The cooling of solids by optical means only using anti-Stokes emission has a long history of research and achievements. Such cooling methods have many advantages ranging from no moving parts or fluids through to operation in vacuum and may have applications to cryosurgery. However, achieving large optical cryocooling powers has been difficult to manage except in certain rare-earth crystals but these are mostly toxic and not biocompatible. Through study of the emission and absorption cross sections we find that diamond, containing either nitrogen vacancy (NV) or silicon vacancy defects, shows potential for optical cryocooling and, in particular, NV doping shows promise for optical refrigeration. We study the optical cooling of doped diamond microcrystals ranging 10–250 $\mu \mathrm{m}$ in diameter trapped either in vacuum or in water. For the vacuum case we find NV-doped microdiamond optical cooling below room temperature could exceed $\left|\mathrm{\Delta }T\right|>10$ K for irradiation powers of $P_{\text{in}}<100$ mW. We predict that such temperature changes should be easily observed via large alterations in the diffusion constant for optically cryocooled microdiamonds trapped in water in an optical tweezer or via spectroscopic signatures such as the zero-phonon line width or Raman line.

We present here a generalization of the thermal discrete dipole approximation (TDDA) that allows us to describe the near-field radiative heat transfer between finite objects of arbitrary shape that exhibit magneto-optical (MO) activity. We also extend the TDDA approach to describe the thermal emission of a finite object with and without MO activity. Our method is also valid for optically anisotropic materials described by an arbitrary permittivity tensor and we provide simple closed formulas for the basic thermal quantities that considerably simplify the implementation of the TDDA method. Moreover, we show that by employing our TDDA approach one can rigorously demonstrate Kirchhoff's radiation law relating the emissivity and absorptivity of an arbitrary MO object. Our work paves the way for the theoretical study of the active control of emission and radiative heat transfer between MO systems of arbitrary size and shape.

The ordering wave vector $\mathbf{Q}$ of a spin density wave (SDW), stabilized within the superconducting state of ${\mathrm{CeCoIn}}_5$ in a high magnetic field, has been shown to be hypersensitive to the direction of the field. $\mathbf{Q}$ can be switched from a nodal direction of the $d$-wave superconducting order parameter to a perpendicular node by rotating the in-plane magnetic field through the antinodal direction within a fraction of a degree. Here, we address the dynamics of the switching of $\mathbf{Q}$. We use a free-energy functional based on the magnetization density, which describes the condensation of magnetic fluctuations of nodal quasiparticles, and show that the switching process includes closing of the SDW gap at one $\mathbf{Q}$ and then reopening the SDW gap at another $\mathbf{Q}$ perpendicular to the first one. The magnetic field couples to $\mathbf{Q}$ through the spin-orbit interaction. Our calculations show that the width of the hysteretic region of switching depends linearly on the deviation of magnetic field from the critical field associated with the SDW transition, consistent with our thermal conductivity measurements. The agreement between theory and experiment supports our scenario of the hypersensitivity of the $Q$ phase on the direction of magnetic field, as well as the magnon condensation as the origin of the SDW phase in ${\mathrm{CeCoIn}}_5$.

Charge-density-wave (CDW) correlations feature prominently in the phase diagram of the cuprates, motivating competing theories of whether fluctuating CDW correlations aid superconductivity or whether static CDW order coexists with superconductivity in inhomogeneous or spatially modulated states. Here we report Cu $L$-edge resonant x-ray photon correlation spectroscopy measurements of CDW correlations in superconducting ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$, $x=0.11$. Static CDW order is shown to exist in the superconducting state at low temperatures and to persist up to at least 85% of the CDW transition temperature. We discuss the implications of our observations for how nominally competing order parameters can coexist in the cuprates.

