We have investigated the nature of the short-range magnetic phase region of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{IrO}}_4$ ($0\le x\le 0.14$). This so-called spin-orbit Mott insulator's predicted transition to superconductivity with electron doping has remained elusive. Instead, short-range magnetism prevails over an extensive doping range up to the La solubility limit. We find evidence for quantum critical-like fluctuations of the short-range magnetic phase, in both a scaling relation of the ac susceptibility $\chi T^{\alpha }$ and the magnetization $M{T}^{\alpha -1}$ with $\alpha =0.4–0.5$, as well as in the time-field scaling ($t/H^{\gamma }$) of the $\mu \mathrm{SR}$ asymmetry function. Our combined study of ac susceptibility and $\mu \mathrm{SR}$, on the one hand, uncovers a two-stage weak symmetry breaking to spin freezing or proximate spin glass and to a subsequent spin glass within the short-range magnetic phase. On the other hand, we demonstrate that magnetic correlations become more static above $x=0.06$ in spite of an apparent dilution of magnetic moments, indicative of enhanced inhomogeneities in a higher La-doping regime.

Chiral domain walls in ultrathin perpendicularly magnetized layers have a Néel structure stabilized by a Dzyaloshinskii-Moriya interaction (DMI) that is generated at the interface between the ferromagnet and a heavy metal. Different interface materials or properties are required above and below a ferromagnetic film in order to generate the structural inversion asymmetry needed to ensure that the DMI arising at the two interfaces does not cancel. Here we report on the magnetic properties of epitaxial Pt/Co/${\mathrm{Au}}_x{\mathrm{Pt}}_{1-x}$ trilayers grown by sputtering onto sapphire substrates with 0.6 nm thick Co. As $x$ rises from 0 to 1, a structural inversion asymmetry is progressively generated. We characterize the epilayer structure with x-ray diffraction and cross-sectional transmission electron microscopy, revealing (111) stacking. The saturation magnetization falls as the proximity magnetization in Pt is reduced, whilst the perpendicular magnetic anisotropy $K_{\mathrm{u}}$ rises. The micromagnetic DMI strength $D$ was determined using the bubble expansion technique and also rises from a negligible value when $x=0$ to $\sim 1$ mJ/${\mathrm{m}}^2$ for $x=1$. The depinning field at which field-driven domain wall motion crosses from the creep to the depinning regime rises from $\sim 40$ to $\sim 70$ mT, attributed to greater spatial fluctuations of the domain wall energy with increasing Au concentration. Meanwhile, the increase in DMI causes the Walker field to rise from $\sim 10$ to $\sim 280$ mT, meaning that only in the $x=1$ sample is the steady flow regime accessible. The full dependence of domain wall velocity on driving field bears little resemblance to the prediction of a simple one-dimensional model, but can be described very well using micromagnetic simulations with a realistic model of disorder. These reveal a rise in Gilbert damping as $x$ increases.

We use phenomenological symmetry arguments to demonstrate that while chiral magnetic (Dzyaloshinskii-Moriya) interactions are conventionally restricted by symmetry to appear in a limited set of noncentrosymmetric materials, they may arise in a material with any symmetry when coupled to a strain field. We derive point-group specific free-energy functionals that capture the relationship between an applied strain field and chiral magnetic couplings, demonstrating how strain may offer highly selective control over magnetic textures. Finally, we discuss several examples of common strain configurations that may lead to out-of-plane modulation in the magnetic moment of a quasi-two-dimensional film as required for applications in magnetic devices.

We report on the superconducting properties of noncentrosymmetric superconductor LaPtGe, which crystallizes in noncentrosymmetric $\alpha -{\mathrm{ThSi}}_2$ structure. The magnetization, resistivity, and specific-heat measurements confirm that LaPtGe is a type-II bulk superconductor with a transition temperature $T_c=3.05\pm 0.05$ K. Muon-spin relaxation/rotation measurements suggest that time-reversal symmetry is preserved in the superconducting ground state.

When the electron density of highly crystalline thin films is tuned by chemical doping or ionic-liquid gating, interesting effects appear including unconventional superconductivity, sizable spin-orbit coupling, competition with charge-density waves, and a debated low-temperature metallic state that seems to avoid the superconducting or insulating fate of standard two-dimensional electron systems. Some experiments also find a marked tendency to a negative electronic compressibility. We suggest that this indicates an inclination for electronic phase separation resulting in a nanoscopic inhomogeneity. Although the mild modulation of the inhomogeneous landscape is compatible with a high electron mobility in the metallic state, this intrinsically inhomogeneous character is highlighted by the peculiar behavior of the metal-to-superconductor transition. Modeling the system with superconducting puddles embedded in a metallic matrix, we fit the peculiar resistance vs temperature curves of systems like ${\mathrm{TiSe}}_2$, ${\mathrm{MoS}}_2$, and ZrNCl. In this framework also the low-temperature debated metallic state finds a natural explanation in terms of the pristine metallic background embedding nonpercolating superconducting clusters. An intrinsically inhomogeneous character naturally raises the question of the formation mechanism(s). We propose a mechanism based on the interplay between electrons and the charges of the gating ionic liquid.

