We experimentally studied the dynamics of optically excited hotspots in current-carrying WSi superconducting nanowires as a function of bias current, bath temperature, and excitation wavelength. We observed that the hotspot relaxation time depends on bias current, temperature, and wavelength. We explained this effect with a model based on quasiparticle recombination, which provides insight into the quasiparticle dynamics of superconductors.

MnP, a superconductor under pressure, exhibits a ferromagnetic order below $T_{\mathrm{C}}\sim 290$ K followed by a helical order with the spins lying in the $ab$ plane and the helical rotation propagating along the $c$ axis below $T_{\mathrm{s}}\sim 50$ K at ambient pressure. We performed single-crystal neutron diffraction experiments to determine the magnetic ground states under pressure. Both $T_{\mathrm{C}}$ and $T_{\mathrm{s}}$ are gradually suppressed with increasing pressure and the helical order disappears at $\sim 1.2$ GPa. At intermediate pressures of 1.8 and 2.0 GPa, the ferromagnetic order first develops and changes to a conical or two-phase (ferromagnetic and helical) structure with the propagation along the $b$ axis below a characteristic temperature. At 3.8 GPa, a helical magnetic order appears below 208 K, which hosts the spins in the $ac$ plane and the propagation along the $b$ axis. The period of this $b$ axis modulation is shorter than that at 1.8 GPa. Our results indicate that the magnetic phase in the vicinity of the superconducting phase may have a helical magnetic correlation along the $b$ axis.

We investigate the role of symmetries in determining the random matrix class describing quantum thermalization in a periodically driven many-body quantum system. Using a combination of analytical arguments and numerical exact diagonalization, we establish that a periodically driven “Floquet” system can be in a different random matrix class from the instantaneous Hamiltonian. A periodically driven system can thermalize even when the instantaneous Hamiltonian is integrable. A Floquet system that thermalizes in general can display integrable behavior at commensurate driving frequencies. When the instantaneous Hamiltonian and the Floquet operator both thermalize, the Floquet problem can be in the unitary class while the instantaneous Hamiltonian is always in the orthogonal class, and vice versa. We extract general principles regarding when a Floquet problem can thermalize to a different symmetry class from the instantaneous Hamiltonian. A (finite-sized) Floquet system can even display crossovers between different random matrix classes as a function of driving frequency.

The triangular-lattice Heisenberg antiferromagnet (HAF) is known to carry topological ${\mathbb{Z}}_2$ vortex excitations which form a gas at finite temperatures. Here we show that the spin-orbit interaction, introduced via a Kitaev term in the exchange Hamiltonian, condenses these vortices into a triangular ${\mathbb{Z}}_2$ vortex crystal at zero temperature. The cores of the ${\mathbb{Z}}_2$ vortices show abrupt, soliton-like magnetization modulations and arise by a special intertwining of three honeycomb superstructures of ferromagnetic domains, one for each of the three sublattices of the ${120}^{\circ }$ state of the pure HAF. This is an example of a nucleation transition, analogous to the spontaneous formation of magnetic domains, Abrikosov vortices in type-II superconductors, blue phases in cholesteric liquid crystals, and skyrmions in chiral helimagnets. As the mechanism relies on the interplay of geometric frustration and spin-orbital anisotropies, such vortex mesophases can materialize as a ground state property in spin-orbit coupled correlated systems with nearly hexagonal topology, as in triangular or strongly frustrated honeycomb iridates.

Low-temperature specific heat has been measured in FeS superconductor with the transition temperature of 4.55 K. It is found that the low-temperature electronic specific heat $C_e/T$ can be fitted to a linear relation in the low-temperature region, but fails to be described by an exponential relation as expected by an $s$-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat coefficient $\mathrm{\Delta }{\gamma }_e=[C_e\left(H\right)-C_e\left(0\right)]/T$ shows the Volovik relation $\mathrm{\Delta }{\gamma }_e\left(H\right)\propto \sqrt{H}$, suggesting the presence of nodal gap(s) in this material.

