also suggests that $d$-wave superconductivity, which is usually caused by antiferromagnetic fluctuations of the $d_{x^2-y^2}$ orbital, is also unlikely in 326-LNO. These conclusions are supported by our many-body RPA calculations of the pairing behavior. In addition, for the bilayer cuprate ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_6$, we find a strong self-doping effect of the $d_{x^2-y^2}$ orbital under pressure, with the charge of Cu being reduced by approximately 0.13 electrons from 0 GPa to 25 GPa. In contrast, we do not observe such a change in the electronic density in 326-LNO under pressure, establishing another important difference between the nickelates and the cuprates.