Eight years since the discovery of the binary antiferromagnetic (AFM) quasicrystal approximants $\mathrm{C}{\mathrm{d}}_{6}R$ [R. Tamura et al., Phys. Rev. B 82, 220201 (2010)], we report the observation of an AFM transition in quasicrystal approximants Au-Al-R ($R=\mathrm{Gd}$ and Tb), which represent examples of AFM in ternary quasicrystal approximants. Magnetic susceptibility, magnetization, and specific heat coherently show that the Au-Al-R ($R=\mathrm{Gd}$ and Tb) approximants undergo an AFM transition at $T_{\mathrm{N}}=9.4$ and 11.8 K, respectively. The AFM transitions are found near the phase boundary of the wide single-phase region where the chemical disorder is substantially low. This finding of ternary AFM approximants has far deeper implications compared with that of the binary AFM approximants: It now indicates the possibility to synthesize new AFM approximants in many other systems by the appropriate replacement of Cd with two other elements, and thus enables exploration of various magnetic orders over AFM spin icosahedra in solids, in particular, their evolution as a function of the electron-per-atom (e/a) ratio. Moreover, the finding provides us a missing route to synthesize an AFM quasicrystal by similar composition tuning.

Recent experiments show how a bilayer graphene twisted around a certain magic angle becomes superconducting as it is doped into a region with approximate flat bands. We investigate the mean-field $s$-wave superconducting state in such a system and show how the state evolves as the twist angle is tuned, and as a function of the doping level. We argue that part of the experimental findings could well be understood to result from an attractive electron-electron interaction mediated by electron-phonon coupling, but the flat-band nature of the excitation spectrum also makes the superconductivity quite unusual. For example, as the flat-band states are highly localized around certain spots in the structure, also the superconducting order parameter becomes strongly inhomogeneous.

The origin of the structural phase transition of $\mathrm{TaT}{\mathrm{e}}_2$ has been investigated through the measurements of the temperature-dependent Raman spectroscopy and first-principles calculations. Based on the experimental and the calculation results, some interpretations are present. Firstly, abnormal redshifts of few Raman peaks are observed around the structural distortion temperature $T_S\sim 170\mathrm{K}$, which is thought to originate from the clustering of Ta atoms below $T_S$. Secondly, new modes of Raman spectrum below $T_S$ are present, which is suggested to be related to the possible transition of charge density wave (CDW) in $\mathrm{TaT}{\mathrm{e}}_2$ from the room temperature to the low temperature. The ground state of the $\mathrm{TaT}{\mathrm{e}}_2$ is the special charge modulation, which is related to the formation of Ta-Ta dimer structure at low temperature. All explanations have been supported by first-principles calculations. Our results suggest that $\mathrm{TaT}{\mathrm{e}}_2$ is an alternative platform to explore the charge-lattice coupling with rich structural and charge configurations in $M{X}_2$ systems, where $M$ is the transition metal ion (Nb, Ta, Mo, etc.) and $X$ is the chalcogenide one (S, Se, or Te).

We introduce a solvable model of driven fermions that elucidates the role of the localization transition in driven disordered magnets, as used in the context of dynamic nuclear polarization. Instead of spins, we study a set of noninteracting fermions that are coupled locally to nuclear spins and tend to hyperpolarize them. The induced hyperpolarization is a fingerprint of the driven steady state of the fermions, which undergo an Anderson localization (AL) transition upon increasing the disorder. Our central result is that the maximal hyperpolarization level is always found close to the localization transition. In the limit of small nuclear moments the maximum is pinned to the transition, and the hyperpolarization is strongly enhanced by multifractal correlations in the critical state of the nearly localized driven system, its magnitude reflecting multifractal scaling.

