We report on the experimental observation of a long-range antiferromagnetic structure in the metamagnetic Ni-Co-Mn-Ga Heusler alloys. The accurate magnetic symmetry analysis based on experimental neutron diffraction data, exploiting the Shubnikov theory, allows the determination of the correct magnetic space group of the system. A two-step process, featuring the ordering of the Ni and Mn sublattices at different temperatures, leads to the antiferromagnetic structure in martensite. A perfect, constrained by the symmetry, antiferromagnetic ordering of the Ni sublattice in the “paramagnetic gap” is observed, followed by the ordering of the Mn sublattice at lower temperatures. The observation of such antiferromagnetic structure clarifies the current debate on the presence of antiferromagnetic interactions in the (Ni,Co)-Mn-$X$ ($X=\mathrm{Ga}$, Sn, Sb, and In) ferromagnetic shape memory alloys and yields new insights in understanding the magnetostructural properties of this relevant class of materials.

Spin waves are collective excitations propagating in the magnetic medium with ordered magnetizations. Magnonics, utilizing the spin wave (magnon) as an information carrier, is a promising candidate for low-dissipation computation and communication technologies. We discover that, due to the Dzyaloshinskii-Moriya interaction, the scattering behavior of the spin wave at a magnetic domain wall follows a generalized Snell's law, where two magnetic domains work as two different mediums. Similar to optical total reflection that occurs at water-air interfaces, spin waves may experience total reflection at the magnetic domain walls when their incident angle is larger than a critical value. We design a spin-wave fiber using a magnetic domain structure with two domain walls, and demonstrate that such a spin-wave fiber can transmit spin waves over long distances by total internal reflections, in analogy to an optical fiber.

Magnon drag is shown to dominate the thermopower of elemental Fe from 2 to 80 K and of elemental Co from 150 to 600 K; it is also shown to contribute to the thermopower of elemental Ni from 50 to 500 K. Two theoretical models are presented for magnon-drag thermopower. One is a hydrodynamic theory based purely on nonrelativistic, Galilean, spin-preserving electron-magnon scattering. The second is based on spin-motive forces, where the thermopower results from the electric current pumped by the dynamic magnetization associated with a magnon heat flux. In spite of their very different microscopic origins, the two give similar predictions for pure metals at low temperature, allowing us to semiquantitatively explain the observed thermopower of elemental Fe and Co without adjustable parameters. We also find that magnon drag may contribute to the thermopower of Ni. A spin-mixing model is presented that describes the magnon-drag contribution to the anomalous Nernst effect in Fe, again enabling a semiquantitative match to the experimental data without fitting parameters. Our paper suggests that particle nonconserving processes may play an important role in other types of drag phenomena and also gives a predicative theory for improving metals as thermoelectric materials.

We study the dynamics of excitons in GaAs/(Al,Ga)As core-shell nanowires by continuous-wave and time-resolved photoluminescence and photoluminescence excitation spectroscopy. Strong Al segregation in the shell of the nanowires leads to the formation of Ga-rich inclusions acting as quantum dots. At 10 K, intense light emission associated with these shell quantum dots is observed. The average radiative lifetime of excitons confined in the shell quantum dots is 1.7 ns. We show that excitons may tunnel toward adjacent shell quantum dots and nonradiative point defects. We investigate the changes in the dynamics of charge carriers in the shell with increasing temperature, with particular emphasis on the transfer of carriers from the shell to the core of the nanowires. We finally discuss the implications of carrier localization in the (Al,Ga)As shell for fundamental studies and optoelectronic applications based on core-shell III-As nanowires.

In highly viscous electron systems such as high-quality graphene above liquid nitrogen temperature, a linear response to applied electric current becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena including negative nonlocal resistance and current whirlpools. It has also been shown that, although both effects originate from high electron viscosity, a negative voltage drop does not principally require current backflow. In this work, we study the role of geometry on viscous flow and show that confinement effects and relative positions of injector and collector contacts play a pivotal role in the occurrence of whirlpools. Certain geometries may exhibit backflow at arbitrarily small values of the electron viscosity, whereas others require a specific threshold value for whirlpools to emerge.

