The Zeeman interaction results in spontaneous current through a Josephson contact with a spin-orbit coupled normal metal, even in the absence of any voltage, or phase bias. In the case of the Rashba spin-orbit coupling of electrons in a two-dimensional (2D) electron gas this effect takes place for the Zeeman field which is parallel to the 2D system and to superconducting contacts. At the same time, the spontaneous current is absent when this field is perpendicular to the contacts. It is shown that in the latter case it may manifest itself in oscillations of the critical Josephson current at varying Zeeman energy. These oscillations have a form of the Fraunhofer diffraction pattern. The Josephson current under the phase bias was calculated based on the semiclassical Green's functions for a disordered 2D electron gas with the strong spin-orbit coupling, as well as for surface electrons of a three-dimensional topological insulator. In the latter case the diffraction pattern was found to be most pronounced, while in the Rashba gas the oscillations of the critical current are weaker.

Lanthanum (La) is the first member of the lanthanoid that has recently raised considerable interest because of its unique La superhydride ${\mathrm{LaH}}_{10}$ and its own superconducting properties. There has been a lack of experimental evidence for the equation of state (EOS) and superconductivity of La at pressures exceeding one megabar. Here, we extend the pressure range up to 140 GPa to explore the EOS and superconductivity of La via the electrical resistance and x-ray diffraction measurements. We identified the phase transition sequences by the La Bail refinements of the experimental XRD patterns and discovered a distorted fcc-La phase (space group Fmmm) above 78 GPa with nonhydrostatic pressure transmitting media. All the experimental pressure-volume data were fitted by the third-order Birch-Murnaghan equation: $V_0=37.5{\AA }^3, B_0=14.5\left(1\right)\mathrm{GPa}$, and $B_0^{\prime }=5$. Superconductivity still existed in the distorted fcc-La with an onset critical temperature $T_c$ of 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. We calculated the superconducting parameters of La at several pressure points, and discussed the difference from experimetal $T_c$.

We study the magneto-optical Kerr effect (MOKE) in ${\mathrm{SrRuO}}_3$ thin films, uncovering wide regimes of wavelength, temperature, and magnetic field where the Kerr rotation is not simply proportional to the magnetization but instead displays two-component behavior. One component of the MOKE signal tracks the average magnetization, while the second “anomalous” component bears a resemblance to anomalies in the Hall resistivity which have been previously reported in skyrmion materials. We present a theory showing that the MOKE anomalies arise from the nonmonotonic relation between the Kerr angle and the magnetization, when we average over magnetic domains which proliferate near the coercive field. Our results suggest that inhomogeneous domain formation, rather than skyrmions, may provide a common origin for the observed MOKE and Hall resistivity anomalies.

The synthesis of new materials that can host magnetic skyrmions and their thorough experimental and theoretical characterization are essential for future technological applications. The $\beta$-Mn-type compound ${\mathrm{FePtMo}}_3\mathrm{N}$ is one such novel material that belongs to the chiral space group $P{4}_{1}32$, where the antisymmetric Dzyaloshinskii-Moriya interaction is allowed due to the absence of inversion symmetry. We report the results of small-angle neutron scattering (SANS) measurements of ${\mathrm{FePtMo}}_3\mathrm{N}$ and demonstrate that its magnetic ground state is a long-period spin helix with a Curie temperature of 222 K. The magnetic field-induced redistribution of the SANS intensity showed that the helical structure transforms to a lattice of skyrmions at $\sim 13$ mT at temperatures just below $T_{\text{C}}$. Our key observation is that the skyrmion state in ${\mathrm{FePtMo}}_3\mathrm{N}$ is robust against field cooling down to the lowest temperatures. Moreover, once the metastable state is prepared by field cooling, the skyrmion lattice exists even in zero field. Furthermore, we show that the skyrmion size in ${\mathrm{FePtMo}}_3\mathrm{N}$ exhibits high sensitivity to the sample temperature and can be continuously tuned between 120 and 210 nm. This offers different prospects in the control of topological properties of chiral magnets.

The phase coexistence present through a first-order phase transition means there will be finite regions between the two phases where the structure of the system will vary from one phase to the other, known as a phase boundary wall. This region is said to play an important but unknown role in the dynamics of the first-order phase transitions. Here, by using both x-ray photon correlation spectroscopy and magnetometry techniques to measure the temporal isothermal development at various points through the thermally activated first-order metamagnetic phase transition present in the near-equiatomic FeRh alloy, we are able to isolate the dynamic behavior of the domain walls in this system. These investigations reveal that relaxation behavior of the domain walls changes when phase coexistence is introduced into the system and that the domain-wall dynamics is different to the macroscale behavior. We attribute this to the effect of the exchange coupling between regions of either magnetic phase changing the dynamic properties of domain walls relative to bulk regions of either phase. We also believe this behavior comes from the influence of the phase boundary wall on other magnetic objects in the system.