Temperature rise ($\mathrm{\Delta }T$) associated with shear banding of metallic glasses is of great importance for their performance. However, experimental measurement of $\mathrm{\Delta }T$ is difficult due to temporal and spatial localization of shear bands and, as a result, our understanding of the mechanism of $\mathrm{\Delta }T$ is limited. Here, based on molecular dynamics simulations, we observe a spectrum of $\mathrm{\Delta }T$, which depends on both sample size and strain rate, in the shear bands of CuZr metallic glass under tension. More importantly, we find that the maximum sliding velocity of the shear bands correlates linearly with the corresponding $\mathrm{\Delta }T$, ranging from $\sim 25$ K up to near the melting point for the samples studied. Taking heat diffusion into account, we expect $\mathrm{\Delta }T$ to be lower than 25 K for the lower end of sliding velocity. At high temperature, shear band bifurcation and/or multiplication can occur as a negative feedback mechanism that prevents temperature rising well above the melting point.

We present a nonequilibrium quantum field theory approach to the initial-state dynamics of spin models based on two-particle irreducible (2PI) functional integral techniques. It employs a mapping of spins to Schwinger bosons for arbitrary spin interactions and spin lengths. At next-to-leading order (NLO) in an expansion in the number of field components, a wide range of nonperturbative dynamical phenomena are shown to be captured, including relaxation of magnetization in a 3D long-range interacting system with quenched disorder, different relaxation behavior on both sides of a quantum phase transition, and the crossover from relaxation to arrest of dynamics in a disordered spin chain previously shown to exhibit many-body localization. Where applicable, we employ alternative state-of-the-art techniques and find rather good agreement with our 2PI NLO results. As our method can handle large system sizes and converges relatively quickly to its thermodynamic limit, it opens the possibility to study these phenomena in higher dimensions in regimes in which no other efficient methods exist. Furthermore, the approach to classical dynamics can be investigated as the spin length is increased.

${}^{55}\mathrm{Mn}$ and ${}^{27}\mathrm{Al}$ NMR measurements are performed on a single crystal of ${\mathrm{YMn}}_4{\mathrm{Al}}_8$, which is known as one of the rare examples of pseudogap opening in transition intermetallic compounds, to investigate the formation of this pseudogap. The NMR spectrum, Knight shift, linewidth, and spin-lattice relaxation rate $1/T_1$ are measured as functions of the temperature down to 4 K for the $c$ axis, which is parallel and perpendicular to a magnetic field of 8 T. The temperature dependencies of the Knight shifts and $1/\left(T_{1}T\right)$ for both NMRs as well as the magnetic susceptibility show significant decrease as the temperature decreases below room temperature, indicating the opening of a pseudogap down to ∼100 K, where these data become nearly constant, confirming that the formation of the spin pseudogap is incomplete and a finite density of states remains below. By analyzing ${}^{55}K\left(T\right)$ and ${}^{27}K\left(T\right)$ thoroughly, we decompose the orbital and spin shifts for both Mn and Al sites, from which the anisotropies of the Korringa ratios are extracted. The Korringa ratios at the Mn sites are temperature dependent and highly anisotropic above 100 K, and then decrease approaching unity and become isotropic and constant below 100 K. From the temperature dependencies of $1/\left(T_{1}T\right)$ for both NMRs, the pseudogap temperature, $T^{*}$ is found to be approximately 270 K. From the anisotropy of ${}^{55}1/\left(T_{1}T\right)$ during the opening of the pseudogap, we observed that the antiferromagnetic spin fluctuation is anisotropic and stronger by up to $\sim 4.2$ times in the basal plane perpendicular to the quasi-one-dimensional (1D) manganese chain. This significant anisotropy must originate from the 1D nature of the Mn magnetic interaction.

We present experimental results for the in-plane resistivity of the electron-doped cuprate superconductor $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{C}{\mathrm{e}}_x\mathrm{Cu}{\mathrm{O}}_4$ above its transition temperature $T_c$ as a function of Ce doping $x$ and temperature. For the doping $x$ between 0.11 and 0.17, where $T_c$ varies from 30 K ($x=0.11$) to 5 K ($x=0.17$), we find that the resistivity shows a $T^2$ behavior for all values of doping over the measurement range from 70 to 250 K. The coefficient of the $T^2$ resistivity term decreases with increasing $x$ following the trend in $T_c$. We analyze our data theoretically and posit that $n$-type cuprates are better thought of as strange metals. Although the quadratic temperature dependence appears to be in naive agreement with the Fermi-liquid (FL) expectations, the facts that the measured resistivity is large and approximate $T^2$ scattering dominates the resistivity even up to 400 K argue against a standard normal-metal FL picture being applicable. We discuss possible origins of the strange-metal behavior.

