The ferroelectric phase transition behavior in the highly ordered $\mathrm{Pb}\left(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3$ has been investigated by the dielectric and Brillouin spectroscopy. The dielectric permittivity ${\varepsilon }_r$ exhibits a sharp maximum without any frequency dispersion at its Curie temperature $T_c$. In the temperature range far above $T_c$, it was noted that the dielectric permittivity exhibits a noticeable deviation from the Curie-Weiss law below the characteristic intermediate temperature $T^{*}=500\mathrm{K}$, which is common to most of the Pb-based oxide perovskite relaxors. Upon cooling, the frequency of the longitudinal acoustic phonon mode exhibits a remarkable softening towards $T_c$. The relaxation time of the order parameter calculated using the Landau-Khalatnikov approach was determined to be more than one order of magnitude lower than that of the disordered $\mathrm{Pb}(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_3$ and is very close to that observed in the paraelectric phase of the classical ferroelectric, $\mathrm{BaTi}{\mathrm{O}}_3$. The observed dielectric and relaxation features are qualitatively discussed in terms of the difference in the strength of the random electric fields.

Cu(pz)${}_2\left({\mathrm{ClO}}_4\right){}_2$ (with pz denoting pyrazine, ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2)$ is among the best realizations of a two-dimensional spin-$\frac{1}{2}$ square-lattice antiferromagnet. Below $T_{\mathrm{N}}=4.21$ K, its weak interlayer couplings induce a three-dimensional magnetic order, strongly influenced by external magnetic fields and/or hydrostatic pressure. Previous work, focusing on the $[H,T]$ phase diagram, identified a spin-flop transition, resulting in a field-tunable bicritical point. However, the influence of external pressure has not been investigated yet. Here we explore the extended $[p,H,T]$ phase diagram of Cu(pz)${}_2\left({\mathrm{ClO}}_4\right){}_2$ under pressures up to 12 kbar and magnetic fields up to 7.1 T via magnetometry and ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance (NMR) measurements. The application of magnetic fields enhances $T_{XY}$, the crossover temperature from the Heisenberg to the $XY$ model, thus pointing to an enhancement of the effective anisotropy. The applied pressure has an opposite effect $\mathrm{[}d{T}_{\mathrm{N}}/dp=-0.050\left(8\right)$ K/kbar], as it modifies marginally the interlayer couplings but likely changes more significantly the orbital reorientation and the square-lattice deformation. This results in a remodeling of the effective Hamiltonian, whereby the field and pressure effects compensate each other. Finally, by comparing the experimental data with numerical simulations we estimate $T_{\mathrm{BKT}}$, the temperature of the Berezinskii-Kosterlitz-Thouless topological transition, and argue why it is inaccessible in our case.

Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. Such a description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axis spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the “spin” of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of freedoms of an individual skyrmion can couple to each other, and give rise to unusual behavior that is absent for the skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. The derived Ginzburg-Landau theory provides a convenient and general framework to discuss skyrmion physics and will facilitate the search for skyrmions in inversion-symmetric magnets.

The Higgs amplitude mode in superconductors is the condensed-matter analogy of Higgs bosons in particle physics. We investigate the time evolution of Higgs amplitude mode in massless Dirac systems induced by a weak quench of an attractive interaction. We find that the Higgs amplitude mode in the half-filled honeycomb lattice has a logarithmic decaying behavior, qualitatively different from the $1/\sqrt{t}$ decay in the normal superconductors. Our study is also extended to the doped cases in honeycomb lattices. As for the three-dimensional Dirac semimetal at half filling, we obtain an undamped oscillation of the amplitude mode. Our finding is not only an important supplement to the previous theoretical studies on normal fermion systems but also provides an experimental signature to characterize the superconductivity in two- or three-dimensional Dirac systems.