We perform a spin-polarized neutron-diffraction study to investigate the nature of the high-field magnetic state of the trilayered ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$. The analysis indicates that a high field applied within the $ab$ plane leads to an unbalance of the spin and orbital moments with a spatial profile that is strongly tied to the layers where the electrons are located in the unit cell. We provide evidence of a layer dependent magnetic anisotropy with the inner layers having larger spin and orbital magnetic moments than the outer ones and show that such behavior is robust to temperature variation being persistent above the Curie temperature. By means of an effective model that includes the coupling between the spin-orbital degrees of freedom at inequivalent Ru sites we ascribe the origin of the layer anisotropy to the cooperative effects between octahedral distortions, spin orbit, and Coulomb interactions.

We investigate topological properties in the Fe(Te,Se) thin films. We find that the single layer ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ has nontrivial $Z_2$ topological invariance which originates from the parity exchange at the $\mathrm{\Gamma }$ point of the Brillouin zone. The nontrivial topology is mainly controlled by the Te(Se) height. Adjusting the anion height, which can be realized as the function of lattice constants and $x$ in ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$, can drive a topological phase transition. In a bulk material, the two-dimensional $Z_2$ topology invariance is extended to a strong three-dimensional one. In a thin film, we predict that the topological invariance oscillates with the number of layers. The results can also be applied to iron pnictides. Our research establishes ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ as a unique system to integrate high-$T_c$ superconductivity and topological properties in a single electronic structure.

We theoretically model the electronic dynamics of a coupled quantum dot pair in a static electric field. We then investigate the possibility of polarization-entangled photon emission from the radiative cascade of the molecular biexciton state. Through numerical simulations, we analyze the dependence of entanglement fidelity on temperature and electric field, as well as tunnel coupling. We establish a regime of direct-indirect exciton detunings for which coupled quantum dots are superior to single dots for entangled photon generation, yielding near-unit fidelity over a larger range of exchange splittings.

Doping semiconductor nanocrystals (NCs) is a promising way to tailor the optical and electronic behavior of these materials to enable their use in (opto)electronic applications. Yet the practical exploitation of doping requires an understanding of its efficiency, and dependence on external environment, and of the electronic localization of dopant states due to confinement effects. Here, we experimentally probe the efficiency of doping of Si NCs grown in amorphous ${\mathrm{SiO}}_2$ by means of phase segregation method. We estimate a P doping efficiency of these Si NCs of about 30% and from this we infer that most P dopants are incorporated at substitutional sites of the NCs lattice and thus act as donors. We further show that the doping efficiency in Si NCs varies by several orders of magnitude depending on their external environment. Charge traps associated with air molecules adsorbed to the NCs surface give rise to a strong compensation of donors. We observe that this process can be reverted by desorbing the molecules from the NCs surface under vacuum. Moreover, we experimentally assess the confinement energy of isolated donors in Si NCs from the temperature dependence of their magnetic resonance. From this, we provide experimental evidence for the confinement-induced increase of ionization energy of dopants with decreasing NC size previously predicted with ab initio calculations of doped Si NCs.

We present angle-dependent measurements of the effective $g$ factor $g^{☆}$ in a Ge-Si core-shell nanowire quantum dot. $g^{☆}$ is found to be maximum when the magnetic field is pointing perpendicularly to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces $g^{☆}$ significantly. $g^{☆}$ is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field.

We show that Jastrow-Slater wave functions, in which a density-density Jastrow factor is applied onto an uncorrelated fermionic state, may possess long-range order even when all symmetries are preserved in the wave function. This fact is mainly related to the presence of a sufficiently strong Jastrow term (also including the case of full Gutzwiller projection, suitable for describing spin models). Selected examples are reported, including the spawning of Néel order and dimerization in spin systems, and the stabilization of charge and orbital order in itinerant electronic systems.

