Rhombohedral stacked multilayer graphene displays the occurrence of a magnetic surface state at low temperatures. Recent angular resolved photoemission experiments demonstrate the robustness of the magnetic state in long sequences of ABC graphene. Here, by using first-principles calculations, we show that field-effect doping of these graphene multilayers induces a nonrigid electronic structure deformation and stabilizes a perfect half-metallic behavior with nearly 100% of spin current polarization and sizable conductivity already at dopings attainable in conventional field-effect transistors with solid state dielectrics. At high doping, magnetism disappears and a flat electronic band prone to correlated, charge density wave, or superconducting instabilities occurs at the Fermi level. Our work demonstrates the realizability of a new kind of spintronic devices where the transition between the low resistance and the high resistance state is driven only by electric fields.

We theoretically introduce the fundamentally fastest induction of a significant population and valley polarization in a monolayer of a transition metal dichalcogenide (i.e., ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$). This may be extended to other two-dimensional materials with the same symmetry. This valley polarization can be written and read out by a pulse consisting of just a single optical oscillation with a duration of a few femtoseconds and an amplitude of $\sim 0.25\mathrm{V}/\text{Å}$. Under these conditions, we predict an effect of topological resonance, which is due to the Bloch motion of electrons in the reciprocal space where electron population textures are formed due to non-Abelian Berry curvature. The predicted phenomena can be applied for information storage and processing in PHz-band optoelectronics.

The combination of non-Hermitian physics and strong correlations can give rise to new effects in open quantum many-body systems with balanced gain and loss. We propose a generalized Anderson impurity model that includes non-Hermitian hopping terms between an embedded quantum dot and two wires. These non-Hermitian hopping terms respect a parity-time ($\mathcal{PT}$) symmetry. In the regime of a singly occupied localized state, we map the problem to a $\mathcal{PT}$-symmetric Kondo model and study the effects of the interactions using a perturbative renormalization group approach. We find that the Kondo effect persists if the couplings are below a critical value that corresponds to an exceptional point of the non-Hermitian Kondo interaction. On the other hand, in the regime of spontaneously broken $\mathcal{PT}$ symmetry, the Kondo effect is suppressed and the low-energy properties are governed by a local-moment fixed point with vanishing conductance.

Barium titanate (BTO) nanoparticles (sizes 10–500 nm) exhibit a displacement of the Ti atom from the center of the perovskite unit cell as inferred from synchrotron x-ray diffraction patterns (XRD) analyzed using atomic pair distribution functions (PDFs). Fits to PDFs acquired at temperatures of 20 °C–220 °C indicate that these Ti displacements (∼0.1 Å) are comparable to or even greater than those in the bulk material. Moreover, these displacements persist at temperatures well above 120 °C, where the tetragonal-to-pseudocubic phase transition occurs in the bulk. Tetragonal Raman spectral lines were observed for all sizes of these BTO nanoparticles and confirm a distorted unit cell up to 120 °C. Above 120 °C, the small BTO nanoparticles (10, 50, 100 nm) continue to display tetragonal Raman lines, though with slowly decreasing amplitudes as the temperature rises. In contrast, the tetragonal Raman lines of large BTO nanoparticles (300, 400, 500 nm) disappear abruptly above 120 °C, suggestive of bulk material. Indeed, fits to large-particle x-ray PDFs over the range 20–60 Å reveal a sharp, long-range structural change toward a cubic lattice at 120 °C, again consistent with bulk material. This sharp, long-range structural change is absent in the small particles. In fact, laboratory XRD Bragg peak profiles for the small BTO particles appear to be singlets at 20 °C, indicating that significant long-range cubic order already exists at room temperature. As temperature rises, this long-range cubic order is gradually reinforced, as inferred from long-range fits of the small particle PDFs. By combining information from x-ray PDFs, Raman spectra, and Bragg peak profiles, we conclude that small BTO nanoparticles exhibit both short-range (unit-cell) distortion and long-range (mesoscale) cubic order from 20 °C to 220 °C, while the large nanoparticles behave as bulk material, differing from small particles only by exhibiting long-range tetragonal order below 120 °C and a mesoscale structural phase change at 120 °C.

The three-dimensional (3D) topological insulator (TI) p-n junction under magnetic fields presents an interesting transport property which is investigated both theoretically and numerically in this paper. Transport in this device can be tuned by the axial magnetic field. Specifically, the scattering coefficients between incoming and outgoing modes oscillate with axial magnetic flux at the harmonic form. In the condition of horizontal mirror symmetry, the initial phase of the harmonic oscillation is dependent on the parities of incoming and outgoing modes. This symmetry is broken when a vertical bias is applied which leads to a kinetic phase shift added to the initial phase. On the other hand, the amplitude of oscillation is suppressed by the surface disorder while it has no influence on the phase of oscillation. Furthermore, with the help of the vertical bias, a special $(1,-2)$ 3D TI p-n junction can be achieved, leading to an unusual spin precession phenomenon.

