The van-der-Waals gap of iron chalcogenide superconductors can be intercalated with a variety of inorganic and organic compounds that modify the electron doping level of the iron layers. In ${\mathrm{Li}}_x{\left({\mathrm{C}}_3{\mathrm{N}}_2{\mathrm{H}}_{10}\right)}_{0.37}\mathrm{FeSe}$, a dome in the superconducting transition temperature $T_c$ has been reported to occur in the doping range of $x=0.06$ to $x=0.68$. We use a combination of density functional theory and spin fluctuation theory to capture the evolution of superconducting transition temperatures theoretically. We clearly demonstrate how the changing electronic structure supports an increasing superconducting $T_c$. The suppression of $T_c$ at high doping levels can, however, only be understood by analyzing the magnetic tendencies, which evolve from stripe-type at low doping to bicollinear at high doping.

We have performed magnetic susceptibility $\left(\chi \right)$, heat capacity $\left(C_p\right)$, and nuclear magnetic resonance (NMR) measurements on the layered compound ${\mathrm{Na}}_2{\mathrm{Mn}}_3{\mathrm{O}}_7$ in which the magnetic ${\mathrm{Mn}}^{4+}$ ions constitute a maple-leaf lattice. At high temperature, $\chi$ follows the Curie-Weiss (CW) law with CW temperature as high as 152 K and ${\mathrm{Mn}}^{4+}$ ions are found to be in the high-spin state $\left(S=\frac{3}{2}\right)$. With decreasing temperature, $\chi$ passes through a broad maximum at 115 K and decreases rapidly at low temperature. A similar broad peak appears at around 62 K in the magnetic contribution to heat capacity $\left(C_m\right)$. The temperature dependence of $\chi$ and $C_m$ are the characteristics of an one-dimensional Heisenberg antiferromagnetic spin chain with a nonmagnetic ground state. The obtained values of spin-gap energy $\left({\mathrm{\Delta }}_s\right)$ and nearest-neighbor antiferromagnetic exchange coupling $\left(J\right)$ are 28 and 27 K, respectively. There is no evidence of a long-range magnetic ordering down to 2 K in heat capacity data, in agreement with spin-singlet ground state. These observations are further corroborated by ${}^{23}\mathrm{Na}$ NMR studies through measurements of both the NMR shift and the spin-lattice relaxation rate.

The charge density wave (CDW) instability in $1T\text{−}{\mathrm{TiSe}}_2$ is revisited to investigate its electronic and structural origins using angle resolved photoemission spectroscopy (ARPES), neutron diffraction, and density functional theory (DFT) calculations. The evidence for Jahn-Teller (JT)-like distortions in the CDW state is provided from the local structure analysis that shows splitting of the Ti-Se bonds into short and long. The magnitude of the split is inconsistent with the commonly accepted Di Salvo model for the structural distortions in the CDW phase. From the APRES data, it is deduced that a nontrivial temperature-dependent energy shift of the Se $4p$ valence bands occurs, but is absent for the Ti $3d$ conduction bands. Collectively, these observations suggest that a JT-like mechanism is most likely central to the CDW instability in $1T\text{−}{\mathrm{TiSe}}_2$.

We report the magnetic structure of nominally 10% Cd-doped ${\mathrm{CeIrIn}}_5,\mathrm{CeIr}{\left({\mathrm{In}}_{0.9}{\mathrm{Cd}}_{0.1}\right)}_5$, determined by elastic neutron scattering. Magnetic intensity was observed only at the ordering wave vector $Q_{\text{AF}}=(1/2,1/2,1/2)$, commensurate with the crystal lattice. A staggered moment of 0.47(3)${\mu }_B$ at 1.8 K resides on the Ce ion. The magnetic moments are found to be aligned along the crystallographic $c$ axis. This is further confirmed by magnetic susceptibility data, which suggest the $c$ axis to be the easy magnetic axis. The determined magnetic structure is strikingly different from the incommensurate antiferromagnetic ordering of the closely related compound ${\mathrm{CeRhIn}}_5$, in which the magnetic moments are antiferromagnetically aligned within the tetragonal basal plane.