In this work, we develop a coupled layer construction of fracton topological orders in $d=3$ spatial dimensions. These topological phases have subextensive topological ground-state degeneracy and possess excitations whose movement is restricted in interesting ways. Our coupled layer approach is used to construct several different fracton topological phases, both from stacked layers of simple $d=2$ topological phases and from stacks of $d=3$ fracton topological phases. This perspective allows us to shed light on the physics of the X-cube model recently introduced by Vijay, Haah, and Fu, which we demonstrate can be obtained as the strong-coupling limit of a coupled three-dimensional stack of toric codes. We also construct two new models of fracton topological order: a semionic generalization of the X-cube model, and a model obtained by coupling together four interpenetrating X-cube models, which we dub the ‘four color cube model”. The couplings considered lead to fracton topological orders via mechanisms we dub “p-string condensation” and “p-membrane condensation”, in which strings or membranes built from particle excitations are driven to condense. This allows the fusion properties, braiding statistics, and ground-state degeneracy of the phases we construct to be easily studied in terms of more familiar degrees of freedom. Our work raises the possibility of studying fracton topological phases from within the framework of topological quantum field theory, which may be useful for obtaining a more complete understanding of such phases.

Tensor network states, and in particular projected entangled pair states, play an important role in the description of strongly correlated quantum lattice systems. They do not only serve as variational states in numerical simulation methods, but also provide a framework for classifying phases of quantum matter and capture notions of topological order in a stringent and rigorous language. The rapid development in this field for spin models and bosonic systems has not yet been mirrored by an analogous development for fermionic models. In this work, we introduce a tensor network formalism capable of capturing notions of topological order for quantum systems with fermionic components. At the heart of the formalism are axioms of fermionic matrix-product operator injectivity, stable under concatenation. Building upon that, we formulate a Grassmann number tensor network ansatz for the ground state of fermionic twisted quantum double models. A specific focus is put on the paradigmatic example of the fermionic toric code. This work shows that the program of describing topologically ordered systems using tensor networks carries over to fermionic models.

We study the electric field and temperature gradient driven magnetoconductivity of a Weyl semimetal system. To analyze the responses, we utilize the kinetic equation with semiclassical equations of motion modified by the Berry curvature and orbital magnetization of the wave packet. Apart from the known positive quadratic magnetoconductivity, we show that due to the chiral anomaly, the magnetoconductivity can become a nonanalytic function of the magnetic field, proportional to the $\frac{3}{2}$ power of the magnetic field at finite temperatures. We also show that time-reversal symmetry breaking tilt of the Dirac cones results in linear magnetoconductivity. This is due to the one-dimensional chiral anomaly for which the tilt is responsible.

Optically induced dynamic nuclear spin polarization (DNP) in a semiconductor quantum dot (QD) requires many cycles of excitation of spin polarized carriers and carrier recombination. As such, the radiative lifetime of the exciton containing the electron becomes one of the limiting factors of DNP. In principle, changing the radiative lifetime of the exciton will affect DNP and thus the nuclear spin polarization. Here, we demonstrate the manipulation of DNP in single QDs through the engineering of the photonic environment using two-dimensional photonic crystals. We find that the achievable degree of nuclear spin polarization can be controlled through the modification of exciton radiative lifetime. Our results show the promise of achieving a higher degree of nuclear spin polarization via photonic environment engineering, with implications on spin-based quantum information processing.

We study dissipation in palladium (Pd) nanomechanical resonators at low temperatures in the linear response regime. Metallic resonators have shown characteristic features of dissipation due to tunneling two-level systems (TLS). The system described here offers a unique tunability of the dissipation scenario by adsorbing hydrogen (${\text{H}}_2$), which induces a compressive stress. The intrinsic stress is expected to alter TLS behavior. We find a sublinear $\sim T^{0.4}$ dependence of dissipation in a limited temperature regime. As seen in TLS dissipation scenarios, we find a logarithmic increase of frequency from the lowest temperatures till a characteristic temperature $T_{\text{co}}$ is reached. In samples without $H_2,T_{\text{co}}\sim 1\text{K}$ was seen, whereas with ${\text{H}}_2$ it is clearly reduced to $\sim 700\text{mK}$. Based on standard TLS phenomena, we attribute this to enhanced phonon-TLS coupling in samples with compressive strain. We also find that with ${\text{H}}_2$ there is a saturation in low-temperature dissipation, which may possibly be due to super-radiant interaction between TLS and phonons. We discuss the data in the scope of TLS phenomena and similar data for other systems.