We have performed high field magnetoabsorption spectroscopy on silicon doped with a variety of single and double donor species. The magnetic field provides access to an experimental magnetic length, and the quadratic Zeeman effect, in particular, may be used to extract the wave-function radius without reliance on previously determined effective mass parameters. We were, therefore, able to determine the limits of validity for the standard one-band anisotropic effective mass model. We also provide improved parameters and use them for an independent check on the accuracy of effective mass theory. Finally, we show that the optically accessible excited-state wave functions have the attractive property that interactions with neighbors are far more forgiving of position errors than (say) the ground state.

We prove that all eigenstates of many-body localized symmetry-protected topological systems with time reversal symmetry have fourfold degenerate entanglement spectra in the thermodynamic limit. To that end, we employ unitary quantum circuits where the number of sites the gates act on grows linearly with the system size. We find that the corresponding matrix product operator representation has similar local symmetries as matrix product ground states of symmetry-protected topological phases. Those local symmetries give rise to a ${\mathbb{Z}}_2$ topological index, which is robust against arbitrary perturbations so long as they do not break time reversal symmetry or drive the system out of the fully many-body localized phase.

We argue that recent measurements on both the superfluid density and the optical conductivity of high-quality ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) films can be understood almost entirely within the theory of disordered BCS $d$-wave superconductors. The large scattering rates deduced from experiments are shown to arise predominantly from weak scatterers, probably the Sr dopants out of the ${\mathrm{CuO}}_2$ plane, and correspond to a significant suppression of $T_c$ relative to a pure reference state with the same doping. Our results confirm the “conventional” viewpoint that the overdoped side of the cuprate phase diagram can be viewed as approaching the BCS weak-coupling description of the superconducting state, with significant many-body renormalization of the plasma frequency. They suggest that, while some of the decrease in $T_c$ with overdoping may be due to weakening of the pairing, disorder plays an essential role.

We present magnetic penetration depth and electrical transport data in single crystals of quasi-one-dimensional (q1D) ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$, which reveal a $1\mathrm{D}\to 3\mathrm{D}$ superconducting dimensional crossover. The $c$-axis penetration depth shows the onset of superconducting fluctuations below $T_{1D}^{ons}=6.7$ K, whereas signatures of superconductivity in the $ab$-plane penetration depth (a uniquely sensitive probe of the transverse phase stiffness) only emerge below $T_{3D}^{ons}=4.9$ K. An anomalously low superfluid density persists down to $\sim 3$ K before rising steeply, in agreement with a theoretical model for crossovers in q1D superconductors. Our data analysis suggests that a sequence of pairing and phase fluctuation regimes controls the unusually broad superconducting transition. In particular, the electrical resistivity below $T_{3D}^{ons}$ is quantitatively consistent with the establishment of phase coherence through gradual binding of Josephson vortex strings to form 3D loops. This dimensional crossover within the superconducting state occurs despite the relatively large transverse hopping predicted from the band structure. Our results have important consequences for the low-temperature normal state in ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ and similar q1D metals, which may retain one-dimensional behavior to lower temperatures than expected from theory.

We analyze a simple model of quantum dynamics, which is a discrete-time deterministic version of the Fredrickson-Andersen model. This model is integrable, with a quasiparticle description related to the classical hard-rod gas. Despite the integrability of the model, commutators of physical operators grow with a diffusively broadening front, in this respect resembling generic chaotic models. In addition, local operators behave consistently with the eigenstate thermalization hypothesis (ETH). However, large subsystems violate ETH; as a function of subsystem size, eigenstate entanglement first increases linearly and then saturates at a scale that is parametrically smaller than half the system size.

Domain walls (DWs), as interfaces separating different magnetic domains, are widely regarded as classical objects, even though the underlying constituents, namely, electron spins and spin-spin exchange interaction, are intrinsically of quantum nature. One intriguing question is, how do DWs behave when the size of the magnetic domains is shrunk to the atomistic level? Would quantum fluctuations destroy the stability of the DWs? These questions are partly addressed for ferromagnets, but seldom studied for antiferromagnets. Here we present a microscopic quantum spin model of antiferromagnets where the ground states not only share several common properties with the classical domain walls, but also exhibit nontrivial quantum properties. Specifically, the magnetization profile highly resembles its classical counterparts, even though the spins inside the domain walls are highly entangled. Furthermore, a nonzero spin angular momentum of a quantum DW emerges and it does not depend on the system size. Finally, we show that quantum DWs can be generated through a quantum phase transition from a quantum domain and the critical point can be characterized by the global entanglement. These results provide additional physical insights on the quantum effects in spintronics, and can be experimentally verified within the current technologies in superconducting circuits and trapped ions.

We study the nonequilibrium phase structure of the three-state random quantum Potts model in one dimension. This spin chain is characterized by a non-Abelian $D_3$ symmetry recently argued to be incompatible with the existence of a symmetry-preserving many-body localized (MBL) phase. Using exact diagonalization and a finite-size scaling analysis, we find that the model supports two distinct broken-symmetry MBL phases at strong disorder that either break the ${\mathbb{Z}}_3$ clock symmetry or a ${\mathbb{Z}}_2$ chiral symmetry. In a dual formulation, our results indicate the existence of a stable finite-temperature topological phase with MBL-protected parafermionic end zero modes. While we find a thermal symmetry-preserving regime for weak disorder, scaling analysis at strong disorder points to an infinite-randomness critical point between two distinct broken-symmetry MBL phases.