The Bloch theorem mathematically proves that in a periodic crystal, electrons can acquire a negative mass. The present work aims to provide a physical understanding for why this is so. We successively analyze the consequences of the threefold orbital valence-state coupling to (i) a nondegenerate orbital level in the conduction band, (ii) a threefold orbital level in the conduction band, and (iii) spin states through spin-orbit interaction. We show that it is not at all trivial for valence electrons to acquire a negative mass for whatever their momentum with respect to the crystal axes: It is necessary to not only have a coupling to a degenerate orbital conduction level but also a symmetry breaking of the threefold valence subspace by the spin quantization axis, as induced by spin-orbit interaction. Due to the relativistic origin of this interaction, the existence of negative valence masses thus constitutes an unexpected signature of quantum relativity.

We report a photoluminescence study of high-quality Ge samples at temperatures $12\mathrm{K}\le T\le 295\mathrm{K}$, over a spectral range that covers phonon-assisted emission from the indirect gap (between the lowest conduction band at the $L$ point of the Brillouin zone and the top of the valence band at the Γ point), as well as direct gap emission (from the local minimum of the conduction band at the Γ point). The spectra display a rich structure with a rapidly changing line shape as a function of $T$. A theory is developed to account for the experimental results using analytical expressions for the contributions from LA, TO, LO, and TA phonons. Coupling of states exactly at the Γ and $L$ points is forbidden by symmetry for the latter two phonon modes, but becomes allowed for nearby states and can be accounted for using wave-vector dependent deformation potentials. Excellent agreement is obtained between predicted and observed photoluminescence line shapes. A decomposition of the predicted signal in terms of the different phonon contributions implies that near-room temperature indirect optical absorption and emission are dominated by “forbidden” processes, and the deformation potentials for allowed processes are smaller than previously assumed.

Weak-field magnetoresistance has seen a revived interest due to the distinct role played by the momentum-space Berry curvature of Bloch electrons. While most previous studies in this regard focus on the inter-scattering motion of semiclassical Bloch electrons in electromagnetic fields, the intra-scattering effects of the semiclassical dynamics augmented by the Berry curvature, magnetic moment, and shift vector on the magnetoresistance have been largely overlooked. Here, we uncover that these intra-scattering effects, which are neglected in the field-independent relaxation time approximation to the Boltzmann collision integral, can be as important as the inter-scattering ones. Concrete calculations on the two-dimensional gapped Dirac model show that the sign of the negative linear magnetoresistance given by the Berry curvature alone is reversed when one considers the magnetic moment and shift vector.

A possible metal-insulator transition in the $\mathrm{C}{\mathrm{r}}_{1-x}\mathrm{A}{\mathrm{l}}_x\mathrm{N}$ alloy system is explored using electronic transport and optical measurements on epitaxial $\mathrm{C}{\mathrm{r}}_{1-x}\mathrm{A}{\mathrm{l}}_x\mathrm{N}\left(001\right)/\mathrm{MgO}\left(001\right)$ layers which form a cubic solid-solution rocksalt structure for $0\le x\le 0.75$. The optical band gap $E_g$ which is due to direct Γ-point transitions from the $\mathrm{N}\text{−}2p$-like valence-band top to the majority spin $e_g$ conduction-band minimum increases approximately linearly with Al content: $E_g=0.64+2.81x\mathrm{eV}$. The room-temperature resistivity increases by a factor of 400, from $\rho =0.070$ to 26 Ω cm for $x=0–0.46$, indicating an increasing carrier localization due to a decreasing Cr-Cr nearest-neighbor hybridization. However, ρ drops again to $<0.38\mathrm{\Omega }\mathrm{cm}$ for $x\ge 0.62$ which is attributed to midgap states that are also detected by optical absorption. Cooling to 77 K causes an increase in ρ by approximately one order of magnitude for all alloys $\left(x=0–0.75\right)$, suggesting that the decreasing localization length is compensated by an increasing density of states at the Fermi level. Oxygen exposure reduces the resistivity for $x\le 0.46$ due to the formation of a conductive surface oxynitride, suggesting $n$-type doping by oxygen. X-ray-diffraction analyses including reciprocal-space maps indicate that the relaxed lattice constant decreases from $a_o=0.4175$ to 0.4089 nm for $x=0.0$ to 0.75, and that an increase to $x=0.85$ leads to a breakdown of the cubic solid solution which is attributed to the nucleation of wurtzite-structure nanocrystals, causing also a steep increase in $E_g$ and in the low-temperature resistivity due to enhanced carrier localization. The subgap refractive index at $h\nu =0.5\mathrm{eV}$ decreases from 4.8 for CrN to 3.1 for $\mathrm{C}{\mathrm{r}}_{0.25}\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{N}$, due to the decreasing valence electron concentration. The overall results demonstrate that the quasibinary $\mathrm{C}{\mathrm{r}}_{1-x}\mathrm{A}{\mathrm{l}}_x\mathrm{N}$ system in the rocksalt structure retains the semiconducting properties of its parent binaries, cubic rocksalt structure CrN and AlN.

