We found a large and unique magnetoresistance (MR) effect for ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$. When the magnetic field was applied along the crystallographic $c$ axis at low temperatures, the resistivity increased with the magnetic field and the MR ratio reached several hundred percent, but then it decreased with further increasing the applied field. In addition, the angle dependence measurement revealed a strong anisotropy. This compound is an antiferromagnetic semiconductor with a narrow band gap, and Mn atoms form a corrugated honeycomb lattice. Therefore, a frustration among the magnetic moments is expected, and we propose that our observations can be understood by a nonmonotonic modulation of magnetic fluctuation under the magnetic field.

Water inside a nanocapillary exhibits unconventional structural and dynamical behavior due to its ordered structure. The confining walls, density, and lateral pressures control profoundly the microscopic structure of trapped water. Here we study the electrostriction of confined water subjected to pressures of the order of GPa for two different setups: (i) a graphene nanochannel containing a constant number of water molecules independent of the height of the channel, (ii) an open nanochannel where water molecules can be exchanged with those in a reservoir. For the former case, a square-rhombic structure of confined water is formed when the height of the channel is $d=6.5$ Å having a density of $\rho =1.42{\mathrm{g}\mathrm{cm}}^{-3}$. By increasing the height of the channel, a transition from a flat to a buckled state occurs, whereas the density rapidly decreases and reaches the bulk density for $d\cong 8.5$ Å. When a perpendicular electric field is applied, the water structure and the lateral pressure change. For strong electric fields $(\sim 1$ V/Å), the square-rhombic structure is destroyed. For an open setup, a solid phase of confined water consisting of an imperfect square-rhombic structure is formed. By applying a perpendicular field, the density and phase of confined water change. However, the density and pressure inside the channel decrease as compared to the first setup. Our study is closely related to recent experiments on confined water, and it reveals the sensitivity of the microscopic structure of confined water to the size of the channel, the external electric field, and the experimental setup.

Low-dimensional magnetic materials with spin-$\frac{1}{2}$ moments can host a range of exotic magnetic phenomena due to the intrinsic importance of quantum fluctuations to their behavior. Here, we report the structure, magnetic structure, and magnetic properties of copper ii thiocyanate, ${\mathrm{Cu}\left(\mathrm{NCS}\right)}_2$, a one-dimensional coordination polymer which displays low-dimensional quantum magnetism. Magnetic susceptibility, electron paramagnetic resonance spectroscopy, ${}^{13}\mathrm{C}$ magic-angle spinning nuclear magnetic resonance spectroscopy, and density functional theory investigations indicate that ${\mathrm{Cu}\left(\mathrm{NCS}\right)}_2$ behaves as a two-dimensional array of weakly coupled antiferromagnetic spin chains $[J_2=133\left(1\right)\mathrm{K}$, $\alpha =J_1/J_2=0.08]$. Powder neutron-diffraction measurements confirm that ${\mathrm{Cu}\left(\mathrm{NCS}\right)}_2$ orders as a commensurate antiferromagnet below $T_{\mathrm{N}}=12\mathrm{K}$, with a strongly reduced ordered moment ($0.3{\mu }_{\mathrm{B}}$) due to quantum fluctuations.

In a Dirac nodal line semimetal, the bulk conduction and valence bands touch at extended lines in the Brillouin zone. To date, most of the theoretically predicted and experimentally discovered nodal lines derive from the bulk bands of two- and three-dimensional materials. Here, based on combined angle-resolved photoemission spectroscopy measurements and first-principles calculations, we report the discovery of node-line-like surface states on the (001) surface of LaBi. Our first-principles calculations show that these surface states become topologically trivial because of band hybridization. Our calculations also reveal that these “nodal lines” have a tiny gap, which is beyond typical experimental resolution. These results may provide important information to understand the extraordinary physical properties of LaBi, such as the extremely large magnetoresistance and resistivity plateau.

We provide a thermodynamically consistent description of energy, charge, and spin transfers in a thermoelectric quantum-dot spin valve in the collinear configuration based on nonequilibrium Green's function and full counting statistics. We use the fluctuation theorem symmetry and the concept of entropy production to characterize the efficiency with which thermal gradients can transduce charges or spins against their chemical potentials, arbitrary far from equilibrium. Close to equilibrium, we recover the Onsager reciprocal relations and the connection to linear response notions of performance such as the figure of merit. We also identify regimes where work extraction is more efficient far then close from equilibrium.