We theoretically consider the spin-wave mode- and wavelength-dependent enhancement of the Gilbert damping in magnetic insulator–normal metal bilayers due to spin pumping as well as the enhancement's relation to direct and alternating inverse spin Hall voltages in the normal metal. In the long-wavelength limit, including long-range dipole interactions, the ratio of the enhancement for transverse volume modes to that of the macrospin mode is equal to two. With an out-of-plane magnetization, this ratio decreases with both an increasing surface anisotropic energy and mode number. If the surface anisotropy induces a surface state, the enhancement can be an order of magnitude larger than for the macrospin. With an in-plane magnetization, the induced dissipation enhancement can be understood by mapping the anisotropy parameter to the out-of-plane case with anisotropy. For shorter wavelengths, we compute the enhancement numerically and find good agreement with the analytical results in the applicable limits. We also compute the induced direct- and alternating-current inverse spin Hall voltages and relate these to the magnetic energy stored in the ferromagnet. Because the magnitude of the direct spin Hall voltage is a measure of spin dissipation, it is directly proportional to the enhancement of Gilbert damping. The alternating spin Hall voltage exhibits a similar in-plane wave-number dependence, and we demonstrate that it is greatest for surface-localized modes.

Thermodynamic properties and low-energy magnon excitations of $S=1$ honeycomb-layered ${\mathrm{Na}}_3{\mathrm{Ni}}_2{\mathrm{SbO}}_6$ have been investigated by high-resolution dilatometry, static magnetization, and high-frequency electron spin resonance studies in magnetic fields up to 16 T. At $T_{\mathrm{N}}$ = 16.5 K, there is a tricritical point separating two distinct antiferromagnetic phases, AF1 and AF2, from the paramagnetic regime. In addition, our data imply short-range antiferromagnetic correlations at least up to $\sim 5T_{\mathrm{N}}$. Well below $T_{\mathrm{N}}$, the magnetic field $B_{\text{C}1}\approx 9.5$ T is needed to stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies at $T_{\mathrm{N}}$ and $B_{\mathrm{C}1}$ imply significant magnetoelastic coupling, both of which are associated with a sign change of $\partial L/\partial B$. The transition at $B_{\mathrm{C}1}$ is associated with softening of the antiferromagnetic resonance modes observed in the electron-spin-resonance spectra. The anisotropy gap $\mathrm{\Delta }=360$ GHz implies considerable uniaxial anisotropy. We deduce the crucial role of axial anisotropy favoring the AF1 spin structure over the AF2 one. While the magnetostriction data disprove a simple spin-flop scenario at $B_{\mathrm{C}1}$, the nature of a second transition at $B_{\mathrm{C}2}$ $\approx$ 13 T remains unclear. Both the sign of the magnetostriction and Grüneisen analysis suggest that the short-range correlations at high temperatures are of AF2 type.

The layered compound ${\mathrm{KCu}}_3{\mathrm{As}}_2{\mathrm{O}}_7{\left(\mathrm{O}\mathrm{D}\right)}_3$, comprising distorted kagome planes of $S=1/2{\mathrm{Cu}}^{2+}$ ions, is a recent addition to the family of type-II multiferroics. Previous zero-field neutron diffraction work has found two helically ordered regimes in ${\mathrm{KCu}}_3{\mathrm{As}}_2{\mathrm{O}}_7{\left(\mathrm{O}\mathrm{D}\right)}_3$, each showing a distinct coupling between the magnetic and ferroelectric order parameters. Here, we extend this work to magnetic fields up to 20 T using neutron powder diffraction, capacitance, polarization, and high-field magnetization measurements, hence determining the $H-T$ phase diagram. We find metamagnetic transitions in both low-temperature phases around ${\mu }_0{H}_c\sim 3.7$ T, which neutron powder diffraction reveals to correspond to rotations of the helix plane away from the easy plane, as well as a small change in the propagation vector. Furthermore, we show that the sign of the ferroelectric polarization is reversible in a magnetic field, although no change is observed (or expected on the basis of the magnetic structure) due to the transition at 3.7 T. We finally justify the temperature dependence of the polarization in both zero-field ordered phases by a symmetry analysis of the free energy expansion, and attempt to account for the metamagnetic transition by adding anisotropic exchange interactions to our existing model for ${\mathrm{KCu}}_3{\mathrm{As}}_2{\mathrm{O}}_7{\left(\mathrm{O}\mathrm{D}\right)}_3$.

