The spin-half pyrochlore Heisenberg antiferromagnet (PHAF) is one of the most challenging problems in the field of highly frustrated quantum magnetism. Stimulated by the seminal paper of M. Planck [M. Planck, Verhandl. Dtsch. phys. Ges. 2, 202 (1900)] we calculate thermodynamic properties of this model by interpolating between the low- and high-temperature behavior. For that we follow ideas developed in detail by B. Bernu and G. Misguich and use for the interpolation the entropy exploiting sum rules [the “entropy method” (EM)]. We complement the EM results for the specific heat, the entropy, and the susceptibility by corresponding results obtained by the finite-temperature Lanczos method (FTLM) for a finite lattice of $N=32$ sites as well as by the high-temperature expansion (HTE) data. We find that due to pronounced finite-size effects the FTLM data for $N=32$ are not representative for the infinite system below $T\approx 0.7$. A similar restriction to $T\gtrsim 0.7$ holds for the HTE designed for the infinite PHAF. By contrast, the EM provides reliable data for the whole temperature region for the infinite PHAF. We find evidence for a gapless spectrum leading to a power-law behavior of the specific heat at low $T$ and for a single maximum in $c\left(T\right)$ at $T\approx 0.25$. For the susceptibility $\chi \left(T\right)$ we find indications of a monotonous increase of $\chi$ upon decreasing of $T$ reaching ${\chi }_0\approx 0.1$ at $T=0$. Moreover, the EM allows us to estimate the ground-state energy to $e_0\approx -0.52$.

Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot-electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot-electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer, reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the subpicosecond timescale, it excites coherent high-frequency magnons, as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot-electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spin-transfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spin-current pulse is short enough to excite magnons with frequencies beyond $1\mathrm{THz}$, a frequency range out of reach for current-induced spin-transfer torques. We demonstrate the formation of thickness-dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spin-transfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of $1\mathrm{nm}$, confirming experimental findings reporting penetration depths below $2\mathrm{nm}$.

An excitonic magnet hosts a condensate of spin-triplet excitons composed of conduction-band electrons and valence-band holes and may be described by the two-orbital Hubbard model. When the Hamiltonian has the nearest-neighbor interorbital hopping integrals with $d$-wave symmetry and the number of electrons is slightly away from half filling, the $\mathit{k}$-space spin texture appears in the excitonic phase with a broken time-reversal symmetry. We then show that, applying electric field to this doped excitonic magnet along a particular direction, a pure spin current emerges along its orthogonal direction. We discuss possible experimental realization of this type of the pure spin current in actual materials.

By combining hydrogen and sulfur within diamond-anvil cells we synthesize ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ at 5 GPa and 373 K. Through a series of Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction experiments we have constrained the phase diagram of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ within a wide $P\text{−}T$ range. On compression we observe the phase transition sequence of ${\text{I-II-II}}^{\prime }\text{-III}$, where ${\mathrm{II}}^{\prime }$ is a previously unreported phase; at room temperature this sequence spans from 5 to 47 GPa, while the application of low temperatures stabilizes this sequence to 127 GPa $(<80\mathrm{K})$. Above these pressures we propose that phase III of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ transforms to a nonmolecular ${\mathrm{H}}_3\mathrm{S}$ network. Our Raman and infrared measurements indicate that the transition from ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ to ${\mathrm{H}}_3\mathrm{S}$ is reversible at room temperature. X-ray diffraction reveals that the symmetry of the underlying S lattice of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ and ${\mathrm{H}}_3\mathrm{S}$ is retained along this compression path up to at least 135 GPa.

We report the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting an acoustic eigenmode in nanometric-sized thin films. The efficiency of the coupling is quadratic in the terahertz electric field strength within the range of investigation. Owing to a quantitative comparison between terahertz and near-infrared ultrashort pulse excitations, we show that the process of acoustic phonon generation by terahertz radiation is mainly driven by thermoelastic stress. While for the near-infrared light excitation the lattice temperature increase comes from a rapid energy transfer from the hot carriers to the phonon bath during carrier intraband relaxation, the thermoelastic stress induced by the terahertz electric field is linked to the scattering of the accelerated electrons leading to an ultrafast Joule effect.