Recent studies show that several metal oxides and dichalcogenides $\left(M{X}_2\right)$, which exist in nature, can be stable in two-dimensional (2D) form and each year several new $M{X}_2$ structures are explored. The unstable structures in $H$ (hexagonal) or $T$ (octahedral) forms can be stabilized through Peierls distortion. In this paper, we propose new 2D forms of ${\mathrm{RuS}}_2$ and ${\mathrm{RuSe}}_2$ materials. We investigate in detail the stability, electronic, magnetic, optical, and thermodynamic properties of 2D $\mathrm{Ru}{X}_2$ $(X=\text{S},\text{Se})$ structures from first principles. While their $H$ and $T$ structures are unstable, the distorted $T$ structures $\left(T^{\prime }\text{−}\mathrm{Ru}{X}_2\right)$ are stable and have a nonmagnetic semiconducting ground state. The molecular dynamic simulations also confirm that $T^{\prime }\text{−}\mathrm{Ru}{X}_2$ systems are stable even at 500 K without any structural deformation. $T^{\prime }\text{−}{\mathrm{RuS}}_2$ and $T^{\prime }\text{−}{\mathrm{RuSe}}_2$ have indirect band gaps with 0.745 eV (1.694 eV with HSE) and 0.798 eV (1.675 eV with HSE) gap values, respectively. We also examine their bilayer and trilayer forms and find direct and smaller band gaps. We find that AA stacking is more favorable than the AB configuration. The new 2D materials obtained can be good candidates with striking properties for applications in semiconductor electronic, optoelectronic devices, and sensor technology.

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical $A_{1g}$ coherent-phonon mode in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the $\overline{\mathrm{\Gamma }K}$ and the $\overline{\mathrm{\Gamma }M}$ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$.

We report on the quenching of single Ni adatom moments on Te-terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ topological insulator surfaces. The effect is noted as a missing x-ray magnetic circular dichroism for resonant $L_{3,2}$ transitions into partially filled Ni $3d$ states of theory-derived occupancy $n_d=9.2$. On the basis of a comparative study of Ni and Fe using scanning tunneling microscopy and ab initio calculations, we are able to relate the element specific moment formation to a local Stoner criterion. Our theory shows that while Fe adatoms form large spin moments of $m_s=2.54{\mu }_B$ with out-of-plane anisotropy due to a sufficiently large density of states at the Fermi energy, Ni remains well below an effective Stoner threshold for local moment formation. With the Fermi level remaining in the bulk band gap after adatom deposition, nonmagnetic Ni and preferentially out-of-plane oriented magnetic Fe with similar structural properties on ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ surfaces constitute a perfect platform to study the off-on effects of time-reversal symmetry breaking on topological surface states.

Symmetry protected topological (SPT) phases in free fermion and interacting bosonic systems have been classified, but the physical phenomena of interacting fermionic SPT phases have not been fully explored. Here, employing large-scale quantum Monte Carlo simulation, we investigate the edge physics of a bilayer Kane-Mele-Hubbard model with zigzag ribbon geometry. Our unbiased numerical results show that the fermion edge modes are gapped out by interaction, while the bosonic edge modes remain gapless at the $(1+1)d$ boundary, before the bulk quantum phase transition to a topologically trivial phase. Therefore, finite fermion gaps both in the bulk and on the edge, together with the robust gapless bosonic edge modes, prove that our system becomes an emergent bosonic SPT phase at low energy, which is directly observed in an interacting fermion lattice model.

The presence of several kinds of carriers at the Fermi surface results in interesting complex dielectric properties of the bulk Pd in the low-energy excitation range. A most spectacular manifestation of this is the presence of a collective electronic excitation characterized by a soundlike dispersion, termed acoustic plasmon (AP). Due to the characteristic dispersion reaching zero energy in the long-wavelength limit, the question of the thermal stability of the excitation spectrum arises. In this work we explore this problem investigating the thermal effect on the electronic excitation spectrum in this material, tracing how the AP properties vary with the temperature increase. The main effect consists in the gradual destruction of AP in the energy range corresponding to the temperature.