Long-range quantum lattice systems often exhibit drastically different behavior than their short-range counterparts. In particular, because they do not satisfy the conditions for the Lieb-Robinson theorem, they need not have an emergent relativistic structure in the form of a light cone. Adopting a field-theoretic approach, we study the one-dimensional transverse-field Ising model with long-range interactions, and a fermionic model with long-range hopping and pairing terms, explore their critical and near-critical behavior, and characterize their response to local perturbations. We deduce the dynamic critical exponent, up to the two-loop order within the renormalization group theory, which we then use to characterize the emergent causal behavior. We show that beyond a critical value of the power-law exponent of the long-range couplings, the dynamics effectively becomes relativistic. Various other critical exponents describing correlations in the ground state, as well as deviations from a linear causal cone, are deduced for a wide range of the power-law exponent.

We investigate second harmonic generation from individual silicon nanowires and study the influence of resonant optical modes on the far field nonlinear emission. We find that the polarization of the second harmonic has a size-dependent behavior and explain this phenomenon by considering different surface and bulk nonlinear susceptibility contributions. We show that the second harmonic generation has an entirely different origin, depending on the nanowire diameter and on whether the incident illumination is polarized parallel or perpendicular to the nanowire axis. The results open perspectives for further geometry-based studies on the origin and control of second harmonic generation in nanostructures of high-refractive index centrosymmetric dielectrics.

Some anisotropy in both mechanical and thermodynamical properties of bismuth is expected. A combination of density functional theory total energy calculations and density functional perturbation theory in the local density approximation is used to compute the elastic constants at 0 K using a finite strain approach and the thermal expansion tensor in the quasiharmonic approximation. The overall agreement with experiment is good. Furthermore, the anisotropy in the thermal expansion is found to arise from the anisotropy in both the directional compressibilities and the directional Grüneisen functions.

The room temperature magnetocaloric material ${\mathrm{MnFe}}_4{\mathrm{Si}}_3$ was investigated with nuclear inelastic scattering (NIS) and resonant ultrasound spectroscopy (RUS) at different temperatures and applied magnetic fields in order to assess the influence of the magnetic transition and the magnetocaloric effect on lattice dynamics. The NIS data give access to phonons with energies above 3 meV, whereas RUS probes the elasticity of the material in the MHz frequency range and thus low-energy, $\sim$ neV, phonon modes. A significant influence of the magnetic transition on the lattice dynamics is observed only in the low-energy, long-wavelength limit. ${\mathrm{MnFe}}_4{\mathrm{Si}}_3$ and other compounds in the ${\mathrm{Mn}}_{5-\mathrm{x}}{\mathrm{Fe}}_{\mathrm{x}}{\mathrm{Si}}_3$ series were also investigated with vibrating sample magnetometry, resistivity measurements, and Mössbauer spectroscopy in order to study the magnetic transitions and to complement the results obtained on the lattice dynamics.

Based on the Kitaev-Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions, we studied the nature of possible magnetic phases in the frustrated hyperkagome iridate, ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ (Na-438). Using Monte Carlo simulation, we showed that the phase diagram is mostly covered by two competing magnetic ordered phases, the $Z_2$ symmetry breaking (SB) phase and the $Z_6$ SB phase, latter of which is stabilized by the classical order by disorder. These two phases are intervened by a first-order phase-transition line with $Z_8$-like symmetry. The critical nature at the $Z_6$ SB ordering temperature is characterized by the three-dimensional $XY$ universality class, below which U(1) to $Z_6$ crossover phenomenon appears: the $Z_6$ spin anisotropy becomes irrelevant in a length scale shorter than a crossover length ${\mathrm{\Lambda }}_{*}$ while it becomes relevant otherwise. A possible phenomenology of polycrystalline Na-438 is discussed based on this crossover phenomenon.