We report on the epitaxial growth and surface structure of infinite-layer cuprate ${\mathrm{Sr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$ films on ${\mathrm{SrTiO}}_3$(001) substrates by combining ozone-assisted molecular beam epitaxy and in situ scanning tunneling microscopy. Careful substrate temperature and flux control has been used to achieve single-phase, stoichiometric, and $c$-axis oriented films. The surface of the films is usually characterized by a mixed ${\mathrm{CuO}}_2$ surface and gridlike superstructure. The superstructure exhibits a periodicity of 3.47 nm that corresponds to a coincidence lattice between the overlayer peroxide ${\mathrm{SrO}}_2$ and underlying ${\mathrm{CuO}}_2$ plane, and gives rise to a conductance spectrum that is distinct from the Mott-Hubbard band structure of ${\mathrm{CuO}}_2$. At a higher Nd composition $x>0.1$, a $(2\times 2)$ surface characteristic of the hole-doped ${\mathrm{CuO}}_2$ emerges, which we ascribe to the intake of apical oxygens in the intervening Sr planes.

Layered copper oxides have the highest superconducting transition temperatures at ambient pressure. Its mechanism remains a major challenge in condensed matter physics. The essential physics lying in two-dimensional copper-oxygen layers is well described by a single-band Hubbard model or its strong-coupling limit $t-J$ model in a two-dimensional square lattice. The recently discovered high-temperature superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ with $\delta \sim 0.2$ has a different crystal structure with a large portion of in-plane oxygen vacancies. We observe that an oxygen vacancy breaks the bond of its two neighboring copper atoms, and propose the ordered vacancies in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ lead to an extended $t-J$ model on an effective brick-wall lattice. For the nearest-neighbor hopping, the brick-wall model can be mapped onto the $t-J$ model on a honeycomb lattice. Our theory explains the superconductivity of ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ at a high charge carrier density, and predicts a time-reversal symmetry-broken pairing state.

We present a comprehensive study on structural and electronic properties of lanthanum intermetallic compounds ($M_x{\mathrm{La}}_y$, $M=\text{Be}$, Mg, Al, Ga, In, Tl, Pb, and Bi) under high pressure. By using a swarm intelligence structure search method combined with first-principles calculations, pressure-induced phase transitions of $M_x{\mathrm{La}}_y$ were investigated, with several new structures predicted. A universal yet intriguing phenomenon was found; that is, all of these compounds will decompose into elemental solids at certain pressures, which is against the general intuition that extreme pressure always stabilizes and densifies materials. Mechanical analysis suggests that this anomalous behavior is associated to the elastic moduli and interatomic interaction in $M_x{\mathrm{La}}_y$, and their changes under extreme pressure. A low bulk modulus and larger atomic volume of La result in a smaller volume for the elemental mixture compared to their compound at high pressures, which leads to an energetically favorable $PV$ work and enthalpy for the elemental mixture. Furthermore, the external pressure tends to weaken the La-$M$ electrostatic interaction in compounds as evidenced by the reduced charge transfer between La and $M$, which in turn modifies the electronegativity of La and $M$ and destabilizes the compounds. Our results shed light on the high-pressure behaviors of La-based intermetallic compounds and provide important guidance for understanding other La-like intermetallic compounds at high pressures.