We report the experimental details of how mechanical detwinning can be implemented in tandem with high-sensitivity nuclear magnetic resonance measurements and use this setup to measure the in-plane anisotropy of the spin-lattice relaxation rate in underdoped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ with $x=0.048$. The anisotropy reaches a maximum of 30% at $T_N$, and the recovery data reveal that the glassy behavior of the spin fluctuations present in the twinned state persist in the fully detwinned crystal. A theoretical model is presented to describe the spin-lattice relaxation rate in terms of anisotropic nematic spin fluctuations.

Using density-functional perturbation theory and the Grüneisen formalism, we directly calculate the linear thermal expansion coefficients (TECs) of a hexagonal bulk system ${\mathrm{MoS}}_2$ in the crystallographic $a$ and $c$ directions. The TEC calculation depends critically on the evaluation of a temperature-dependent quantity $I_i\left(T\right)$, which is the integral of the product of heat capacity and ${\mathrm{\Gamma }}_i\left(\nu \right)$, of frequency $\nu$ and strain type $i$, where ${\mathrm{\Gamma }}_i\left(\nu \right)$ is the phonon density of states weighted by the Grüneisen parameters. We show that to determine the linear TECs we may use minimally two uniaxial strains in the $z$ direction and either the $x$ or $y$ direction. However, a uniaxial strain in either the $x$ or $y$ direction drastically reduces the symmetry of the crystal from a hexagonal one to a base-centered orthorhombic one. We propose to use an efficient and accurate symmetry-preserving biaxial strain in the $xy$ plane to derive the same result for $\mathrm{\Gamma }\left(\nu \right)$. We highlight that the Grüneisen parameter associated with a biaxial strain may not be the same as the average of Grüneisen parameters associated with two separate uniaxial strains in the $x$ and $y$ directions due to possible preservation of degeneracies of the phonon modes under a biaxial deformation. Large anisotropy of TECs is observed where the linear TEC in the $c$ direction is about 1.8 times larger than that in the $a$ or $b$ direction at high temperatures. Our theoretical TEC results are compared with experiment. The symmetry-preserving approach adopted here may be applied to a broad class of two lattice-parameter systems such as hexagonal, trigonal, and tetragonal systems, which allows many complicated systems to be treated on a first-principles level.

The spin-wave modes of ferromagnetic films have been studied for a long time experimentally as well as theoretically, either in the magnetostatic approximation or also considering the exchange interaction. A theoretical method is presented that allows one to determine with ease the exact frequency dispersion relations of dipole-exchange modes under general conditions: an obliquely applied magnetic field, and surface boundary conditions that allow for partial pinning, which may be of different origins. The method is a generalization of Green's theorem to the problem of solving the linear dynamics of ferromagnetic spin-wave modes. Convolution integral equations for the magnetization and the magnetostatic potential of the modes are derived on the surfaces of the film. For the translation-invariant film these become simple local algebraic equations at each in-plane wave vector. Eigenfrequencies result from imposing a $6\times 6$ determinant to be null, and spin-wave modes follow everywhere through solving linear $6\times 6$ inhomogeneous systems. An interpretation of the results is that the Green's functions represent six independent plane-wave solutions to the equations of motion, with six associated complex perpendicular wave vectors: volume modes correspond to the cases in which two of these are purely real at a given frequency. Furthermore, the convolution extinction equations enforce the boundary conditions: this is possible at specific eigenfrequencies for a given in-plane wave vector. Magnetostatic modes may also be obtained in detail. At low frequencies and for some obliquely applied magnetic fields, magnetostatic and dipole-exchange volume modes may have forward or backward character depending on the frequency range.

