New frustrated antiferromagnetic compounds $\mathrm{Cu}{R}_2{\mathrm{Ge}}_2{\mathrm{O}}_8$ ($R=\mathrm{Pr}$, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic, and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a $S=1/2$ two-dimensional triangular antiferromagnetic lattice oriented in the $ac$ plane with geometrical frustration. On the other hand, the Pr, Nd, and Sm compounds show a three-dimensional honeycomb-tunnel-like lattice made of $R^{3+}$ running along the $a$ axis with the characteristic behavior of frustrated antiferromagnets.

The intrinsically hole-doped ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ exhibits bulk superconductivity at $T_{\mathrm{sc}}=36.5$ K and ferromagnetic ordering in the Eu sublattice at $T_{\mathrm{m}}=15$ K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ with $T_{\mathrm{sc}}>T_{\mathrm{m}}$. With the Ni doping, $T_{\mathrm{sc}}$ decreases rapidly, and the Eu-spin ferromagnetism and its $T_{\mathrm{m}}$ remain unchanged. Consequently, the system $\mathrm{RbEu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ transforms into a superconducting ferromagnet with $T_{\mathrm{m}}>T_{\mathrm{sc}}$ for $0.07\le x\le 0.08$. The occurrence of superconducting ferromagnets is attributed to the decoupling between ${\mathrm{Eu}}^{2+}$ spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.

The spin-induced electric polarization in multiferroics with long-period modulated magnetic structure and some polar crystals is difficult to calculate directly due to the modeling limitation or the lack of a reference paraelectric phase. We propose a general method to investigate spin-induced polarization by combining symmetry arguments and first-principles calculations, and demonstrate it in the triangular-lattice delafossite structure. Two special spin models based on the four-sublattice orthorhombic supercell were used to calculate the polarization induced by spin-current and $p\text{−}d$ hybridization mechanisms, respectively. All the polarization components appearing in various spiral spin orders can be explained by the two different spin-current mechanisms through analyzing the symmetry of ${\mathbf{S}}_a\times {\mathbf{S}}_b$ vectors, where ${\mathbf{S}}_a$ and ${\mathbf{S}}_b$ represents the neighbor spins. We confirm that the $p\text{−}d$ hybridization mechanism only plays a minor role in the origins of polarization in addition to causing the $2q$ lattice modulation, where $q$ is the magnetic modulation wave number.

We present a systematic investigation of the structural, magnetic, electronic, mechanical, and thermodynamic properties of ${\mathrm{CmO}}_2$ with the local density approximation (LDA)+$U$ and the generalized gradient approximation (GGA)+$U$ approaches. The strong Coulomb repulsion and the spin-orbit coupling (SOC) effects on the lattice structures, electronic density of states, and band gaps are carefully studied, and compared with other $A{\mathrm{O}}_2$ ($A=\mathrm{U}$, Np, Pu, and Am). The ferromagnetic configuration with half-metallic character is predicted to be energetically stable while a charge-transfer semiconductor is predicted for the antiferromagnetic configuration. The elastic constants and phonon spectra show that the fluorite structure is mechanically and dynamically stable. Based on the first-principles phonon density of states, the lattice vibrational energy is calculated using the quasiharmonic approximation. Then, the Gibbs free energy, thermal expansion coefficient, specific heat, and entropy are obtained and compared with experimental data. The mode Grüneisen parameters are presented to analyze the anharmonic properties. The Slack relation is applied to obtain the lattice thermal conductivity in temperature range of 300–1600 K. The phonon group velocities are also calculated to investigate the heat transfer. For all these properties, if available, we compare the results of ${\mathrm{CmO}}_2$ with other $A{\mathrm{O}}_2$.

A calculational method based on Fourier expansion is developed and applied to the study of the strong-field galvanomagnetic thermoelectric properties of a free-electron metal, inside of which is embedded a simple cubic array of identical spheres or cylinders, which have different thermoelectric and conductivity tensors. When the magnetic field is strong enough, the effective galvanomagnetic thermoelectric properties of such composites exhibit very strong variations with the direction of the applied magnetic field with respect to the symmetry axes of the composite microstructure. This is qualitatively similar to the predicted magnetoinduced angular magnetoresistance anisotropy [D. J. Bergman and Y. M. Strelniker, Phys. Rev. B 49, 16256 (1994)] which was verified experimentally [M. Tornow et al., Phys. Rev. Lett. 77, 147 (1996)]. This is a purely classical effect, even though it is qualitatively similar to what is observed in some metallic crystals which have a noncompact Fermi surface. The current results can be useful for studying the possibility of increasing the thermoelectric figure of merit in periodic composites by application of a strong magnetic field. As follows from our very preliminary results, the figure of merit can be increased by application of a strong magnetic field to the composite.