We investigated Weyl semimetal (WSM) phases in bismuth antimony (${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$) alloys by combination of atomic composition and arrangement. Via first-principles calculations, we found two WSM states with Sb concentrations of $x=0.5$ and 0.83 with specific inversion-symmetry-broken elemental arrangement. The Weyl points are close to the Fermi level in both of these two WSM states. Therefore, it is likely to obtain Weyl points in Bi-Sb alloy. The WSM phase provides a reasonable explanation for the current transport study of Bi-Sb alloy with the violation of Ohm's law [D. Shin, Y. Lee, M. Sasaki, Y. H. Jeong, F. Weickert, J. B. Betts, H.-J. Kim, K.-S. Kim, and J. Kim, Nat. Mater. 16, 1096 (2017)]. This paper shows that the topological phases in Bi-Sb alloys depend on both elemental composition and their specific arrangement.

Topological nodal-line semimetals are characterized by one-dimensional Dirac nodal rings that are protected by the combined symmetry of inversion $\mathcal{P}$ and time-reversal $\mathcal{T}$. The stability of these Dirac rings is guaranteed by a quantized $\pm \pi$ Berry phase and their low-energy physics is described by a one-parameter family of (2+1)-dimensional quantum field theories exhibiting the parity anomaly. Here we study the Berry-phase supported topological transport of $\mathcal{P}\mathcal{T}$-invariant nodal-line semimetals. We find that small inversion breaking allows for an electric-field-induced anomalous transverse current, whose universal component originates from the parity anomaly. Due to this Hall-like current, carriers at opposite sides of the Dirac nodal ring flow to opposite surfaces when an electric field is applied. To detect the topological currents, we propose a dumbbell device, which uses surface states to filter charges based on their momenta. Suggestions for experiments and device applications are discussed.

${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ has been reported to exhibit a paramagnetic to ferromagnetic transition with a transition temperature of $\sim 260$ K. However, most of its magnetic properties, such as the saturation magnetization and even the transition temperature, appear to vary considerably among different reports. This is possibly because the crystallographic structure as well as the extent of the anti-site disorder (ASD) at the Ni/Mn sites are strongly influenced by the choice of synthesis routes. There are diverse reports connecting the extent of ASD to the valencies of Ni and Mn ions, such as ${\mathrm{Ni}}^{2+}\text{−}{\mathrm{Mn}}^{4+}$ and ${\mathrm{Ni}}^{3+}\text{−}{\mathrm{Mn}}^{3+}$, including suggestions of thermally induced valence transitions. Consequently, these reports arrive at very different conclusions on the mechanism behind the magnetic properties of ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$. To address the correlation between ASD and valency, we have carried out a comparative study of two monoclinic ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ polycrystals with different degrees of ASD. Using a combination of x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetometry, we conclude that the valency of the transition metal ions, and the transition temperature, are insensitive to the extent of ASD. However, we find the magnetic moment decreases strongly with an increasing ASD. We attribute this effect to the introduction of antiferromagnetic interactions in the anti-site disordered regions.

The coupling between longitudinal optical (LO) phonons and plasmons plays a fundamental role in determining the performance of doped semiconductor devices. In this work, we report a comparative investigation into the dependence of the coupling on temperature and doping in $n$- and $p$-type GaAs by using ultrafast coherent phonon spectroscopy. A suppression of coherent oscillations has been observed in $p$-type GaAs at lower temperature, strikingly different from $n$-type GaAs and other materials in which coherent oscillations are strongly enhanced by cooling. We attribute this unexpected observation to a cooling-induced elongation of the depth of the depletion layer which effectively increases the screening time of the surface field due to a slow diffusion of photoexcited carriers in $p$-type GaAs. Such an increase breaks the requirement for the generation of coherent LO phonons and, in turn, LO phonon-plasmon coupled modes because of their delayed formation in time.