We present an ab initio study of Ru substitution in two different compounds, ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and LaFeAsO, pure and F doped. Despite the many similarities among them, Ru substitution has very different effects on these compounds. By means of an unfolding technique, which allows us to trace back the electronic states into the primitive cell of the pure compounds, we are able to disentangle the effects brought by the local structural deformations and by the impurity potential to the states at the Fermi level. Our results are compared with available experiments and show (i) satisfying agreement of the calculated electronic properties with experiments, confirming the presence of a magnetic order on a short-range scale, and (ii) Fermi surfaces strongly dependent on the internal structural parameters, more than on the impurity potential. These results enter a widely discussed field in the literature and provide a better understanding of the role of Ru in iron pnictides: although isovalent to Fe, the Ru-Fe substitution leads to changes in the band structure at the Fermi level mainly related to local structural modifications.

We theoretically demonstrate natural optical activity in disordered ensembles of nonchiral plasmonic resonators. We show that the statistical distributions of rotatory power and spatial dichroism are strongly dependent on the scattering mean free path in diffusive random media. This result is explained in terms of the intrinsic geometric chirality of disordered media, as they lack mirror symmetry. We argue that chirality and natural optical activity of disordered systems can be quantified by the standard deviation of both rotatory power and spatial dichroism. Our results are based on microscopic electromagnetic wave transport theory coupled to vectorial Green's matrix method for pointlike scatterers and are independently confirmed by full-wave simulations.

Multiple reverberation compression can achieve higher pressure, higher temperature, but lower entropy. It is available to provide an important validation for the elaborate and wider planetary models and simulate the inertial confinement fusion capsule implosion process. In the work, we have developed the thermodynamic and optical properties of helium from shock-adiabatic to quasi-isentropic compression by means of a multiple reverberation technique. By this technique, the initial dense gaseous helium was compressed to high pressure and high temperature and entered the warm dense matter (WDM) region. The experimental equation of state (EOS) of WDM helium in the pressure-density-temperature (P-$\rho$-T) range of $1–150\mathrm{GPa}$, $0.1–1.1\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, and 4600–24 000 K were measured. The optical radiations emanating from the WDM helium were recorded, and the particle velocity profiles detecting from the sample/window interface were obtained successfully up to 10 times compression. The optical radiation results imply that dense He has become rather opaque after the 2nd compression with a density of about $0.3\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ and a temperature of about 1 eV. The opaque states of helium under multiple compression were analyzed by the particle velocity measurements. The multiple compression technique could efficiently enhanced the density and the compressibility, and our multiple compression ratios $({\eta }_i={\rho }_i/{\rho }_0,i=1–10)$ of helium are greatly improved from 3.5 to 43 based on initial precompressed density $\left({\rho }_0\right)$. For the relative compression ratio $({\eta }_i^{\prime }={\rho }_i/{\rho }_{i-1})$, it increases with pressure in the lower density regime and reversely decreases in the higher density regime, and a turning point occurs at the 3rd and 4th compression states under the different loading conditions. This nonmonotonic evolution of the compression is controlled by two factors, where the excitation of internal degrees of freedom results in the increasing compressibility and the repulsive interactions between the particles results in the decreasing compressibility at the onset of electron excitation and ionization. In the P-$\rho$-T contour with the experiments and the calculations, our multiple compression states from insulating to semiconducting fluid (from transparent to opaque fluid) are illustrated. Our results give an elaborate validation of EOS models and have applications for planetary and stellar opaque atmospheres.