Recently, the bilayer kagome lattice material ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ has been shown to be a quasi-two-dimensional quantum spin liquid (QSL) where the frustration arises from a balance between competing ferromagnetic and antiferromagnetic exchange within a bilayer. In an attempt to understand what happens when this balance is disturbed, we present a magnetic dilution study. Specifically, we have synthesized ${\mathrm{Ca}}_{10}{\left({\mathrm{Cr}}_{1-x}{\mathrm{V}}_x\right)}_7{\mathrm{O}}_{28}$ $(0\le x\le 0.5)$ where magnetic ${\mathrm{Cr}}^{5+}$ ($S=1/2$) is partially replaced by nonmagnetic ${\mathrm{V}}^{5+}$ ($S=0$). We also synthesized the fully nonmagnetic isostructural material ${\mathrm{Ca}}_{10}{\mathrm{V}}_7{\mathrm{O}}_{27.5}$. We report a detailed structural, magnetic and heat capacity study on these materials. A monotonic increase in the unit cell parameters is found for the ${\mathrm{Ca}}_{10}{\left({\mathrm{Cr}}_{1-x}{\mathrm{V}}_x\right)}_7{\mathrm{O}}_{28}$ materials with increasing $x$. An order of magnitude decrease in the Curie-Weiss temperature from 4 to $0.5$ K is found for the partial V substituted samples, which indicates a relative increase in antiferromagnetic exchange with increase in V content. However, despite this change in the relative balance in the exchange interactions and the large disorder introduced, no magnetic ordering or spin-glass state is observed down to 2 K in the V substituted samples. The QSL state of the parent compound thus seems surprisingly robust against these large perturbations.

Magnetic frustration in ${\mathrm{Fe}}_3{\mathrm{PO}}_4{\mathrm{O}}_3$ produces an unusual magnetic state below ${\mathrm{T}}_N=163\mathrm{K}$, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that ${\mathrm{Fe}}_3{\mathrm{PO}}_4{\mathrm{O}}_3$ does not exhibit a preferred ordering wave vector direction in the $ab$ plane despite having a well-defined ordering wave vector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wave vector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor ($J_1$) and second-neighbor ($J_2$) interactions, which produce a quasidegenerate manifold of ordering wave vectors. This state appears to be similar to the partially ordered phase of MnSi, but in ${\mathrm{Fe}}_3{\mathrm{PO}}_4{\mathrm{O}}_3$ arises in a frustrated antiferromagnet rather than a chiral ferromagnet.

Magnetic cylindrical nanowires are promising candidates for future three-dimensional nanotechnology. Domain walls (DWs) in magnetic nanowires play the role of information carriers, and the development of applications requires proper description of their dynamics. Here we perform a detailed analytical and numerical analysis of the DW motion along a bent magnetic nanowire under the action of tangential magnetic fields. Our results show that the DW velocity, precession, and oscillation frequencies can be controlled by the interplay between the curvature and the external magnetic field. Small magnetic fields induce a DW motion without precession and oscillatory behavior, while higher magnetic fields yield a Walker breakdown regime, in which an oscillatory forward and backward DW motion is observed. Controlled DW motion under the Walker breakdown regime makes magnetic nanowires potential candidates for nanoscale microwave generation and sensing.

We study the interplay between two nontrivial boundary effects: (1) the two-dimensional ($2d$) edge states of three-dimensional ($3d$) strongly interacting bosonic symmetry-protected topological states, and (2) the boundary fluctuations of $3d$ bulk disorder-to-order phase transitions. We then generalize our study to $2d$ gapless states localized at an interface embedded in a $3d$ bulk, when the bulk undergoes a quantum phase transition. Our study is based on generic long-wavelength descriptions of these systems and controlled analytic calculations. Our results are summarized as follows: (i) The edge state of a prototype bosonic symmetry-protected state can be driven to a new fixed point by coupling to the boundary fluctuations of a bulk quantum phase transition; (ii) the states localized at a $2d$ interface of a $3d\mathrm{SU}\left(N\right)$ quantum antiferromagnet may be driven to a new fixed point by coupling to the bulk quantum critical modes. Properties of the new fixed points identified are also studied.

Recent demonstration of efficient transport and manipulation of spin information by magnon currents have opened exciting prospects for processing information in devices. Magnon currents can be driven both electrically and thermally, even in magnetic insulators, by applying charge currents in an adjacent metal layer. Earlier reports in thin yttrium iron garnet (YIG) films suggested that the diffusion length of magnons is independent of the biasing method, but different values were obtained in thicker films. Here, we study the magnon diffusion length for electrically and thermally driven magnon currents in the linear regime in a 2-μm-thick YIG film as a function of temperature and magnetic field. Our results show a decrease in the magnon diffusion length with magnetic field for both biasing methods and at all temperatures from 5 to 300 K, indicating that subthermal magnons dominate long-range transport. Moreover, we demonstrate that the value of the magnon diffusion length depends on the driving mechanism, suggesting that different nonequilibrium magnon distributions are biased for each method. Finally, we demonstrate that the magnon diffusion length for thermally driven magnon currents is independent of the YIG thickness and material growth conditions, confirming that this quantity is an intrinsic parameter of YIG.