The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spin-photon coupling, an essential ingredient for the generation of long-range entanglement. Furthermore, we employ input-output theory to identify observable signatures of spin-photon coupling in the cavity output field, which may provide guidance to the experimental search for strong coupling in such spin-photon systems and opens the way to cavity-based readout of the spin qubit.

The recent upswing in attention for the thermoelectric properties of organic semiconductors (OSCs) adds urgency to the need for a quantitative description of the range and energetics of hopping transport in organic semiconductors under relevant circumstances, i.e., around room temperature (RT). In particular, the degree to which hops beyond the nearest neighbor must be accounted for at RT is still largely unknown. Here, measurements of charge and energy transport in doped OSCs are combined with analytical modeling to reach the univocal conclusion that variable-range hopping is the proper description in a large class of disordered OSC at RT. To obtain quantitative agreement with experiment, one needs to account for the modification of the density of states by ionized dopants. These Coulomb interactions give rise to a deep tail of trap states that is independent of the material's initial energetic disorder. Insertion of this effect into a classical Mott-type variable-range hopping model allows one to give a quantitative description of temperature-dependent conductivity and thermopower measurements on a wide range of disordered OSCs. In particular, the model explains the commonly observed quasiuniversal power-law relation between the Seebeck coefficient and the conductivity.

We consider an interface separating the Moore-Read state and Halperin 331 state in a half-filled Landau level, which can be realized in a double quantum well system with varying interwell tunneling and/or interaction strengths. In the presence of electron tunneling and strong Coulomb interactions across the interface, we find that all charge modes localize and the only propagating mode left is a chiral Majorana fermion mode. Methods to probe this neutral mode are proposed. A quantum phase transition between the Moore-Read and Halperin 331 states is described by a network of such Majorana fermion modes. In addition to a direct transition, they may also be separated by a phase in which the Majorana fermions are delocalized, realizing an incompressible state which exhibits quantum Hall charge transport and bulk heat conduction.

We present nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements performed on single crystalline ${\mathrm{Ce}}_2{\mathrm{CoAl}}_7{\mathrm{Ge}}_4$, a member of a recently discovered family of heavy fermion materials ${\mathrm{Ce}}_{2}M{\mathrm{Al}}_7{\mathrm{Ge}}_4$ ($M=\mathrm{Co}$, Ir, Ni, or Pd). Previous measurements indicated a strong Kondo interaction as well as magnetic order below $T_M=1.8\mathrm{K}$. Our NMR spectral measurements show that the Knight shift $K$ is proportional to the bulk magnetic susceptibility $\chi$ at high temperatures. A clear Knight shift anomaly ($K\neg \propto \chi$) is observed at coherence temperatures $T^{*}\sim 17.5\mathrm{K}$ for $H_0\parallel \widehat{c}$ and 10 K for $H_0\parallel \widehat{a}$ at the ${}^{59}\mathrm{Co}$ site, and $T^{*}\sim 12.5\mathrm{K}$ at the ${}^{27}\mathrm{Al}$(3) site for $H_0\parallel \widehat{a}$ characteristic of the heavy fermion nature of this compound. At high temperatures, the ${}^{59}\mathrm{Co}$ NMR spin-lattice relaxation rate $T_1^{-1}$ is dominated by spin fluctuations of the $4f$ local moments with a weak metallic background. The spin fluctuations probed by ${}^{59}\mathrm{Co}$ NMR are anisotropic and larger in the basal plane than in the $c$ direction. Furthermore, we find ${\left(T_{1}TK\right)}^{-1}\propto T^{-1/2}$ at the ${}^{59}\mathrm{Co}$ site as expected for a Kondo system for $T>T^{*}$ and $T>T_K$. ${}^{59}\mathrm{Co}$ NQR $T_1^{-1}$ measurements at low temperatures indicate slowing down of spin fluctuations above the magnetic ordering temperature $T_M\sim 1.8\mathrm{K}$. A weak ferromagnetic character of fluctuations around $\mathbf{q}=0$ is evidenced by an increase of $\chi T$ versus $T$ above the magnetic ordering temperature. We also find good agreement between the observed and calculated electric field gradients at all observed sites.