The recently discovered $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\text{OHFeSe}$ superconductor provides a new platform for exploiting the microscopic mechanisms of high-$T_c$ superconductivity in FeSe-derived systems. Using density functional theory calculations, we first show that substitution of Li by Fe not only significantly strengthens the attraction between the $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\text{OH}$ spacing layers and the FeSe superconducting layers along the $c$ axis but also minimizes the lattice mismatch between the two in the $ab$ plane, both favorable for stabilizing the overall structure. Next, we explore the electron injection into FeSe from the spacing layers and unambiguously identify the ${\mathrm{Fe}}_{0.2}$ components to be the origin of the dramatically enhanced interlayer charge transfer. We further reveal that the system strongly favors collinear antiferromagnetic ordering in the FeSe layers, but the spacing layers can be either antiferromagnetic or ferromagnetic depending on the ${\mathrm{Fe}}_{0.2}$ spatial distribution. Based on these insights, we predict $\left({\mathrm{Li}}_{0.8}{\mathrm{Co}}_{0.2}\right)\text{OHFeSe}$ to be structurally stable with even larger electron injection and potentially higher $T_c$.

Heavy fermion (HF) materials exhibit a rich array of phenomena due to the strong Kondo coupling between their localized moments and itinerant electrons. A central question in their study is to understand the interplay between magnetic order and charge transport, and its role in stabilizing new quantum phases of matter. Particularly promising in this regard is a family of tetragonal intermetallic compounds $\mathrm{Ce}T{X}_2$ (where $T$ denotes transition metal and $X$ denotes pnictogen), which includes a variety of HF compounds showing $T$-linear electronic specific heat ${\mathbf{C}}_e\sim \gamma T$, with $\gamma \sim 20–500\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$, reflecting an effective-mass enhancement ranging from small to modest. Here, we study the low-temperature field-tuned phase diagram of high-quality ${\mathrm{CeAgBi}}_2$ using magnetometry and transport measurements. We find an antiferromagnetic transition at $T_N=6.4\mathrm{K}$ with weak magnetic anisotropy and the easy axis along the $c$ axis, similar to previous reports $\left(T_N=6.1\mathrm{K}\right)$. This scenario, along with the presence of two anisotropic Ruderman-Kittel-Kasuya-Yosida interactions, leads to a rich field-tuned magnetic phase diagram, consisting of five metamagnetic transitions of both first and second order. In addition, we unveil an anomalous Hall contribution for fields $H<54\mathrm{kOe}$, which is drastically altered when $H$ is tuned through a trio of transitions at 57, 78, and 84 kOe, suggesting that the Fermi surface is reconstructed in a subset of the metamagnetic transitions.

Thermoelectric transport in strongly spin-orbit coupled two-dimensional Rashba systems is studied using the exact solution of the linearized Boltzmann equation. Some unusual transport behaviors are revealed. We show that the electrical conductivity as a function of the Fermi energy ${\epsilon }_F$ behaves differently between the two sides of the band crossing point (BCP). The deviation from the Mott relation occurs when ${\epsilon }_F$ lies in the vicinity of the BCP and is attributed to the topological change of Fermi surface varying across the BCP. It is shown that the thermopower and thermoelectric figure of merit are strongly enhanced when ${\epsilon }_F$ downs below the BCP. This enhancement is attributed to not only the one-dimensional-like density of state, but also the unconventional intraband elastic scattering below the BCP. The differences between these results and those obtained by the relaxation time approximation (RTA) are discussed in detail, revealing the necessity of going beyond the RTA in the considered system.

Nonradiative resonance energy transfer (RET) provides the ability to transfer excitation energy between contiguous nanowires (NWs) with high efficiency under certain conditions. Nevertheless, the well-established Förster formalism commonly used to represent RET was developed for energy transfer primarily between molecular blocks (i.e., from one molecule, or part of a molecule, to another). Although deviations from Förster theory for functional blocks such as NWs have been studied previously, the role of the relative distance, the orientation of transition dipole moment pairs, and the passively interacting matter on electronic energy transfer are to a large extent unknown. Thus, a comprehensive theory that models RET in NWs is required. In this context, analytical insights to give a deeper and more intuitive understanding of the distance and orientation dependence of RET in NWs is presented within the framework of quantum electrodynamics. Additionally, the influence of an included intermediary on the rate of excitation energy transfer is illustrated, embracing indirect energy transfer rate and quantum interference. The results deliver equations that afford new intuitions into the behavior of virtual photons. In particular, results indicate that RET efficiency in a NW system can be explicitly expedited or inhibited by a neighboring mediator, depending on the relative spacing and orientation of NWs.