Noncollinear and noncoplanar magnetic textures including Skyrmions and vortices act as emergent electromagnetic fields and give rise to novel electronic and transport properties. We here report a unified understanding of noncoplanar magnetic orderings emergent from the spin-charge coupling in itinerant magnets. The mechanism has its roots in effective multiple spin interactions beyond the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, which are ubiquitously generated in itinerant electron systems with local magnetic moments. By carefully examining the higher-order perturbations in terms of the spin-charge coupling, we construct a minimal effective spin model composed of the bilinear and biquadratic interactions with particular wave numbers dictated by the Fermi surface. Taking two-dimensional systems as examples, we find that our effective model captures the underlying physics of the instability toward noncoplanar multiple-$Q$ states discovered recently: a single-$Q$ helical state expected from the RKKY theory is replaced by a double-$Q$ vortex crystal with chirality density waves even for an infinitely small spin-charge coupling on generic lattices [R. Ozawa et al., J. Phys. Soc. Jpn. 85, 103703 (2016)], and a triple-$Q$ Skyrmion crystal with a high topological number of two appears while increasing the spin-charge coupling on a triangular lattice [R. Ozawa, S. Hayami, and Y. Motome, Phys. Rev. Lett. 118, 147205 (2017)]. We find that by introducing an external magnetic field, our effective model exhibits a plethora of multiple-$Q$ states. Our effective model will serve as a guide for exploring further exotic magnetic orderings in itinerant magnets, not only in two dimensions but also in three dimensions.

We demonstrate a close connection between observed field-induced antiferromagnetic (AFM) order and quantum critical fluctuation (QCF) in the Zn7%-doped heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. Magnetization, specific heat, and electrical resistivity at low temperatures all show the presence of new field-induced AFM order under the magnetic field $B$ of 5–10 T, whose order parameter is clearly distinguished from the low-field AFM phase observed for $B<5\mathrm{T}$ and the superconducting phase for $B<3\mathrm{T}$. The 4f electronic specific heat divided by the temperature, $C_e/T$, exhibits $-lnT$ dependence at $B\sim 10\mathrm{T}$ $\left(\equiv B_0\right)$, and furthermore, the $C_e/T$ data for $B\ge B_0$ are well scaled by the logarithmic function of $B$ and $T$: $ln[(B-B_0)/T^{2.7}]$. These features are quite similar to the scaling behavior found in pure ${\mathrm{CeCoIn}}_5$, strongly suggesting that the field-induced QCF in pure ${\mathrm{CeCoIn}}_5$ originates from the hidden AFM order parameter equivalent to high-field AFM order in Zn7%-doped ${\mathrm{CeCoIn}}_5$.

We successfully synthesize single crystals of the verdazyl radical $\alpha \text{−}2,3,5\text{−}{\mathrm{Cl}}_3$-V. Ab initio molecular orbital calculations indicate that the two dominant antiferromagnetic interactions, $J_1$ and $J_2$ $\left(\alpha =J_2/J_1\simeq 0.56\right)$, form an $S=1/2$ distorted square lattice. We explain the magnetic properties based on the $S=1/2$ square lattice Heisenberg antiferromagnet using the quantum Monte Carlo method, and examine the effects of the lattice distortion and the interplane interaction contribution. In the low-temperature regions below 6.4 K, we observe anisotropic magnetic behavior accompanied by a phase transition to a magnetically ordered state. The electron spin resonance signals exhibit anisotropic behavior in the temperature dependence of the resonance field and the linewidth. We explain the frequency dependence of the resonance fields in the ordered phase using a mean-field approximation with out-of-plane easy-axis anisotropy, which causes a spin-flop phase transition at approximately 0.4 T for the field perpendicular to the plane. Furthermore, the anisotropic dipole field provides supporting information regarding the presence of the easy-axis anisotropy. These results demonstrate that the lattice distortion, anisotropy, and interplane interaction of this model are sufficiently small that they do not affect the intrinsic behavior of the $S=1/2$ square lattice Heisenberg antiferromagnet.

In strong magnetic field, the longitudinal magnetoconductivity in three-dimensional chiral materials is shown to exhibit a new type of quantum oscillations arising from the chiral magnetic effect (CME). These quantum CME oscillations are predicted to dominate over the Shubnikov–de Haas (SdH) ones in chiral materials with an approximately conserved chirality of quasiparticles at strong magnetic fields. The phase of quantum CME oscillations differs from the phase of the conventional SdH oscillations by $\pi /2$.