We implemented various DFT+$U$ schemes, including the Agapito, Curtarolo, and Buongiorno Nardelli functional (ACBN0) self-consistent density-functional version of the $\mathrm{DFT}+U$ method [Phys. Rev. X 5, 011006 (2015)] within the massively parallel real-space time-dependent density functional theory (TDDFT) code octopus. We further extended the method to the case of the calculation of response functions with real-time TDDFT+$U$ and to the description of noncollinear spin systems. The implementation is tested by investigating the ground-state and optical properties of various transition-metal oxides, bulk topological insulators, and molecules. Our results are found to be in good agreement with previously published results for both the electronic band structure and structural properties. The self-consistent calculated values of $U$ and $J$ are also in good agreement with the values commonly used in the literature. We found that the time-dependent extension of the self-consistent DFT+$U$ method yields improved optical properties when compared to the empirical TDDFT+$U$ scheme. This work thus opens a different theoretical framework to address the nonequilibrium properties of correlated systems.

The extraordinary properties and the novel applications of black phosphorene induce the research interest in the monolayer group-IV monochalcogenides. Here using first-principles calculations, we systematically investigate the electronic, transport, and optical properties of monolayer $\alpha$- and $\beta$-GeSe, revealing a direct band gap of 1.61 eV for monolayer $\alpha$-GeSe and an indirect band gap of 2.47 eV for monolayer $\beta$-GeSe. For monolayer $\beta$-GeSe, the electronic/hole transport is anisotropic, with an extremely high electron mobility of $2.93\times {10}^4{\text{cm}}^2/\text{V}\text{s}$ along the armchair direction, comparable to that of black phosphorene. Furthermore, for $\beta$-GeSe, robust band gaps nearly independent of the applied tensile strain along the armchair direction are observed. Both monolayer $\alpha$- and $\beta$-GeSe exhibit anisotropic optical absorption in the visible spectrum.

We report a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) and their outcomes based on first-principles calculations and classical molecular-dynamics simulations. We find a strong attractive interaction between nanopores and GNR edges that drives the pores to migrate toward and coalesce with the GNR edges, which can be exploited to form GNR edge patterns that impact the GNR electronic band structure and tune the GNR band gap. Our analysis introduces a viable physical processing strategy for modifying GNR properties by combining defect engineering and thermal annealing.

This in situ neutron-diffraction study on antiferroelectric (AFE) $\mathrm{P}{\mathrm{b}}_{0.99}\left(\mathrm{N}{\mathrm{b}}_{0.02}\mathrm{Z}{\mathrm{r}}_{0.65}\mathrm{S}{\mathrm{n}}_{0.28}\mathrm{T}{\mathrm{i}}_{0.05}\right){\mathrm{O}}_3$ polycrystalline materials describes systematic structural and associated preferred orientation changes as a function of applied electric field and temperature. It is found that the pristine AFE phase can be poled into the metastable ferroelectric (FE) phase at room temperature. At this stage, both AFE and FE phases consist of modes associated with octahedral rotation and $A$-site ionic displacements. The temperature-induced phase transition indicates that the octahedral rotation and ionic displacements are weakly coupled in the room-temperature FE phase and decoupled in the high-temperature FE phase. However, both temperature and $E$-field-induced phase transitions between the AFE and high-temperature FE phase demonstrate the critical role of coupling between octahedral rotation and $A$-site ionic displacements in stabilizing the AFE structure, which provides not only experimental evidence to support previous theoretical calculations, but also an insight into the design and development of AFE materials. Moreover, the associated preferred orientation evolution in both AFE and FE phases is studied during the phase transitions. It is found that the formation of the preferred orientation can be controlled to tune the samples’ FE and AFE properties.