Quantum excitations in lattice systems always propagate at a finite maximum velocity. We probe this mechanism by considering a defect traveling at a constant velocity in the quantum Ising spin chain in transverse field. Independently of the microscopic details of the defect, we characterize the expectation value of local observables at large times and large distances from the impurity, where a local quasistationary state (LQSS) emerges. The LQSS is strongly affected by the defect velocity: for superluminal defects, it exhibits a growing region where translational invariance is spontaneously restored. We also analyze the behavior of the friction force exerted by the many-body system on the moving defect, which reflects the energy required by the LQSS formation. Exact results are provided in the two limits of extremely narrow and very smooth impurity. Possible extensions to more general free-fermion models and interacting systems are discussed.

Recently, it was reported by MacLaughlin et al. in Phys. Rev. B 93, 214421 (2016) that $\alpha \text{−}{\mathrm{YbAl}}_{0.986}{\mathrm{Fe}}_{0.014}{\mathrm{B}}_4$ exhibits an anomalous temperature dependence in the relaxation rate $1/T_1$ of $\mu \mathrm{SR}$, and stressed that such temperature dependence cannot be understood by the scenario based on the quantum critical valence transition (QCVT) while this compound exhibits a series of the non-Fermi-liquid behaviors explained by the theory of the QCVT. In this paper, we point out that the anomalous temperature dependence in $1/T_1$ can be understood semiquantitatively by assuming that the attraction of a screening cloud of conduction electrons about the ${\mu }^{+}$ induces a local magnetic moment arising from a $4f$ hole on the Yb ion, giving rise to the Kondo effect between heavy quasiparticles.

We apply the self-consistent renormalized perturbation theory to the Hubbard model on the square lattice at finite temperatures to study the evolution of the Fermi surface (FS) as a function of temperature and doping. Previously, a nematic phase for the same model has been reported to appear at weak coupling near a Lifshitz transition from closed to open FS at zero temperature where the self-consistent renormalized perturbation theory was shown to be sensitive to small deformations of the FS. We find that the competition with the superconducting order leads to a maximal nematic order appearing at nonzero temperature. We explicitly observe the two competing phases near the onset of nematic instability, and by comparing the grand canonical potentials, we find that the transitions are first order. We explain the origin of the interaction-driven spontaneous symmetry breaking to a nematic phase in a system with several symmetry-related Van Hove points and discuss the required conditions.

We report on a comprehensive experimental and theoretical study of optical third harmonic generation (THG) on the exciton-polariton resonances in the zinc-blende semiconductors GaAs, CdTe, and ZnSe subject to an external magnetic field, representing a topic that had remained unexplored so far. In these crystals, crystallographic THG is allowed in the electric-dipole approximation, so that substantial magnetic-field-induced changes of the THG are unexpected: the symmetry reduction due to magnetic field, corresponding change of the selection rules, and the Zeeman effect are expected to play a minor role. Surprisingly, we observe a strong enhancement of the THG intensity by a factor of 50 for the $1s$ exciton-polariton in GaAs in magnetic fields up to 10 T. In contrast, the corresponding enhancement is moderate in CdTe and almost absent in ZnSe. In order to explain this strong variation, we develop a microscopic theory accounting for the optical harmonics generation on exciton-polaritons and analyze the THG mechanisms induced by the magnetic field. The calculations show that the increase of THG intensity is dominated by the magnetic field enhancement of the exciton oscillator strength, which is particularly strong for GaAs in the studied range of field strengths. The much weaker increase of THG intensity in CdTe and ZnSe is explained by the considerably larger exciton binding energies, leading to a weaker modification of their oscillator strengths by the magnetic field.

A mechanism for perfect reflection is proposed for the dielectric medium beyond the total internal reflection and band gaps. It arises from the coherence of multiple propagating modes, and can be determined by the topological vortex of a transmission coefficient with a nonzero winding number in parameter space. Based on the coherent perfect reflection, a generalized waveguide condition is derived analytically. In a photonic crystal slab, the modes that satisfy this condition are precisely bound states in the continuum. Our findings may have many potential applications in guided-wave optics.

Highly entangled “graph” states of photons have applications in universal quantum computing and in quantum communications. In the latter context, they have been proposed as the key ingredient in the establishment of long-distance entanglement across quantum repeater networks. Recently, a general deterministic approach to generate repeater graph states from quantum emitters was given. However, a detailed protocol for the generation of such states from realistic systems is still needed in order to guide experiments. Here, we provide such explicit protocols for the generation of repeater graph states from two types of quantum emitters: NV centers in diamond and self-assembled quantum dots. A crucial element of our designs is an efficient controlled-$Z$ gate between the emitter and a nuclear spin, used as an ancilla qubit. Additionally, a fast protocol for using pairs of exchange-coupled quantum dots to produce repeater graph states is described. Our focus is on near-term experiments feasible with existing experimental capabilities.