We numerically simulated quantum turbulence in superfluid ${}^4\mathrm{He}$ to investigate the emission of vortex rings from a localized vortex tangle. Turbulence is characterized by some universal statistical laws. Although there are a lot of studies on statistical laws in bulk quantum turbulence, studies in inhomogeneous or localized turbulence are scarce. We first investigate the statistical laws of localized quantum turbulence, referring to two statistical laws deduced from the vibrating wire experiments in Yano et al., J. Low Temp. Phys. 196, 184 (2019). The first law is the Poisson process for the detection of vortex rings; the vortex tangle emits vortex rings with frequencies depending on their sizes. The second law is the power law between the frequency and the size of the emitted vortex rings, showing the self-similarity of the tangle. To study these statistical laws numerically, we developed a system similar to experiments. First, we generate a localized statistically steady vortex tangle by injecting vortex rings from two opposite sides and causing collisions. We investigated the conditions that aid the formation of the tangles and the anisotropy of the emission of vortex rings from the tangle. Second, from the data on emitted rings, we reconstruct the two statistical laws. Results from our numerical investigations are consistent with the known self-similarity of emitted vortex rings and localized tangles.

We propose a platform for engineering helical fermions in a hybridized double-quantum-wire setup. When our setup is proximity coupled to an $s$-wave superconductor, it can become a class $D$ topological superconductor exhibiting Majorana zero modes. The goal of this proposal is to expand the group of available Hamiltonians to those without strong Rashba spin-orbit interactions which are essential to many other approaches. Furthermore, we show that there exist electron-electron interactions that stabilize fractional excitations which obey ${\mathbb{Z}}_3^{}$ parafermionic algebra.

The knowledge of the charge carrier distribution among the different orbitals of Cu and O is a precondition for the understanding of the physical properties of various Cu-O frameworks. We employ electron energy loss spectroscopy to elucidate the charge carrier plasmon dispersion in (La, ${\mathrm{Ca})}_x{\mathrm{Sr}}_{14-x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ in dependency of $x$ as well as temperature. We observe that the energy of the plasmon increases upon increasing Ca content, which signals an internal charge redistribution between the two Cu-O subsystems. Moreover, contrary to an uncorrelated model we come to the conclusion that the holes transferred to the ${\mathrm{Cu}}_2{\mathrm{O}}_3$ ladders are mainly located in the bonding and not in the antibonding band. This is caused by an orbital dependent Mott transition.

We report our direct study of the compressibility on an ultrahigh mobility two-dimensional electron system (${\mu }_e\sim$ $1\times {10}^7{\mathrm{cm}}^2$/V s) in GaAs/AlGaAs quantum wells under microwave irradiation. The field penetration current results show that the quantum capacitance oscillates with microwave-induced resistance oscillations, however, the trend is opposite with respect to the compressibility of equilibrium states in previous theoretical explanations. The anomalous phenomena provide a platform for study on the nonequilibrium system under microwaves. Moreover, the quantum capacitance indication for a multiphoton process around $j=1/2$ is detected under an intensive microwave below 30 GHz.

We study nanomachines whose relevant (effective) degrees of freedom $f\gg 1$ but smaller than $f$ of proteins. In these machines, both the entropic and the quantum effects over the whole system play the essential roles in producing nontrivial functions. We therefore call them entropic quantum machines (EQMs). We propose a systematic protocol for designing the EQMs, which enables a rough sketch, accurate design of equilibrium states, and accurate estimate of response time. As an illustration, we design a novel EQM, which shows two characteristic shapes. One can switch from one shape to the other by changing temperature or by applying a pulsed external field. We discuss two potential applications of this example of an EQM.