We theoretically investigate the unusual features of the magnetotransport in a monolayer phosphorene ferromagnetic/normal/ferromagnetic (F/N/F) hybrid structure. We find that the charge conductance can feature a minimum at parallel (P) configuration and a maximum near the antiparallel (AP) configuration of magnetization in the F/N/F structure with $n$-doped F and $p$-doped N regions and also a finite conductance in the AP configuration with the N region of $n$-type doping. In particular, the proposed structure exhibits giant magnetoresistance, which can be tuned to unity. This perfect switching is found to show strong robustness with respect to increasing the contact length and tuning the chemical potential of the N region with a gate voltage. We also explore the oscillatory behavior of the charge conductance or magnetoresistance in terms of the size of the N region. We further demonstrate the penetration of the spin-transfer torque into the right F region and show that, unlike graphene structure, the spin-transfer torque is very sensitive to the chemical potential of the N region as well as the exchange field of the F region.

We provide a theory of how heat tunnels between two parallel metal surfaces separated by an air gap. Two contributions are calculated: (1) electron-electron interactions and (2) photon fields from surface plasmons. Both contributions can transfer energy through low energy electron pairs. Our concern is with heat flow for small air gaps, on the order of nanometers. In that case the contribution from electron-electron interactions is most important. The contribution from photons is more important at larger separations.

Magneto-quantum oscillation experiments in high-temperature superconductors show a strong thermally induced suppression of the oscillation amplitude approaching the critical dopings [B. J. Ramshaw et al., Science 348, 317 (2014); H. Shishido et al., Phys. Rev. Lett. 104, 057008 (2010); P. Walmsley et al., Phys. Rev. Lett. 110, 257002 (2013)]—in support of a quantum-critical origin of their phase diagrams. We suggest that, in addition to a thermodynamic mass enhancement, these experiments may directly indicate the increasing role of quantum fluctuations that suppress the quantum oscillation amplitude through inelastic scattering. We show that the traditional theoretical approaches beyond Lifshitz-Kosevich to calculate the oscillation amplitude in correlated metals result in a contradiction with the third law of thermodynamics and suggest a way to rectify this problem.

Half-filled Landau levels form a zoo of strongly correlated phases. These include non-Fermi-liquids (NFLs), fractional quantum Hall (FQH) states, nematic phases, and FQH nematic phases. This diversity begs the following question: what keeps the balance between the seemingly unrelated phases? The answer is elusive because the Halperin-Lee-Read description that offers a natural departure point is inherently strongly coupled. However, the observed nematic phases suggest that nematic fluctuations play an important role. To study this possibility, we apply a recently formulated controlled double-expansion approach in large-$N$ composite fermion flavors and small $\epsilon$ nonanalytic bosonic action to the case with both gauge and nematic boson fluctuations. In the vicinity of a nematic quantum critical line, we find that depending on the amount of screening of the gauge- and nematic-mediated interactions controlled by $\epsilon$'s, the renormalization-group flow points to all four mentioned correlated phases. When pairing preempts the nematic phase, NFL behavior is possible at temperatures above the pairing transition. We conclude by discussing measurements at low tilt angles, which could reveal the stabilization of the FQH phase by nematic fluctuations.

The electrical, crystallographic, and magnetic properties of ultrathin magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) have been studied in detail, by employing superlattice structures of ${\mathrm{Fe}}_3{\mathrm{O}}_4$/${\mathrm{MgFe}}_2{\mathrm{O}}_4$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$/MgO on a variety of substrates. By careful analysis of their properties, the influence of substrate stoichiometry, ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin film thickness, antiphase boundaries on the magnetic properties can be separated. In particular, the controversial enhanced magnetic moment in ultrathin films (<5 nm) was confirmed to be related to the substrate stoichiometry, specifically the migration of oxygen vacancies into the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin films. The multilayer concept can be employed with many other such systems and offers methods of tuning the properties of thin magnetic oxides.