We report a bimetallic holmium(III) complex showing a S-shaped magnetic hysteresis at low temperature. The complex is investigated by x-ray crystallography, magnetometry, single crystal microsquid measurements, and first-principles calculations. A model Hamiltonian including electronic and nuclear magnetic moments is used to fit all experimental data. We conclude that the Ho(III) may be described as non-Kramers doublets with respective gaps of ${\mathrm{\Delta }}_A=0.8$ and ${\mathrm{\Delta }}_B=10 {\mathrm{cm}}^{-1}$ and that there is a small ferromagnetic coupling of $J=1 {\mathrm{cm}}^{-1}$ (${\widehat{\mathcal{H}}}_S=-J{\mathit{S}}_A·{\mathit{S}}_B$). As in previous works, the hysteresis arise from the hyperfine structure of the Ho(III) ions. The S-shaped form of the hysteresis reflects the avoided crossing of the electronic states in the non-Kramers doublets.

The magnetic properties of the two-dimensional, $S=1$ honeycomb antiferromagnet ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ have been comprehensively studied using dc susceptibility measurements and inelastic neutron scattering techniques. The magnetic excitation spectrum is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane where it consists of two gapped spin-wave modes. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The first- and second-neighbor magnetic exchange interactions are antiferromagnetic and lie within the ranges $10.90\mathrm{meV}\le J_n\le 13.35$ meV and $0.85\mathrm{meV}\le J_{nn}\le 1.65$ meV, respectively. The interplane coupling $J_{\mathrm{out}}$ is four orders of magnitude weaker than the intraplane interactions, confirming the highly two-dimensional magnetic behavior of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenon.

Coupling a mesoscopic conductor to a microwave cavity can lead to fascinating feedback effects which generate strong correlations between the dynamics of photons and charges. We explore the connection between cavity dynamics and charge transport in a model system consisting of a voltage-biased Josephson junction embedded in a high-$Q$ cavity, focusing on the behavior as the system is tuned through a dynamical critical point. On one side of the critical point the noise is strongly suppressed, signaling the existence of a regime of highly coherent transport, but on the other side it switches abruptly to a much larger value. Using a semiclassical approach we show that this behavior arises because of the strongly nonlinear cavity drive generated by the Cooper pairs. We also uncover an equivalence between charge and photonic current noise in the system which opens up a route to detecting the critical behavior through straightforward microwave measurements.

Recently, hydrogen sulfide has attracted enormous attention due to the record high-temperature superconductivity of ${\text{H}}_3\text{S}$ with $T_c=203$ K at a pressure of 155 GPa. In addition to ${\text{H}}_3\text{S}$, there are many theoretically and experimentally confirmed compositions and crystal structures forming under high pressure. Here, using the ab initio variable-composition evolutionary algorithm USPEX, we performed a comprehensive search for new phases in the $\text{H}-\text{S}$ system and built a refined composition-pressure phase diagram. For ${\text{H}}_3{\text{S}}_2$, a new stable semiconducting $P{2}_1$ phase was found. We predicted a new sulfur-rich stable ${\text{HS}}_2$ phase and metastable compound ${\text{H}}_5{\text{S}}_8$ and calculated their $T_c$ using superconducting density functional theory (11 K at 100 GPa and 9.2 K at 80 GPa, respectively). Our results show that among stable phases only ${\text{H}}_3\text{S}$ is a high-$T_c$ superconductor, and highlight that compositional descriptors alone are insufficient for indicating superconductivity, and crystal structure plays an essential role.

In this Rapid Communication we utilize nuclear magnetic resonance to investigate the hyperpolarization effect of negatively charged nitrogen vacancy (NV) centers on bulk ${}^{13}\mathrm{C}$ nuclei in a diamond single crystal. We were able to identify several polarization peaks of a different sign at different magnetic fields in a region of some tens of Gauss centered around $50\mathrm{mT}$. The bulk ${}^{13}\mathrm{C}$ hyperpolarization in the investigated field range is usually attributed to the excited state level anticrossing of the NV center. However, we found that this bulk hyperpolarization is caused by optically induced cross relaxation and that it takes place in the NV center ground state. The four-spin coupling between the polarized NV electron spin, the electron spin of a substitutional nitrogen impurity (P1), as well as its ${}^{14}\mathrm{N}$ nuclei and the ${}^{13}\mathrm{C}$ nuclear spin have to be considered. We introduce a simple theoretical model which completely fits with the experimental data and which clearly shows that the P1 centers are involved in the polarization process. We expect that the current work has a significant impact on future NV-based polarization applications.