We study the low-temperature properties of the differential response of the current to a temperature gradient at finite voltage in a single-level quantum dot including electron-electron interaction, nonsymmetric couplings to the leads, and nonlinear effects. The calculated response is significantly enhanced in setups with large asymmetries between the tunnel couplings. In the investigated range of voltages and temperatures with corresponding energies up to several times the Kondo energy scale, the maximum response is enhanced nearly an order of magnitude with respect to symmetric coupling to the leads.

We report a study of magnetic susceptibility and electrical resistivity as a function of temperature and magnetic field in superconducting crystals of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ with $x=0.10$ and 0.15 and transition temperature $T_c^{\mathrm{m}}=54$ K (determined from the susceptibility). When an external magnetic field is applied perpendicular to the ${\mathrm{CuO}}_2$ bilayers, the resistive superconducting transition measured with currents flowing perpendicular to the bilayers is substantially lower than that found with currents flowing parallel to the bilayers. Intriguingly, this anisotropic behavior is quite similar to that observed for the magnetic irreversibility points with the field applied either perpendicular or parallel to the bilayers. We discuss the results in the context of other studies that have found evidence for the decoupling of superconducting layers induced by a perpendicular magnetic field.

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the $A_{1g}^2$ mode, which is attributed to quantum interference between continuumlike coherent Dirac plasmons and phonons. By analyzing the time-dependent asymmetric line shape using the two-temperature model (TTM), it was determined that a Fano-like resonance persisted up to $\approx 1$ ps after photo excitation with a relaxation profile dominated by Gaussian decay at $\le 200$ fs. The asymmetry parameter could be well described by the TTM for $\ge 200$ fs, therefore suggesting the coherence time of the Dirac plasmon is $\approx 200$ fs.

Using the classical Monte Carlo calculations, we investigate the effects of longitudinal spin fluctuations on the helimagnetic transition in a Heisenberg magnet with the Dzyaloshinskii-Moriya interaction. We use variable spin amplitudes in the framework of the spin-lattice Hamiltonian. It is this kind of fluctuations that naturally occur in an itinerant system. We show that the basic features of the helical phase transition are not changed much by the longitudinal spin fluctuations though the transition temperature $T_c$ and the fluctuation hump seen in specific heat at $T>T_c$ is significantly affected. We report thermodynamic and structural effects of these fluctuations. By increasing the system size in the Monte Carlo modeling, we are able to reproduce the ring shape scattering intensity above the helimagnetic transition temperature $T_c$, which transforms into the spiral spots seen below $T_c$ in the neutron scattering experiments.

${\mathrm{Rb}}_3{\mathrm{Ni}}_2{\left({\mathrm{NO}}_3\right)}_7$ was obtained by crystallization from anhydrous nitric acid solution of rubidium nitrate and nickel nitrate hexahydrate. The crystal structure determined on single crystals implies isolated spin-1 two-leg ladders of ${\mathrm{Ni}}^{2+}$ ions connected by (${\mathrm{NO}}_3{)}^{-}$ groups as basic elements. Magnetic susceptibility, specific heat in magnetic fields up to 9 T, magnetization, and high-frequency electron spin resonance studies performed on powder samples show the absence of long-range magnetic order at $T\ge 2$ K. Electronic structure calculations and the detailed analysis of the experimental data enable quantitative estimates of the relevant parameters of the $S=1$ ladders in ${\mathrm{Rb}}_3{\mathrm{Ni}}_2{\left({\mathrm{NO}}_3\right)}_7$. The rung coupling $J_1=10.16$ K, the leg coupling $J_2=1.5$ K, and the Ising-type anisotropy $\left|A\right|=8.6$ K are obtained. The scenario of a valence-bond solidlike quantum ground state realized in the two-leg ${\mathrm{Ni}}^{2+}$ ladders is further corroborated by model simulations of the magnetic susceptibility.