We discuss how complex-oxide heterostructures that include high-$T_c$ superconducting cuprates can be used to realize an array of submillimeter cavities that support Josephson plasmon polaritons. These cavities have several attractive features for new types of light-matter interaction studies and we show that they promote “ultrastrong” coupling between THz frequency radiation and Josephson plasmons. Cavity electrodynamics of Josephson plasmons allows us to manipulate the superconducting order-parameter phase coherence. As an example, we discuss how it could be used to cool superconducting phase fluctuations with light.

Angular-dependent magnetoresistance (MR) is considered to be intrinsic to spintronic materials, represented by the classical anisotropic MR (AMR) phenomenon and the recently emerged spin Hall MR (SMR). So far, isotropic AMR, AMR with geometric size effect and interfacial effect, and SMR have been treated separately to explain distinct MR correlations observed in various systems. Current study shows all four types of MR correlations can be reproduced in Fe thin films depending on the film thickness, texture, interface, and morphology. Results suggest previous explanations of the thin-film MR correlations are incomplete and it is inappropriate to use a specific MR angular-dependent correlation as the sole criterion in determining the origin of AMR or ascertaining the exclusive existence of SMR.

The electronic structure of the transparent semiconductor ${\mathrm{In}}_2{\mathrm{O}}_3$ has been studied by angle-resolved photoemission spectroscopy upon deposition of metallic indium and also tin on the surface of the semiconductor. By deposition of metallic indium on ${\mathrm{In}}_2{\mathrm{O}}_3$(111) single crystals, we detected the formation of a free-electron-like band of effective mass $(0.38\pm 0.05)m_0$. At low coverages, metallic In shifts the Fermi level of ${\mathrm{In}}_2{\mathrm{O}}_3$ to higher energies and a new electronic state forms at the metal/semiconductor interface. This state of a two-dimensional character (2D electron gas) is completely responsible for the electrical conduction in ${\mathrm{In}}_2{\mathrm{O}}_3$(111) at the surface region and has a band dispersion, which does not correspond to the previously found surface accumulation layers in this material. Despite the similarities of the electronic properties of In and Sn, a larger downward band bending was observed by Sn coverage, which was not accompanied by the appearance of the surface state.

Nematic order resulting from the partial melting of density waves has been proposed as the mechanism to explain nematicity in iron-based superconductors. An outstanding question, however, is whether the microscopic electronic model for these systems—the multiorbital Hubbard model—displays such an ordered state as its leading instability. In contrast to usual electronic instabilities, such as magnetic and charge order, this fluctuation-driven phenomenon cannot be captured by the standard random phase approximation (RPA) method. Here, by including fluctuations beyond RPA in the multiorbital Hubbard model, we derive its nematic susceptibility and contrast it with its ferro-orbital order susceptibility, showing that its leading instability is the spin-driven nematic phase. Our results also demonstrate the primary role played by the $d_{xy}$ orbital in driving the nematic transition and reveal that high-energy magnetic fluctuations are essential to stabilize nematic order in the absence of magnetic order.

