We propose a realization of perfect valley filters based on the chiral domain-wall channels between a quantum anomalous Hall insulator and a quantum valley Hall insulator. Uniquely, all these channels reside in the same valley and propagate unidirectionally, $100\%$ valley-polarizing passing-by carriers without backscattering. The valley index, the chirality, and the number of the channels are protected by topological charges, controllable by external fields, and detectable by circular dichroism.

We investigate the quantum phases of hard-core bosonic atoms in an extended Hubbard model where particles interact via soft-shoulder potentials in one dimension. Using a combination of field-theoretical methods and strong-coupling perturbation theory, we demonstrate that the low-energy phase can be a conformal cluster Luttinger liquid (CLL) phase with central charge $c=1$, where the microscopic degrees of freedom correspond to mesoscopic ensembles of particles. Using numerical density-matrix renormalization-group methods, we demonstrate that the CLL phase [first predicted in M. Mattioli et al., Phys. Rev. Lett. 111, 165302 (2013)] is separated from a conventional Tomonaga-Luttinger liquid by an exotic critical point with central charge $c=\frac{3}{2}$. The latter is expression of an emergent conformal supersymmetry, which is not present in the original Hamiltonian. We discuss the observability of the CLL phase in realistic experimental settings with weakly dressed Rydberg atoms confined to optical lattices. Using quantum Monte Carlo simulations, we show that the typical features of CLLs are stable up to comparatively high temperatures. Using exact diagonalizations and quantum trajectory methods, we provide a protocol for adiabatic state preparation as well as quantitative estimates on the effects of particle losses.

We propose to continuously monitor a charge qubit by utilizing a T-shaped double quantum dot detector, in which the qubit and double dot are arranged in such a unique way that the detector turns out to be particularly susceptible to the charge states of the qubit. Special attention is paid to the regime where acquisition of qubit information and backaction upon the measured system exhibit nontrivial correlation. The intrinsic dynamics of the qubit gives rise to dynamical blockade of tunneling events through the detector, resulting in a super-Poissonian noise. However, such a pronounced enhancement of the detector's shot noise does not necessarily produce a rising dephasing rate. In contrast, an inhibition of dephasing is entailed by the reduction of information acquisition in the dynamically blockaded regimes. We further reveal the important impact of the charge fluctuations on the measurement characteristics. Noticeably, under the condition of symmetric junction capacitances the noise pedestal of the circuit current is completely suppressed, leading to a divergent signal-to-noise ratio, and eventually to a violation of the Korotkov-Averin bound in quantum measurement. Our study offers the possibility for a double dot detector to reach the quantum limited effectiveness in a transparent manner.

Josephson weak links made of two-dimensional topological insulators (TIs) exhibit magnetic oscillations of the supercurrent that are reminiscent of those in superconducting quantum interference devices (SQUIDs). We propose a microscopic theory of this effect that goes beyond the approaches based on the standard SQUID theory. For long junctions we find a temperature-driven crossover from ${\mathrm{\Phi }}_0$-periodic SQUID-like oscillations to a $2{\mathrm{\Phi }}_0$-quasiperiodic interference pattern with different peaks at even and odd values of the magnetic flux quantum ${\mathrm{\Phi }}_0=ch/2e$. This behavior is absent in short junctions where the main interference signal occurs at zero magnetic field. Both types of interference patterns reveal gapless (protected) Andreev bound states. We show, however, that the usual sawtooth current-flux relationship is profoundly modified by a Doppler-like effect of the shielding current which has been overlooked previously. Our findings may explain recently observed even-odd interference patterns in InAs/GaSb-based TI Josephson junctions and uncover unexplored operation regimes of nano-SQUIDs.

Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide ${\mathrm{B}}_{13}{\mathrm{C}}_2$ has been a controversial topic in the field of icosahedral boron-rich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while the ordered ground state ${\mathrm{B}}_{13}{\mathrm{C}}_2$ is metallic, the configurationally disordered ${\mathrm{B}}_{13}{\mathrm{C}}_2$, modeled with a superatom-special quasirandom structure method, goes through a metal to nonmetal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chain-end carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a ${\mathrm{B}}_{12}$ icosahedron bonded to it, giving rise to a ${\mathrm{B}}_{11}{\mathrm{C}}^e$(BBC) unit. The atomic configuration of the substitutionally disordered ${\mathrm{B}}_{13}{\mathrm{C}}_2$ thus tends to be dominated by a mixture between ${\mathrm{B}}_{12}$(CBC) and ${\mathrm{B}}_{11}{\mathrm{C}}^e$(BBC). Due to splitting of valence states in ${\mathrm{B}}_{11}{\mathrm{C}}^e$(BBC), the electron deficiency in ${\mathrm{B}}_{12}$(CBC) is gradually compensated.