We show that spin $S=1$ system with large and random single-ion anisotropy can be at low energies mapped to a $S=1/2$ system with random magnetic fields. This is, for example, realized in $\mathrm{Ni}({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x{)}_2-4\mathrm{SC}({\mathrm{NH}}_2{)}_2$ compound (DTNX) and therefore it represents a long-sought realization of random local (on-site) magnetic fields in antiferromagnetic systems. We support the mapping by numerical study of $S=1$ and effective $S=1/2$ anisotropic Heisenberg chains and find excellent agreement for static quantities and also for the spin conductivity. Such systems can therefore be used to study the effects of local random magnetic fields on transport properties.

We explore the competition and coupling of vibrational and electronic contributions to the heat capacity of Al and ${\mathrm{Al}}_3\mathrm{Sc}$ at temperatures below 50 K, combining experimental calorimetry with highly converged finite-temperature density functional theory calculations. We find that semilocal exchange-correlation functionals accurately describe the rich feature set observed for these temperatures, including electron-phonon coupling. Using different representations of the heat capacity, we are therefore able to identify and explain deviations from the Debye behavior in the low-temperature limit and in the temperature regime 30–50 K as well as the reduction of these features due to the addition of Sc.

On the basis of fully relativistic Korringa-Kohn-Rostoker calculations and in conjunction with the coherent potential approximation and the linear response formalism, we present a complete ab initio study of the influence of alloy disorder on the static and dynamic (Gilbert damping) magnetic properties and on the electronic structure of the half-metallic full-Heusler alloy ${\mathrm{Co}}_2\mathrm{MnSi}$. We discuss in particular partial atomic disorders intermediate between the main crystal phases $\text{L}{2}_1$, B2, A2, and $\text{D}{0}_3$ of this alloy. We compare our results with homemade experiments and measurements from the literature, and conclude that the presence of a partial $\text{D}{0}_3$-like disorder could explain the relatively high value of the Gilbert damping parameter and the lack of half-metallicity measured in real samples, in which alloy disorder cannot be totally avoided.

Superfluid ${}^3\mathrm{He}$ is a spin-triplet $\left(S=1\right)$, $p$-wave $\left(L=1\right)$ BCS condensate of Cooper pairs with total angular momentum $J=0$ in the ground state. In addition to the breaking of U(1) gauge symmetry, separate spin or orbital rotation symmetry is broken to the maximal subgroup $\mathrm{SO}{\left(3\right)}_S\times \mathrm{SO}{\left(3\right)}_L\to {\mathrm{SO}\left(3\right)}_J$. The fermions acquire mass $m_F\equiv \mathrm{\Delta }$, where $\mathrm{\Delta }$ is the BCS gap. There are also 18 bosonic excitations: 4 Nambu-Goldstone modes and 14 massive amplitude Higgs modes. The bosonic modes are labeled by the total angular momentum $J\in \{0,1,2\}$, and parity under particle-hole symmetry $\mathrm{c}=\pm 1$. For each pair of angular momentum quantum numbers $J,J_z$, there are two bosonic partners with $\mathrm{c}=\pm 1$. Based on this spectrum, Nambu proposed a sum rule connecting the fermion and boson masses for BCS-type theories, which for ${}^3\mathrm{He}\text{−}B$ is $M_{J,+}^2+M_{J,-}^2=4m_F^2$ for each family of bosonic modes labeled by $J$, where $M_{J,c}$ is the mass of the bosonic mode with quantum numbers $(J,\mathrm{c})$. The Nambu sum rule (NSR) has recently been discussed in the context of Nambu-Jona-Lasinio models for physics beyond the standard model to speculate on possible partners to the recently discovered Higgs boson at higher energies. Here, we point out that the Nambu fermion-boson mass relations are not exact. Corrections to the bosonic masses from (i) leading-order strong-coupling corrections to BCS theory, and (ii) polarization of the parent fermionic vacuum lead to violations of the sum rule. Results for these mass corrections are given in both the $T\to 0$ and $T\to T_c$ limits. We also discuss experimental results, and theoretical analysis, for the masses of the $J^{\text{c}}=2^{\pm }$ Higgs modes and the magnitude of the violation of the NSR.