We study with angle-resolved photoemission spectroscopy (ARPES) the renormalization and quasiparticle lifetimes of the $d_{xy}$ and $d_{xz}/d_{yz}$ orbitals in two iron pnictides, LiFeAs and $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}){}_2{\mathrm{As}}_2$ (Co8). We find that both quantities depend on orbital character rather than on the position on the Fermi surface (for example, hole or electron pocket). In LiFeAs, the renormalizations are larger for $d_{xy}$, while they are similar for both types of orbitals in Co8. The most salient feature, which proved robust against all the ARPES caveats we could think of, is that the lifetimes for $d_{xy}$ exhibit a markedly different behavior than those for $d_{xz}/d_{yz}$. They have smaller values near $E_F$ and exhibit larger $\omega$ and temperature dependences. While the behavior of $d_{xy}$ is compatible with a Fermi-liquid description, that is not the case for $d_{xz}/d_{yz}$. This situation should have important consequences for the physics of iron pnictides, which have not been considered up to now. More generally, it raises interesting questions about how a Fermi-liquid regime can be established in a multiband system with small effective bandwidths.

This paper considers a system of two parallel quantum Hall layers with total filling factor 0 or 1. When the distance between the layers is small enough, electrons and holes in opposite layers form interlayer excitons, which have a finite effective mass and interact via a dipole-dipole potential. I present results for the chemical potential $\mu$ of the resulting bosonic system as a function of the exciton concentration $n$ and the interlayer separation $d$. Both $\mu$ and the interlayer capacitance have an unusual nonmonotonic dependence on $d$, owing to the interplay between an increasing dipole moment and an increasing effective mass with increasing $d$. A phase transition between superfluid and Wigner crystal phases is shown to occur at $d\propto n^{-1/10}$. Results are derived first via simple intuitive arguments, and then verified with more careful analytic derivations and numeric calculations.

We develop a Ginzburg-Landau (GL) theory for fcc crystal-melt systems at equilibrium by employing two sets of order parameters that correspond to amplitudes of density waves of principal reciprocal lattice vectors and amplitudes of density waves of a second set of reciprocal lattice vectors. The choice of the second set of reciprocal lattice vectors is constrained by the condition that this set must form closed triangles with the principal reciprocal lattice vectors in reciprocal space to make the fcc-liquid transition first order. The capillary anisotropy of fcc-liquid interfaces is investigated by GL theory with amplitudes of $\langle 111\rangle$ and $\langle 200\rangle$ density waves. Furthermore, we explore the dependence of the anisotropy of the excess free energy of the solid-liquid interface on density waves of higher-order reciprocal lattice vectors such as $\langle 311\rangle$ by extending the two-mode GL theory with an additional mode. The anisotropy calculated using GL theory with input parameters from molecular dynamics (MD) simulations for fcc Ni is compared to that measured in MD simulations.

Metallic glasses are of recent interest worldwide due to their remarkable physicochemical properties which can be put in relation with their crystalline counterparts. Among them, cerium-based metallic glasses (Ce-MGs) have unique features such as the existence of polyamorphism under pressure, which is unexpected in these spatially compact systems. While a phase transition between amorphous phases with change of density and local structure has been previously detected, the corresponding structural variation under pressure was not clearly identified due to difficulties in performing accurate measurements and reliable analysis. In this work, angle dispersive x-ray diffraction experiments of ${\mathrm{Ce}}_{69}{\mathrm{Al}}_{10}{\mathrm{Cu}}_{20}{\mathrm{Co}}_1$ bulk metallic glass have been performed up to 16 GPa along two distinct isotherms (300 and 340 K). All of the diffuse signals have then been processed in order to extract the structure factor $S\left(Q\right)$, the pair distribution $g\left(r\right)$, the atomic density $\rho$, and the compressibility as a function of pressure and temperature. These are crucial probes to fully characterize the phase diagram, and they clearly confirm the existence of a link between polyamorphism in Ce-MGs and the $\gamma$ $\leftrightarrows$ $\alpha$ transition in pure cerium. Finally, owing to the presence of a critical point in pure solid Ce, the existence of such a feature is discussed here for Ce-MGs.