We study the static charge correlation function in a one-band model on a square lattice. The Hamiltonian consists of effective hoppings of the electrons between the lattice sites and the Heisenberg Hamiltonian. Approximating the irreducible charge correlation function by a single bubble yields the ladder approximation for the charge correlation function. In this approximation, one finds, in general, three charge instabilities—two of them are due to nesting, the third one is the flux phase instability. Since these instabilities cannot explain the experiments in hole-doped cuprates, we have included in the irreducible charge correlation function also Aslamasov-Larkin (AL) diagrams where charge fluctuations interact with products of spin fluctuations. We then find at high temperatures a nematic or $d$-wave Pomeranchuk instability with a very small momentum. Its transition temperature decreases roughly linearly with doping in the underdoped region and vanishes near optimal doping. Decreasing the temperature further, a secondary axial charge-density wave (CDW) instability appears with mainly $d$-wave symmetry and a wave vector somewhat larger than the distance between nearest-neighbor hot spots. At still lower temperatures, the diagonal flux phase instability emerges. A closer look shows that the AL diagrams enhance mainly axial and not diagonal charge fluctuations in our one-band model. This is the main reason why axial and not diagonal instabilities are the leading ones in agreement with experiment. The two instabilities due to nesting vanish already at very low temperatures and do not play any major role in the phase diagram. Remarkable is that the nematic and the axial CDW instabilities show a large reentrant behavior.

We derive a model for accurately reproducing the broadband infrared optical response of common engineering metals. Here we use “broadband infrared” to refer to wavelengths beginning at $1\mu \mathrm{m}$ and extending to approximately $100\mu \mathrm{m}$ or more. The model generalizes the Drude theory to account for sources of anomalous intraband absorption. This is accomplished by modeling these sources as elements of disorder that introduce diffusive perturbations into the local field. In the stationary setting, the memory kernel description of the field relaxation leads to a fractional derivative with an order that corresponds to the memory decay strength. We demonstrate that this model is fully consistent with the Drude theory and that the semiclassical theory is recovered under requisite assumptions on the field relaxation or radiative wavelength. The anomalous model component is shown to reproduce empirically observed anomalous absorption that has been traditionally corrected by models possessing empirical components that have not been formally derived. Results are presented for several common metals for which the proposed model accurately reproduces the data over the entire modeled bandwidth. A comparative analysis confirms that the proposed model represents a robust, high fidelity alternative to previously proposed models that do not capture the observed physical response over the extended infrared range.

By means of the first-principles density functional theory ($\mathrm{DFT}+U$) calculations and experiments, we investigate the role of dilution on the structural, magnetic, electronic, and optical properties of the antiferromagnetic (AFM) spinel ${\mathrm{Co}}_3{\mathrm{O}}_4$ having Néel temperature $\left(T_{\text{N}}\right)\sim 30$ K. As the octahedral cobalt site of spinel lattice is diluted with Ge, Al, Ti, Ru, and Sn cations, we observe a substantial increase in the size of the unit cell as well as destruction of the long-range magnetic ordering with a spin-orbit compensation effect. The ferrimagnetic ordering in diluted inverse spinels such as ${\mathrm{Co}}_2\mathrm{\Sigma }{\mathrm{O}}_4$ ($\mathrm{\Sigma }$ = Ti and Sn) emerges due to the difference in the magnetic moments of two sublattices A (3.87 ${\mu }_{\text{B}}$) and B (4.16 ${\mu }_{\text{B}}$ for ${\mathrm{Co}}_2{\mathrm{SnO}}_4$ and 5.19 ${\mu }_{\text{B}}$ for ${\mathrm{Co}}_2{\mathrm{TiO}}_4$). Experiments and DFT calculations indicate antiferromagnetic configuration for ${\mathrm{Co}}_3{\mathrm{O}}_4$, ${\mathrm{Co}}_2{\mathrm{AlO}}_4$ ($T_{\text{N}}\sim 4.8$ K) spinels with an equal and opposite moment of $\sim 2.60$ ${\mu }_{\text{B}}$ in tetrahedral sites of divalent Co ions and negligible contribution from trivalent B-site Co due to the complete filling of $t_{2g}$ levels having a giant crystal field of $\sim 2.5$ and 1.8 eV, respectively. However, in ${\mathrm{Co}}_2{\mathrm{GeO}}_4$ ($T_{\text{N}}\sim 20.4$ K) case AFM behavior originates due to the opposite spins at octahedral sites of divalent Co ions. The remaining spinels ${\mathrm{Co}}_2{\mathrm{TiO}}_4$ ($T_{\text{N}}\sim 47.8$ K), ${\mathrm{Co}}_2{\mathrm{RuO}}_4$ ($T_{\text{N}}\sim 16$ K), and ${\mathrm{Co}}_2{\mathrm{SnO}}_4$ ($T_{\text{N}}\sim 41$ K) are more favorable to ferrimagnetic structure as evident from our magnetization measurements with a different temperature dependence of magnetic moments A(T) and B(T) at tetrahedral A and octahedral B sites, respectively. The variation in the energy band gap ($E_g=1.68\to 3.28$ eV for ${\mathrm{Co}}_2{\mathrm{RuO}}_4\to {\mathrm{Co}}_2{\mathrm{GeO}}_4$) obtained from $\mathrm{DFT}+U$ calculations are in good agreement with our experimental results ($E_g=1.52\to 3.16$ eV) obtained from the diffusive reflectance spectroscopy. The extent of exchange splitting ${\mathrm{\Delta }}_{\text{EX}}^{e_g}$ of tetrahedral ${\mathrm{Co}}^{2+}$ varies between 1.8 and 1.3 eV for ${\mathrm{Co}}_3{\mathrm{O}}_4$ and ${\mathrm{Co}}_2{\mathrm{AlO}}_4$, respectively. However, ${\mathrm{\Delta }}_{\text{EX}}^{t_{2g}}$ exhibits a decreasing trend ($5.2\to 3.6$ eV for ${\mathrm{Co}}_3{\mathrm{O}}_4\to {\mathrm{Co}}_2{\mathrm{SnO}}_4$) with increasing the lattice parameter, except for cobalt-orthogermanate ${\mathrm{Co}}_2{\mathrm{GeO}}_4$.