At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments. Partitioning the overall phonon population for a given transport length is performed examining the mean free paths obtained from the solution of the Boltzmann transport equation and allowing only diffusive phonons to participate in anharmonic phonon-phonon scattering processes. We present results for Si and diamond, discussing the crossover from ballistic to diffusive transport as the length scale and/or the temperature increases and compute the relative contribution of ballistic and diffusive phonons to the thermal conductance in each transport condition.

Novel randomness-induced disordered ground states in two-dimensional (2D) quantum spin systems have been attracting much interest. For quantitative analysis of such random quantum spin systems, one of the most promising numerical approaches is the tensor-network strong-disorder renormalization group (tSDRG), which was basically established for one-dimensional (1D) systems. In this paper, we propose a possible improvement of its algorithm toward 2D random spin systems, focusing on a generating process of the tree network structure of tensors, and precisely examine their performances for the random antiferromagnetic Heisenberg model not only on the 1D chain but also on the square- and triangular-lattices. On the basis of comparison with the exact numerical results up to 36 site systems, we demonstrate that accuracy of the optimal tSDRG algorithm is significantly improved for the 1D and 2D systems in the strong-randomness regime.

Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation ($\mu \mathrm{SR}$) technique to unveil the superconducting phase diagram of the dodecaboride ${\mathrm{ZrB}}_{12}$ and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive $\mu \mathrm{SR}$ signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.

We report a study of magnetization, resistivity, magnetoresistance, and specific heat of the pyrochlore iridate ${\left({\mathrm{Eu}}_{1-x}{\mathrm{Nd}}_x\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ with $x=0.0$, 0.5 and 1.0, where spin-orbit coupling, electronic correlation, magnetic frustration, and Kondo scattering coexist. The metal insulator transition temperature ($T_{MI}$) decreases with increase in Nd content, but always coincides with the magnetic irreversibility temperature (field-induced moment). Resistivity below $T_{MI}$ does not fit with either activated (gap) or any power-law (gapless) dependence. The Curie constant shows the surprising result that Nd induces singlet correlation (reduction of paramoment) in the Ir sublattice. Magnetoresistance is negative at low temperatures below 10 K and increases strongly with increase in $x$, and varies quadratically with field switching over to a linear dependence above 50 kOe. Low-temperature specific heat shows a Schottky peak, coming from Nd moments, showing the existence of a doublet split in the Nd energy level, arising from the $f-d$ exchange interaction. All materials show the presence of a linear specific heat in the insulating region. The coefficient of linear specific heat for $x=0.0$ does not vary with the external magnetic field, but varies superlinearly for $x=1.0$ materials. We argue that linear specific heat probably rules out weakly correlated phases such as Weyl fermions. We propose that with the introduction of Nd at the Eu site, the system evolves from a chiral spin liquid with gapless spinon excitations with a very small charge gap to a Kondo-type interaction superposed on a chiral spin liquid coexisting with long-range antiferromagnetic ordering. A huge increase of magnetoresistance with increase in Nd concentrations shows the importance of Kondo scattering in the chiral spin-liquid material by rare-earth moments.

${\mathrm{Ce}}_3\mathrm{Al}$ is an archetypal heavy-fermion compound with multiple crystalline phases. Here, we try to investigate its electronic structures in the hexagonal phase ($\alpha \text{−}{\mathrm{Ce}}_3\mathrm{Al}$) and cubic phase ($\beta \text{−}{\mathrm{Ce}}_3\mathrm{Al}$) by means of a combination of density functional theory and single-site dynamical mean-field theory. We confirm that the $4f$ valence electrons in both phases are itinerant, accompanied by strong valence state fluctuations. Their $4f$ band structures are heavily renormalized by electronic correlations, resulting in large effective electron masses. The Kondo screening in both phases would be protracted over a wide range of temperature since the single-impurity Kondo temperature $T_K$ is much higher than the coherent Kondo temperature $T_K^{*}$. Especially, the crystal structure of $\alpha \text{−}{\mathrm{Ce}}_3\mathrm{Al}$ forms a layered kagome lattice. We observe conspicuous kagome-derived flat bands and Dirac cones (or gaps) in its quasiparticle band structure. Therefore, it is suggested that the hexagonal phase of ${\mathrm{Ce}}_3\mathrm{Al}$ is a promising candidate for a heavy-fermion kagome metal.