With the changing of physical properties through chemical substitution in mind, this work shows the applicability of this approach to organic based spin-Peierls (SP) systems. To demonstrate this we have used a well known system, potassium TCNQ, that undergoes an SP transition at $T_{\text{SP}}=396$ K. Simply substituting protons for fluorine, using ${\mathrm{TCNQF}}_4$, shows a decrease in the coupling strength between TCNQ anions where the spin-Peierls transition is dramatically reduced in temperature, with ${\mathrm{KTCNQF}}_4$ showing a $T_{\text{SP}}$ at approximately 160 K, which is due to changes of the electron spin density across the molecule. Muon spin relaxation is a more suitable technique for determining the magnetic properties of these systems and measurements were conducted on both KTCNQ and ${\mathrm{KTCNQF}}_4$ in order to study the behavior of the SP transition. This has highlighted and shown that both transitions exhibit the same behavior and so are indeed similar; however, we have also succeeded in being able to tune the transition. Estimates of the dynamic critical exponents from both samples are obtained from the muon data and are found to be $\sim 0.33$, corresponding to a three-dimensional (3D) antiferromagnetic system implying that spin fluctuations associated with the SP state are correlated not just along stacks of dimerized TCNQ anions. This result extends the understanding of the SP phase transition in the KTCNQ system beyond that determined from purely structural studies.

The thermal evolution of structural and magnetic details of the orthovanadate $\mathrm{TmV}{\mathrm{O}}_3$, studied in detail by neutron and synchrotron powder diffraction, is reported. Crystallizing in space group Pnma at room temperature, $\mathrm{TmV}{\mathrm{O}}_3$ undergoes a first structural phase transition to $P{2}_1/a$ at $T_{\mathrm{OO}}=180\mathrm{K}$, where a $G$-type orbital ordered state develops. At $T_{\mathrm{S}}=75\mathrm{K}$, a change back to Pnma occurs, and the establishment of $C$-type orbital order takes place. The ${\mathrm{V}}^{3+}$ ions order antiferromagnetically with a magnetic propagation vector $\mathbf{k}=0$ below $T_{\mathrm{N}1}=105\mathrm{K}$, while the $\mathrm{T}{\mathrm{m}}^{3+}$ sublattice orders at $T_{\mathrm{N}2}=20\mathrm{K}$ following the same propagation vector. Between $T_{\mathrm{N}1}$ and $T_{\mathrm{S}}$, a coexistence of $G$-type ($P{2}_1/a$) and $C$-type (Pnma) orbital ordered states exists. The $P{2}_1/a$ phase is magnetically separated into two fractions, which adopt a $C_x{C}_{y}0$ and $G_{x}00$ coupling, respectively, while the Pnma volume fraction follows a $0G_{y}0$ magnetic structure. At $T_{\mathrm{N}2}$, the appearance of the Tm sublattice magnetization ($0C_{y}0$) leads to a spin flop transition of the V sublattice from $0G_{y}0$ to $G_{x}00$. The results are presented and analyzed in the general context of the series of $R\mathrm{V}{\mathrm{O}}_3$ compounds, and they are used to discuss recent magnetization results.

Experimentally observed ultrafast all-optical magnetization reversal in ferrimagnetic metals and heterostructures based on antiferromagnetically coupled ferromagnetic $d$- and $f$-metallic layers relies on intricate energy and angular momentum flow between electrons, phonons, and spins. Here we treat the problem of angular momentum transfer in the course of ultrafast laser-induced dynamics in a ferrimagnetic metallic system using microscopical approach based on the system of rate equations. We show that the magnetization reversal is supported by a coupling of $d$ and $f$ subsystems to delocalized $s$ or $p$ electrons. The latter can transfer spin between the two subsystems in an incoherent way owing to the $(s;p)\text{−}(d;f)$ exchange scattering. Since the effect of the external excitation in this process is reduced to the transient heating of the mobile electron subsystem, we also discuss the possibility to trigger the magnetization reversal by applying a voltage bias pulse to antiferromagnetically coupled metallic ferromagnetic layers embedded in point contact or tunneling structures. We argue that such devices allow controlling reversal with high accuracy. We also suggest using the anomalous Hall effect to register the reversal, thus playing a role of reading probes.