We study a three-orbital Hubbard model with negative Hund's coupling in infinite dimensions, combining dynamical mean-field theory with continuous time quantum Monte Carlo simulations. This model, which is relevant for the description of alkali-doped fullerides, has previously been shown to exhibit a spontaneous orbital-selective Mott phase in the vicinity of the superconducting phase. Calculating the pair potential and double occupancy in each orbital, we study the competition between different homogeneous ordered states and determine the corresponding finite-temperature phase diagram of the model. We identify two distinct types of spontaneous orbital-selective Mott states and show that an orbital-selective $s$-wave superconducting state with one superconducting and two metallic orbitals is spontaneously realized between the conventional $s$-wave superconducting phase and these two kinds of spontaneously orbital-selective Mott states.

We consider symmetry-protected topological (SPT) phases in two dimensions protected by linear subsystem symmetries, i.e., those that act along rigid lines. There is a distinction between a “strong” subsystem SPT phase and a “weak” one, which is composed of decoupled one-dimensional SPTs with global symmetries. We propose a natural definition for strong equivalence of such phases, in terms of a linearly symmetric local unitary transformation, under which a weak subsystem SPT is equivalent to the trivial phase. This leads to a number of distinct equivalence classes of strong subsystem SPTs, which we show are in one-to-one correspondence with elements of the group $\mathcal{C}\left[G_s\right]={\mathcal{H}}^2[G_s^2,U\left(1\right)]/{\left\{{\mathcal{H}}^2[G_s,U\left(1\right)]\right\}}^3$, where $G_s$ is the finite Abelian on-site symmetry group. We also show that strong subsystem SPTs by our classification necessarily exhibit a spurious topological entanglement entropy on a cylinder.

The electronic stopping power of nickel for protons and alpha particles at velocities below and around the Fermi velocity has been obtained to high accuracy using time-dependent density functional theory. For the wide range of projectile velocities considered, we observed different regimes of electronic stopping due to the alternative participation of $s$- and $d$-band electrons. Despite the sharp discontinuity in the electronic density of states near the Fermi energy characteristic of the nickel band structure, we do not find an anomalous nonlinear electronic stopping power limit as a function of velocity. However, we find a crossover region above $v=0.15\mathrm{a}.\mathrm{u}.$ both for protons and alpha particles, related to the increase in participating host electrons and, in the case of alpha particles, to an increase of the charge state. We compare our calculated results with widely available experimental data and analyze the low velocity limits in the context of Lindhard's linear response theory and previous nonlinear density functional calculations. The comparison shows good accord with the lowest velocity experiments available. This may indicate that the adiabatic local density approximation is already a good theory to calculate electronic stopping power in materials at low velocity.

By analyzing the many-body problem for nonrelativistic electrons strongly coupled to photon modes of a microcavity I derive the exact momentum/force balance equation for cavity quantum electrodynamics. Implications of this equation for the electron self-energy and the exchange-correlation potential of the quantum electrodynamic time-dependent density functional (QED-TDDFT) are discussed. In particular I generalize the concept of $\mathrm{\Phi }$ derivability to construct approximations which ensure the correct momentum balance. It is shown that a recently proposed optimized effective potential approximation for QED-TDDFT is conserving, and its possible improvements are discussed.