We computationally study the Fermi arc states in a Dirac semimetal, both in a semi-infinite slab and in the thin-film limit. We use ${\mathrm{Cd}}_3{\mathrm{As}}_2$ as a model system, and include perturbations that break the $C_4$ symmetry and inversion symmetry while mirror symmetries remain. The surface states are protected by the mirror symmetries present in the bulk states and thus survive these perturbations. The Fermi arc states persist down to very thin films, thinner than presently measured experimentally, but are affected by breaking the symmetry of the Hamiltonian. Recent experimental observations of transport in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ films strongly suggest transport by surface states. Our findings are consistent with those observations in that we show that Fermi arc states can survive to rather thin films, and our results also suggest that symmetry-breaking terms that preserve the Fermi arc states nevertheless can have a profound effect in the thin film limit.

We report measurements and calculations on the properties of the intermetallic compound ${\mathrm{Be}}_5\mathrm{Pt}$. High-quality polycrystalline samples show a nearly constant temperature dependence of the electrical resistivity over a wide temperature range. On the other hand, relativistic electronic structure calculations indicate the existence of a narrow pseudogap in the density of states arising from accidental approximate Dirac cones extremely close to the Fermi level. A small true gap of order $\sim 3$ meV is present at the Fermi level, yet the measured resistivity is nearly constant from low to room temperature. We argue that this unexpected behavior can be understood by a cancellation of the energy dependence of density of states and relaxation time due to disorder, and discuss a model for electronic transport. With applied pressure, the resistivity becomes semiconducting, consistent with theoretical calculations that show that the bandgap increases with applied pressure. We further discuss the role of Be inclusions in the samples.

Polaritonic materials, owing to a strong phonon-polariton resonance in the THz and far-infrared parts of the electromagnetic spectrum, offer both high-index dielectric and metallic response. This complex response makes them suitable candidates for the design of metamaterial-related phenomena and applications. Here we show that one type of polaritonic-material-based structures that are particularly suitable for the achievement of a wide range of metamaterial properties are systems of polaritonic rods. To study the interplay between the material and the structural resonances in such systems, we employ as model systems rods of LiF and SiC and we calculate first the scattering properties of a single rod, identifying and discussing the behavior of the different resonances for different rod diameters. To analyze the response of ensembles of polaritonic rods, we employ an effective medium approach based on the coherent potential approximation (CPA), which is shown to be superior to the simple Maxwell-Garnett approximation for polaritonic and high-index dielectric metamaterials. Calculating and analyzing the CPA effective parameters, we find that our systems exhibit a large variety of interesting metamaterial properties, including hyperbolic dispersion, epsilon-near-zero and negative refractive index response. This rich response, achievable in almost any system of polaritonic rods, is highly engineerable by properly selecting the radius and the filling ratio of the rods, making polaritonic rod systems an ideal platform for demonstration of multifunctional metamaterials.

We propose a linear actuation mechanism for twisted bilayer graphene. Using molecular dynamics simulations, we show that the translational motion of a layer can cause another layer to move in an orthogonal direction. Such an effect depends strongly on the crystallographic orientation of graphene with respect to the direction of displacement. For the cross junction between two graphene nanoribbons, we predict the existence of a linear and a nonlinear actuation regime separated by a critical bilayer twisting angle. This critical angle is determined by the overlap between the van der Waals interaction ranges of the moiré superlattice. Based on this mechanism, a linear nanoactuator with desired transmission efficiency can be designed by adjusting the interlayer twisting angle of bilayer graphene.

Hall resistance $R_{xy}$ is commonly suggested to be linear-in-magnetic-field $B$, provided the field is small. We argue here that at low temperatures this linearity is broken due to weak localization/antilocalization phenomena in inhomogeneous systems, while in a uniform medium the linear-in-field dependence of $R_{xy}\left(B\right)$ is preserved. We calculate the Hall resistance for different two-component media using a mean-field approach and show that this nonlinearity is experimentally observable.

We reveal how to exploit joint spatial-electromagnetic duality symmetries to obtain invariant scattering properties (including extinction, scattering, absorption) of self-dual scattering systems for incident waves of arbitrary polarizations. The electromagnetic duality ensures the helicity preservation along all scattering directions and thus intrinsically eliminates the interferences between the two scattering channels originating from the circularly polarized components of incident waves. This absence of interference directly secures invariant scattering properties for all polarizations located on the same latitude circle of the Poincaré sphere, which are characterized by polarization ellipses of the same eccentricity and handedness. Further incorporations of mirror and/or inversion symmetries would lead to such invariance throughout the whole Poincaré sphere, guaranteeing invariant scattering properties for all polarizations. Simultaneous exploitations of composite symmetries of different natures render an extra dimension of freedom for scattering manipulations, offering new insights for both fundamental explorations and optical device engineering related to symmetry-dictated light-matter interactions.