As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from $\mathrm{L}{\mathrm{a}}_{1.15}\mathrm{S}{\mathrm{r}}_{1.85}\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7-\mathrm{Pb}$ junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.

Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In this work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nanosized $\kappa$ carbides (Fe,Mn)${}_3\mathrm{AlC}$ by atom probe tomography in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magnetic disorder in these carbides. Second, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures.

Thermally fluctuating sheets and ribbons provide an intriguing forum in which to investigate strong violations of Hooke's Law: Large distance elastic parameters are in fact not constant but instead depend on the macroscopic dimensions. Inspired by recent experiments on free-standing graphene cantilevers, we combine the statistical mechanics of thin elastic plates and large-scale numerical simulations to investigate the thermal renormalization of the bending rigidity of graphene ribbons clamped at one end. For ribbons of dimensions $W\times L$ (with $L\ge W$), the macroscopic bending rigidity ${\kappa }_R$ determined from cantilever deformations is independent of the width when $W<{\ell }_{\mathrm{th}}$, where ${\ell }_{\mathrm{th}}$ is a thermal length scale, as expected. When $W>{\ell }_{\mathrm{th}}$, however, this thermally renormalized bending rigidity begins to systematically increase, in agreement with the scaling theory, although in our simulations we were not quite able to reach the system sizes necessary to determine the fully developed power law dependence on $W$. When the ribbon length $L>{\ell }_p$, where ${\ell }_p$ is the $W$-dependent thermally renormalized ribbon persistence length, we observe a scaling collapse and the beginnings of large scale random walk behavior.

We study the two-magnon nonresonant Raman scattering in the $(\pi ,\pi )$ and $(\pi ,0)$ in-plane ordered antiferromagnetic phases of a spin-$S{J}_{1x}\text{−}{J}_{1y}\text{−}{J}_2\text{−}{J}_c$ Heisenberg model on the tetragonal lattice within the framework of the spin-wave theory. We show that magnon-magnon interactions may significantly change the shape of spectra even in large-$S$ cases. We discuss the effects of various tuning factors to the two-magnon Raman spectra. We find that both the magnetic frustration $J_2/J_1$ and the interlayer exchange coupling $J_c$ may significantly affect the spectra in both the $B_{1g}$ and the $A_{1g}^{\prime }$ channels in the $(\pi ,\pi )$ Néel ordered phase. Moreover, we find a splitting of the two-magnon peak in the $(\pi ,0)$ collinear antiferromagnetic phase. We further discuss the implications of our results to the ${\mathrm{BaMnBi}}_2$ and iron pnictide systems.

We study spin transport through a normal metal-spin superconductor junction. A spin-flip reflection is demonstrated at the interface, where a spin-up electron incident from the normal metal can be reflected as a spin-down electron and the spin $2\times \hslash /2$ will be injected into the spin superconductor. When the (spin) voltage is smaller than the gap of the spin superconductor, the spin-flip reflection determines the transport properties of the junction. We consider both graphene-based (linear-dispersion-relation) and quadratic-dispersion-relation normal metal-spin superconductor junctions in detail. For the two-dimensional graphene-based junction, the spin-flip reflected electron can be along the specular direction (retro-direction) when the incident and reflected electron locates in the same band (different bands). A perfect spin-flip reflection can occur when the incident electron is normal to the interface, and the reflection coefficient is slightly suppressed for the oblique incident case. As a comparison, for the one-dimensional quadratic-dispersion-relation junction, the spin-flip reflection coefficient can reach 1 at certain incident energies. In addition, both the charge current and the spin current under a charge (spin) voltage are studied. The spin conductance is proportional to the spin-flip reflection coefficient when the spin voltage is less than the gap of the spin superconductor. These results will help us get a better understanding of spin transport through the normal metal-spin superconductor junction.