We study quasiparticle states on a surface of a topological insulator (TI) with proximity-induced superconductivity under an external magnetic field. An applied magnetic field creates two Majorana bound states: a vortex Majorana state localized inside a vortex core and an exterior Majorana state localized along a circle centered at the vortex core. We calculate the spin-resolved local density of states (LDOS) and demonstrate that the shrinking of the radius of the exterior Majorana state, predicted in R. S. Akzyanov et al., Phys. Rev. B 94, 125428 (2016), under a strong magnetic field can be seen in LDOS without smeared out by nonzero-energy states. The spin-resolved LDOS further reveals that the spin of the exterior Majorana state is strongly spin-polarized. Accordingly, the induced odd-frequency spin-triplet pairs are found to be spin-polarized as well. In order to detect the exterior Majorana states, however, the Fermi energy should be closed to the Dirac point to avoid contributions from continuum levels. We also study a different two-dimensional topological-superconducting system where a two-dimensional electron gas with the spin-orbit coupling is sandwiched between an $s$-wave superconductor and a ferromagnetic insulator. We show that the radius of an exterior Majorana state can be tuned by an applied magnetic field. However, on the contrary to the results at a TI surface, neither the exterior Majorana state nor the induced odd-frequency spin-triplet pairs are spin-polarized. We conclude that the spin polarization of the Majorana state is attributed to the spin-polarized Landau level, which is characteristic for systems with the Dirac-like dispersion.

Type-II Weyl semimetals have recently attracted intensive research interest because they host Lorentz-violating Weyl fermions as quasiparticles. The discovery of type-II Weyl semimetals evokes the study of type-II line-node semimetals (LNSMs) whose linear dispersion is strongly tilted near the nodal ring. We present here a study on the circularly polarized light-induced Floquet states in type-II LNSMs, as well as those in hybrid LNSMs that have a partially overtilted linear dispersion in the vicinity of the nodal ring. We illustrate that two distinct types of Floquet Weyl semimetal (WSM) states can be induced in periodically driven type-II and hybrid LNSMs, and the type of Floquet WSMs can be tuned by the direction and intensity of the incident light. We construct phase diagrams of light-irradiated type-II and hybrid LNSMs which are quite distinct from those of light-irradiated type-I LNSMs. Moreover, we show that photoinduced Floquet type-I and type-II WSMs can be characterized by the emergence of different anomalous Hall conductivities.

Spontaneous pattern formation by energetic ion beams is usually explained in terms of surface-curvature dependent sputtering and atom redistribution in the target. Recently, the effect of ion implantation on surface stability has been studied for nonvolatile ion species, but for the case of noble gas ion beams it has always been assumed that the implanted atoms can be neglected. In this work, we show by molecular dynamics (MD) and Monte Carlo (MC) simulations that this assumption is not valid in a wide range of implant conditions. Sequential-impact MD simulations are performed for 1-keV Ar, 2-keV Kr, and 2-keV Xe bombardments of Si, starting with a pure single-crystalline Si target and running impacts until sputtering equilibrium has been reached. The simulations demonstrate the importance of the implanted ions for crater-function estimates. The atomic volumes of Ar, Kr, and Xe in Si are found to be a factor of two larger than in the solid state. To extend the study to a wider range of energies, MC simulations are performed. We find that the role of the implanted ions increases with the ion energy although the increase is attenuated for the heavier ions. The analysis uses the crater function formalism specialized to the case of sputtering equilibrium.

Here we develop the nonequilibrium Green's function formalism to analyze the fluctuation spectra of the boson tunneling currents. The approach allows us to calculate the noise spectra in both equilibrium and nonequilibrium conditions. The proposed general formalism is applied to several important realizations of boson transport, including the tunneling transport between two reservoirs and the case where the boson current flows through the intermediate region between the reservoirs. Developed theory can be applied for the analysis of the current noise in waveguides, coupled optical resonators, quantum microcavities, etc., where the tunneling of photons, exciton-polaritons, or excitons can be realized.

Relative out-of-plane displacements of the constituent layers of two-dimensional materials give rise to unique low-frequency breathing modes. By computing the height-height correlation functions from molecular dynamics simulations, we show that the layer breathing modes (LBMs) can be mapped consistently to vibrations of a simple linear chain model. Our calculated thickness dependence of LBM frequencies for few-layer (FL) graphene and molybdenum disulfide $\left({\mathrm{MoS}}_2\right)$ are in excellent agreement with available experiments. Our results show a redshift of LBM frequency with an increase in temperature, which is a direct consequence of anharmonicities present in the interlayer interaction. We also predict the thickness and temperature dependence of LBM frequencies for FL hexagonal boron nitride. Our Rapid Communication provides a simple and efficient way to probe the interlayer interaction for layered materials and their heterostructures with the inclusion of anharmonic effects.