We theoretically show that the electric current applied in the plane of a Bloch domain wall introduces a mechanism of its stability or instability that is strongly connected to its topological charge. In the case of a wall in an infinite ferromagnet, applying a current in one direction leads to the instability, while a current in the opposite direction the wall is stable. This property is caused by the toroidal moment of the system and appears to be due to the magnetostatic energy of the domain wall. A more complicated case of a ferromagnetic slab is considered.

Magnetization dynamics in single-domain ferromagnets can be triggered by a charge current if the spin-orbit coupling is sufficiently strong. We apply functional Keldysh theory to investigate spin-orbit torques in metallic two-dimensional Rashba ferromagnets in the presence of spin-dependent disorders. A reactive, antidamping-like spin-orbit torque as well as a dissipative, field-like torque is calculated microscopically, to leading order in the spin-orbit interaction strength. By calculating the first vertex correction we show that the intrinsic antidamping-like torque vanishes unless the scattering rates are spin dependent.

Kitaev's compass model on the honeycomb lattice realizes a spin liquid whose emergent excitations are dispersive Majorana fermions and static ${\mathrm{Z}}_2$ gauge fluxes. We discuss the proper selection of physical states for finite-size simulations in the Majorana representation, based on a recent paper by F. L. Pedrocchi, S. Chesi, and D. Loss [Phys. Rev. B 84, 165414 (2011)]. Certain physical observables acquire large finite-size effects, in particular if the ground state is not fermion-free, which we prove to generally apply to the system in the gapless phase and with periodic boundary conditions. To illustrate our findings, we compute the static and dynamic spin susceptibilities for finite-size systems. Specifically, we consider random-bond disorder (which preserves the solubility of the model), calculate the distribution of local flux gaps, and extract the NMR line shape. We also predict a transition to a random-flux state with increasing disorder.

The representative measurements of the second harmonic in ac magnetization complemented by neutron depolarization have been performed for single-crystal ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{CoO}}_3$ in the temperature range $97\text{K}<T<230\text{K}$, where occurrence of a small fraction $\left(\lesssim {10}^{-3}\right)$ of nanoscale ferromagnetic clusters (FMC) has been found. Magnetic, geometrical, and dynamical parameters of the FMC system have been evaluated in the temperature range $T<140$ K, where superparamagnetic regime installs, by means of the formalism involving the Fokker-Planck equation. With lowering the temperature, the amount of clusters fraction, the cluster size, and magnetic moment along with its diffusion relaxation time strongly increase, each in its own temperature interval. Below 130 K, FMC contribute essentially to the total linear magnetic susceptibility. The damping factor of the order ${10}^{-1}$ proves the importance of precession in thermal relaxation of the cluster magnetic moment. The FMC are a precursor of long-range ferromagnetic correlations seen below 100 K with neutron-scattering techniques.

We calculate the spin-dependent zero-bias conductance $G_{\sigma {\sigma }^{\prime }}$ in armchair graphene nanoribbons with hydrogen adsorbates employing a DFT-based ab initio transport formalism including spin-orbit interaction. We find that the spin-flip conductance $G_{\sigma \overline{\sigma }}$ can reach the same order of magnitude as the spin-conserving one $G_{\sigma \sigma }$, due to exchange-mediated spin scattering. In contrast, the genuine spin-orbit interaction appears to play a secondary role, only.

Thermal fluctuations in hybrid superconductor/ferromagnetic $\mathrm{NbN}/\mathrm{NiCu}$ bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from $4.2\mathrm{to}8\mathrm{K}$. In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a $6\text{−}\text{nm}\text{−}\text{thick}$ NiCu film grown on top of $8\text{−}\text{nm}\text{−}\text{thick}$ NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in $\mathrm{NbN}/\mathrm{NiCu}$ bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.

Precise characterization of a system's Hamiltonian is crucial to its high-fidelity control that would enable many quantum technologies, ranging from quantum computation to communication and sensing. In particular, nonsecular parts of the Hamiltonian are usually more difficult to characterize, even if they can give rise to subtle but non-negligible effects. Here we present a strategy for the precise estimation of the transverse hyperfine coupling between an electronic and a nuclear spin, exploiting effects due to nominally forbidden transitions during the Rabi nutation of the nuclear spin. We applied the method to precisely determine the transverse coupling between a nitrogen-vacancy center electronic spin and its nitrogen nuclear spin. In addition, we show how this transverse hyperfine coupling, which has been often neglected in experiments, is crucial to achieving large enhancements of the nuclear Rabi nutation rate.