Despite its importance to experiments, numerical simulations, and the development of theoretical models, self-averaging in many-body quantum systems out of equilibrium remains underinvestigated. Usually, in the chaotic regime, self-averaging is taken for granted. The numerical and analytical results presented here force us to rethink these expectations. They demonstrate that self-averaging properties depend on the quantity and also on the time scale considered. We show analytically that the survival probability in chaotic systems is not self-averaging at any time scale, even when evolved under full random matrices. We also analyze the participation ratio, Rényi entropies, the spin autocorrelation function from experiments with cold atoms, and the connected spin-spin correlation function from experiments with ion traps. We find that self-averaging holds at short times for the quantities that are local in space, while at long times, self-averaging applies for quantities that are local in time. Various behaviors are revealed at intermediate time scales.

The emergence of complex modulated structures in the magnetization pattern of thin films is a well-established experimental phenomenology caused by the frustrating effects of competing interactions. Using a coarse-grained version of the Ising ferromagnet with dipolar interactions, we develop a method that uses the information from the microscopic Hamiltonian to predict the specific topological phases present in the temperature-external magnetic field phase diagram. This is done by the combination of mean-field variational calculations and the renormalization group equations from the classical theory of two-dimensional melting. In this framework, we are able to distinguish when the orientational and translational symmetries are broken, discriminating between the ordered and disordered states of the system for all temperatures and fields. We observe that the reentrance developed by the $H-T$ phase diagrams in the regime of weak dipolar interactions is directly related with the appearance of anomalous topological transitions. These results motivate the realization of new experiments on magnetic thin films in order to explore the topological properties of the magnetic textures, allowing to identify new exotic phases in these materials.

Optical responses of ferromagnetic materials with spin gauge field that drives intrinsic spin current is theoretically studied. The conductivity tensor is calculated based on a linear response theory to the applied electric field taking account of the nonlinear effects of the spin gauge field to the second order. We consider the case where the spin gauge field is uniform, realized for spiral magnetization structure or uniform spin-orbit interaction. The spin gauge field, or an intrinsic spin current, turns out to give rise to anisotropic optical responses, which is expected to be useful for experimental detection of magnetization structures.

We obtain a quantum dimer model (QDM) containing a Rokhsar-Kivelson (RK) point expressed by spin-$\frac{1}{2}$ Heisenberg antiferromagnets on a diamond-like decorated square lattice. This lattice has macroscopically degenerated nonmagnetic ground states, which are equivalent to the Hilbert space of a square-lattice QDM. Then, a square-lattice QDM containing the RK point as a second-order effective Hamiltonian is obtained by introducing further neighbor couplings as perturbation. Our model can provide a new method for the experimental realization of a resonating valence bond state in the QDM.

We report the existence of a spin-orbit torque generated from charge to spin conversion in perpendicularly magnetized ferromagnetic material through spin-torque ferromagnetic resonance (ST-FMR) measurement. We fabricated a trilayer structure consisting of a Cu spacer layer sandwiched between an in-plane magnetized FeB layer and perpendicularly magnetized Co/Ni multilayers. From the ferromagnetic resonance under DC bias current application, we observed a significant linewidth modulation in the FeB layer attributed to the charge to spin conversion generated from Co/Ni multilayers. In-plane angular dependence and the magnetization polarity dependence of the linewidth modulation reveal that two types of charge to spin conversion with different symmetry coexist in the single Co/Ni multilayers.