We report the results of magnetization and specific heat measurements of ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$ composed of two subsystems A and B, which are magnetically described as an $S=1/2$ triangular-lattice Heisenberg-like antiferromagnet and a $J_1-J_2$ honeycomb-lattice Ising-like antiferromagnet, respectively. These two subsystems were found to be approximately decoupled. ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$ undergoes magnetic phase transitions at $T_{\mathrm{N}1}=12.0$ K and $T_{\mathrm{N}2}=3.0$ K, which can be interpreted as the orderings of subsystems B and A, respectively. Subsystem A exhibits a magnetization plateau at one-third of the saturation magnetization for the magnetic field $H$ perpendicular to the $c$ axis owing to the quantum order-by-disorder, whereas for $H\parallel c$, subsystem B shows three-step metamagnetic transitions with magnetization plateaus at zero, one-third, and one-half of the saturation magnetization. The analysis of the magnetization process for subsystem B shows that the classical ground states at these plateaus are infinitely degenerate within the Ising model.

In this paper, oxygen-deficient $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{S}{\mathrm{r}}_{0.33}\mathrm{Co}{\mathrm{O}}_3$ (LSCO) thin films are prepared by adjusting the oxygen pressure during the deposition. As oxygen vacancies are introduced into the LSCO films, the out-of-plane lattices are elongated, as revealed by x-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Modulated bright and dark stripes observed in the HRTEM images of the oxygen-deficient LSCO films are induced by cation ordering and oxygen vacancy ordering, respectively. Magnetic measurements and first-principles calculations show that the oxygen vacancies depress the magnetism of the LSCO films greatly. We also demonstrate that doping Sr into $\mathrm{LaCo}{\mathrm{O}}_3$ and removing oxygen from LSCO have a similar effect on the magnetic properties, which is correlated with a derived formula.

We have investigated the magnetocaloric effect in single and polycrystalline samples of quantum paraelectric $\mathrm{EuTi}{\mathrm{O}}_3$ by magnetization and heat capacity measurements. Single crystalline $\mathrm{EuTi}{\mathrm{O}}_3$ shows antiferromagnetic ordering due to $\mathrm{E}{\mathrm{u}}^{2+}$ magnetic moments below $T_{\mathrm{N}}=5.6\mathrm{K}$. This compound shows a giant magnetocaloric effect around its Néel temperature. The isothermal magnetic entropy change is $49\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$, the adiabatic temperature change is 21 K, and the refrigeration capacity is $500\mathrm{J}{\mathrm{kg}}^{-1}$ for a field change of 7 T at $T_{\mathrm{N}}$. The single crystal and polycrystalline samples show similar values of the magnetic entropy and adiabatic temperature changes. The large magnetocaloric effect is due to suppression of the spin entropy associated with the localized $4f$ moment of $\mathrm{E}{\mathrm{u}}^{2+}$ ions. The giant magnetocaloric effect, together with negligible hysteresis, suggest that $\mathrm{EuTi}{\mathrm{O}}_3$ could be a potential material for magnetic refrigeration below 40 K.

We demonstrate Si-based light-emitting diodes that continuously emit reddish-yellow visible light at room temperature by utilizing optical transitions between the p-d hybrid orbitals of Mn atoms doped in Si. Our light-emitting diodes show clear visible-light electroluminescence with two peaks at $E_1=1.75$ and $E_2=2.30\mathrm{eV}$, corresponding to optical transitions between p-d hybrid orbitals of Mn atoms. The electrons at the p-d hybrid orbitals of Mn in Si are excited by hot holes that are accelerated by an intensive electric field in the depletion layer of reverse biased Si $p\text{−}n$ junctions containing a Mn-doped Si (Si:Mn) layer. The observed two peaks at $E_1=1.75$ and $E_2=2.30\mathrm{eV}$ are redshifted and blueshifted by 0.14 eV, respectively, from those of GaAs:Mn or ZnS:Mn. Our observations are consistent with the $p\text{−}d$ hybridized electronic structure of Mn atoms doped in Si as predicted by first-principles calculations.