A fundamental challenge in thermoelectric (TE) material research is meeting the simultaneous requirements of high carrier mobility and low thermal conductivity. Simple crystal structures are ideal for maintaining high carrier mobility, but they usually have high thermal conductivity. Here we show by first-principles lattice dynamics and Boltzmann transport calculations that weakly ionic rocksalt structures exhibit strong lattice anharmonicity and low acoustic-phonon group velocity, which combine to produce intrinsic low thermal conductivity. We unveil microscopic mechanisms that explain experimental observations and provide insights for TE material design and discovery.

We predict the occurrence of a novel type of atomic-scale spin lattice in an Fe monolayer on the Ir(001) surface. Based on density functional theory calculations we parametrize a spin Hamiltonian and solve it numerically using Monte Carlo simulations. We find the stabilization of a three-dimensional spin structure arranged on a $(3\times 3)$ lattice. Despite an almost vanishing total spin magnetization we predict the emergence of orbital magnetization and large anomalous Hall effect, to which there is a significant topological contribution purely due to the real space spin texture at the surface.

We show that charge transfer through a small superconducting (S) island of a single-electron transistor is strongly affected by vorticity. This interplay of charge and rotational degrees of freedom in a mesoscopic superconductor occurs through the effect of vorticity on the quantum mechanical spectrum of electron-hole excitations. The subgap quasiparticle levels in vortices can host an extra electron, thus suppressing the so-called parity effect in the S island. We propose to measure the collective dynamics of vorticity and electric charge via the charge pumping effect caused by alternating vortex entry and exit controlled by a periodic magnetic field.

Motivated by recent experiments on heavy fermion materials ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ and ${\mathrm{UBe}}_{13}$, we develop a framework to capture generic properties of multiband superconductors with strong Pauli paramagnetic effect (PPE). In contrast to the single band case, the upper critical field $H_{\mathrm{c}2}$ can remain a second-order transition even for strong PPE cases. The expected first-order transition is hidden inside $H_{\mathrm{c}2}$ and becomes a crossover due to the interplay of multibandness. The present theory based on full self-consistent solutions of the microscopic Eilenberger theory explains several mysterious anomalies associated with the crossover and the “empty” vortex core state which is observed by recent STM experiment on ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$.

We use an ionic liquid-assisted electric-field effect to tune the carrier density in an electron-doped cuprate ultrathin film and cause a two-dimensional superconductor-insulator transition (SIT). The low upper critical field in this system allows us to perform magnetic-field (B)-induced SIT in the liquid-gated superconducting film. Finite-size scaling analysis indicates that SITs induced both by electric and by magnetic fields are quantum phase transitions and the transitions are governed by percolation effects—quantum mechanical in the former and classical in the latter cases. Compared to the hole-doped cuprates, the SITs in the electron-doped system occur at critical sheet resistances $\left(R_c\right)$ much lower than the pair quantum resistance $R_Q=h/{\left(2e\right)}^2=6.45\mathrm{k}\mathrm{\Omega }$, suggesting the possible existence of fermionic excitations at finite temperatures at the insulating phase near the SITs.

X-ray absorption near-edge spectroscopy, photoluminescence, cathodoluminescence, and Raman spectroscopy have been used to investigate the chemical states of nitrogen dopants in ZnO nanowires. It is found that nitrogen exists in multiple states: ${\mathrm{N}}_{\mathrm{O}},{\mathrm{N}}_{\mathrm{Zn}}$, and loosely bound ${\mathrm{N}}_2$ molecule. The results establish a direct link between a donor-acceptor pair emission at 3.232 eV and the concentration of loosely bound ${\mathrm{N}}_2$. This work confirms that ${\mathrm{N}}_2$ at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO as theoretically predicted by Lambrecht and Boonchun [Phys. Rev. B 87, 195207 (2013)]. Additionally, shallow acceptor states arising from ${\mathrm{N}}_{\mathrm{O}}$ complexes have been ruled out in this paper.

We report significant details of the magnetic structure and spin dynamics of ${\mathrm{LiFePO}}_4$ obtained by single-crystal neutron scattering. Our results confirm a previously reported collinear rotation of the spins away from the principal $b$ axis, and they determine that the rotation is toward the $a$ axis. In addition, we find a significant spin-canting component along $c$. The possible causes of these components are discussed, and their significance for the magnetoelectric effect is analyzed. Inelastic neutron scattering along the three principal directions reveals a highly anisotropic hard plane consistent with earlier susceptibility measurements. Using a spin Hamiltonian, we show that the spin dimensionality is intermediate between $XY$- and Ising-like, with an easy $b$ axis and a hard $c$ axis. It is shown that both next-nearest neighbor exchange couplings in the $bc$ plane are in competition with the strongest nearest neighbor coupling.