We perform an exact-diagonalization study of the spin-$\frac{1}{2}$ XXZ Heisenberg antiferromagnet on the kagome lattice at finite magnetization $m=\frac{2}{3}$ with an emphasis on the $XY$ point $\left(J_z=0\right)$ and in the presence of a small chiral term. Recent analytic work by Kumar et al. [K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 90, 174409 (2014)] on the same model, using a newly developed flux attachment transformation, predicts a plateau at this value of the magnetization described by a chiral spin liquid (CSL) with a spin Hall conductance of ${\sigma }_{xy}=\frac{1}{2}$. Such a state is topological in nature, has a ground-state degeneracy, and exhibits fractional excitations. We analyze the degeneracy structure in the low-energy manifold, identify the candidate topological states, and use them to compute the modular matrices and Chern numbers, all of which strongly agree with expected theoretical behavior for the ${\sigma }_{xy}=\frac{1}{2}$ CSL. In the limit of zero chirality, we find on most (not all) clusters that the topological invariants are still those of a CSL.

We report detailed study of angular-dependent magnetoresistance (AMR) with tilting angle $\theta$ from $c$ axis ranging from $0^{\circ }$ to ${360}^{\circ }$ on a high-quality FeSe single crystal. A pronounced AMR with twofold symmetry is observed, which is caused by the quasi-two-dimensional (quasi-2D) Fermi surface. The pronounced AMR is observed only in the orthorhombic phase, indicating that the quasi-2D Fermi surface is induced by the structural transition. Details about the influence of the multiband effect to the AMR are also discussed. Besides, the angular response of a possible Dirac-cone-like band structure is investigated by analyzing the detailed magnetoresistance at different $\theta$. The obtained characteristic field $\left(B^{*}\right)$ can be also roughly scaled in the 2D approximation, which indicates that the Dirac-cone-like state is also 2D in nature.

We study the ground state of the $S=\frac{1}{2}$ Heisenberg model on the checkerboard lattice in a magnetic field by the density matrix renormalization group method with the sine-square deformation. We obtain magnetization plateaus at $M/M_{\mathrm{sat}}=0,\frac{1}{4},\frac{3}{8},\frac{1}{2}$, and $\frac{3}{4}$, where $M_{\mathrm{sat}}$ is the saturated magnetization. The obtained $\frac{3}{4}$ plateau state is consistent with the exact result, and the $\frac{1}{2}$ plateau is found to have a four-spin resonating loop structure similar to the six-spin loop structure of the $\frac{1}{3}$ plateau of the kagome lattice. Different four-spin loop structures are obtained in the $\frac{1}{4}$ and $\frac{3}{8}$ plateaus but no corresponding states exist in the kagome lattice. The $\frac{3}{8}$ plateau has a unique magnetic structure of three types of four-spin local quantum states in a $4\sqrt{2}\times 2\sqrt{2}$ magnetic unit cell with a 16-fold degeneracy.

The full spin density fluctuations (SDF) spectra in $3d$ paramagnetic metals are analyzed from first principles using the linear response technique. Using the calculated complete wave vector and energy dependence of the dynamic spin susceptibility, we obtain the most important, but elusive, characteristic of SDF in solids: on-site spin correlator (SC). We demonstrate that the SDF have a mixed character consisting of interacting collective and single-particle excitations of similar strength spreading continuously over the entire Brillouin zone and a wide energy range up to femtosecond time scales. These excitations cannot be adiabatically separated and their intrinsically multiscale nature should always be taken into account for a proper description of metallic systems. Overall, in all studied systems, despite the lack of local moment, we found a very large SC resulting in an effective fluctuating moment of the order of several Bohr magnetons.

In this Rapid Communication we argue that rounded “hills” or “valleys” demonstrated by the heat capacity, thermal expansion coefficient, and elastic moduli are indications of a smeared second-order phase transition, which is flattened and spread out by the application of a magnetic field. As a result, some of the curves which display a temperature dependence of the corresponding quantities cross almost at a single point. Thus, the Vollhardt crossing point should not be identified with any specific energy scale. The smeared phase transition in MnSi preceding the helical first-order transition most probably corresponds to planar ferromagnetic ordering, with a small or negligible correlation between planes. At lower temperatures, the system of ferromagnetic planes becomes correlated, acquiring a helical twist.