We study energetic, magnetic, and electronic properties of diluted substitutional Mn pairs on reconstructed $\left(001\right)$ GaAs surfaces. The studies are based on first-principles calculations in the framework of density functional theory. We demonstrate that the stability of the systems strongly depends on the position, orientation, and the distance between the Mn atoms constituting the pair. Independently of the considered surface reconstruction pattern, the Mn pairs with Mn atoms being the nearest neighbors (NN) on a cationic sublattice turn out to be energetically more favorable than the pairs with the larger distance between the Mn atoms. However, the preferential buildup orientation of the Mn-NN pair depends on the surface reconstruction and is parallel to either the $\left[110\right]$ or the $\left[1\overline{1}0\right]$ crystallographic direction. We reveal also the mechanisms of the magnetic ordering of Mn-NN pairs. The Mn-NN pairs along the $\left[110\right]$ crystallographic direction exhibit always ferromagnetic alignment of Mn spins, whereas the spins in the Mn-NN pairs along the $\left[1\overline{1}0\right]$ direction are mostly antiferromagnetically aligned. In the electronic structure of the systems containing Mn pairs with ferromagnetically aligned spins, we observe the valence band hole states in the neighborhood of Fermi energy. This indicates that the surface ferromagnetism in this prototype of dilute magnetic semiconductors can be explained in terms of the $p\text{−}d$ Zener model.

Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space [i.e., three-dimensional (3D) Dirac points], such as in $\mathrm{N}{\mathrm{a}}_3\mathrm{Bi}$ and $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$. More recently, new types of Dirac semimetals with robust Dirac line nodes (with nontrivial topology or near the critical point between topological phase transitions) have been proposed that extend the bulk linear touching from discrete points to one-dimensional (1D) lines. In this paper, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the nonsymmorphic crystals $M\text{SiS}$ $(M=\mathrm{Hf}$, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin zone (BZ), we observed two sets of Dirac line-nodes in parallel with the $k_z$ axis and their dispersions. Interestingly, along directions other than the line nodes in the 3D BZ, the bulk degeneracy is lifted by spin-orbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our paper not only experimentally confirms a new Dirac line-node semimetal family protected by nonsymmorphic symmetry but also helps understanding and further exploring the exotic properties, as well as practical applications of the $M\text{SiS}$ family of compounds.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on trigonal tellurium consisting of helical chains in the crystal. Through the band-structure mapping in the three-dimensional Brillouin zone, we found a definitive evidence for the band splitting originating from the chiral nature of crystal. A direct comparison of the band dispersion between the ARPES results and the first-principles band-structure calculations suggests the presence of Weyl nodes and tiny spin-polarized hole pockets around the H point. The present result opens a pathway toward studying the interplay among crystal symmetry, band structure, and exotic physical properties in chiral crystals.

We have investigated the dispersion relation of polariton condensates in a CuBr microcavity with the use of angle-resolved photoluminescence (PL) spectroscopy at 77 K. The polariton condensation was clearly confirmed by the thresholdlike changes in the PL intensity, energy, and bandwidth of the lower polariton at a zero in-plane wave-vector $k_{\parallel }=0$ as a function of excitation power density. A blueshifted flat dispersion of the PL energy suddenly appeared at the condensation threshold in a small $k_{\parallel }$ region accompanied by the dispersion of the noncondensate PL as a background. With increasing excitation power density from the threshold, the intensity of the noncondensate PL became negligible. As a result, we found a dispersive profile of the dispersion relation of the condensate in a large $k_{\parallel }$ region in addition to the flat dispersion corresponding to the diffusive profile. The total dispersion relation of the condensate was explained quantitatively by a theoretical model for nonequilibrium condensation.

We present a quantum Hall effect of magnons in two-dimensional clean insulating magnets at finite temperature. Through the Aharonov-Casher effect, a magnon moving in an electric field acquires a geometric phase and forms Landau levels in an electric field gradient of sawtooth form. At low temperatures, the lowest energy band being almost flat carries a Chern number associated with a Berry curvature. Appropriately defining the thermal conductance for bosons, we find that the magnon Hall conductances get quantized and show a universal thermomagnetic behavior, i.e., are independent of materials, and obey a Wiedemann-Franz law for magnon transport. We consider magnons with quadratic and linear (Dirac-like) dispersions. Finally, we show that our predictions are within experimental reach for ferromagnets and skyrmion lattices with current device and measurement techniques.