The effectiveness of the variational approach à la Feynman is proved in the spin-boson model, i.e., the simplest realization of the Caldeira-Leggett model able to reveal the quantum phase transition from delocalized to localized states and the quantum dissipation and decoherence effects induced by a heat bath. After exactly eliminating the bath degrees of freedom, we propose a trial, nonlocal in time, interaction between the spin and itself simulating the coupling of the two-level system with the bosonic bath. It stems from a Hamiltonian where the spin is linearly coupled to a finite number of harmonic oscillators whose frequencies and coupling strengths are variationally determined. We show that a very limited number of these fictitious modes is enough to get a remarkable agreement, up to very low temperatures, with the data obtained by using an approximation-free Monte Carlo approach, predicting (1) in the Ohmic regime, a Berezinski-Thouless-Kosterlitz quantum phase transition exhibiting the typical universal jump at the critical value; and (2) in the sub-Ohmic regime ($s\le 0.5$), mean-field quantum phase transitions, with logarithmic corrections for $s=0.5$.

Recently, machine learning was applied to extract both the normal and the anomalous components of the self-energy from photoemission data at the antinodal points in $\mathrm{Bi}\text{−}$ based cuprate high-temperature superconductors [Y. Yamaji et al., arXiv:1903.08060]. It was argued that both components do show prominent peaks near $50\mathrm{meV}$, which hold information about the pairing glue, but the peaks are hidden in the actual data, which measure only the total self-energy. We analyze the self-energy within an effective fermion-boson theory. We show that soft thermal fluctuations give rise to peaks in both components of the self-energy at a frequency comparable to the superconducting gap, while they cancel in the total self-energy, all irrespective of the nature of the pairing boson. However, in the quantum limit $T\to 0$ prominent peaks survive only for a very restricted subclass of pairing interactions. We argue that the way to potentially nail down the pairing boson is to determine the thermal evolution of the peaks.

We propose a machine-learning-based method for efficiently predicting atomic diffusivity in crystals, in which the potential energy surface (PES) of a diffusion carrier is partially evaluated by first-principles calculations. To preferentially evaluate the region of interest governing the atomic diffusivity, a statistical PES model based on a Gaussian process (GP-PES) is constructed and updated iteratively from known information on already-computed potential energies. In the proposed method, all local energy minima (stable and metastable sites) and elementary processes of atomic diffusion (atomic jumps) are explored on the predictive mean of the GP-PES. The uncertainty of jump frequency in each elementary process is then estimated on the basis of the variance of the GP-PES. The acquisition function determining the next grid point to be computed is designed to reflect the impacts of the uncertainties of jump frequencies on the uncertainty of the macroscopic atomic diffusivity. A numerical solution of the master equation is here employed to readily estimate the atomic diffusivity, which enables us to design the acquisition function reflecting the centrality of each elementary process.

A generalization of the Drude conductivity for systems which are exposed to periodic driving is presented. The probe bias is treated perturbatively by using the Kubo formula, whereas the external driving is included nonperturbatively using the Floquet theory. Using a different type of four-times Green functions disorder is approached diagrammatically, yielding a fully analytical expression for the Floquet-Drude conductivity. Furthermore, the Floquet Fermi “golden rule” is generalized to $t\text{−}{t}^{\prime }$ Floquet states, connecting the Floquet-Dyson series with scattering theory for Floquet states. It is shown that a low-energy approximation like the parabolic one fails significantly to give the correct conductivity in a system under driving.

Phase transitions for the two-dimensional antiferromagnetic triangular Heisenberg model in a uniform field are primarily analyzed using dynamical scaling analysis. The system has $Z_3$ and $O\left(2\right)$ symmetries. The transition temperatures and types of transition are investigated using the relaxation of order parameters and the asymptotic behaviors of relaxation time around transition temperatures. In the low-field range, the order parameters for the $Z_3$ and $O\left(2\right)$ symmetries exhibit the behavior typical of a second-order transition and Kosterlitz-Thouless (KT) transition, respectively, with distinct transition temperatures. In the high-field range, an order parameter for the $Z_3$ symmetry exhibits the behavior of a second-order transition, while another order parameter related to the components perpendicular to the field demonstrates that the relaxation time diverges with an asymptotic form, indicating a second-order transition rather than a KT transition. These transition temperatures are estimated to be close. Critical exponents for these two order parameters are estimated separately by calculating the relaxation of fluctuations in the high-field range. It was confirmed that the critical exponents for both of these order parameters change continuously as a function of field, and they are distinct from each other.

Spin transfer torque in a two-dimensional electron gas system without space inversion symmetry was theoretically investigated by solving the Pauli-Schrödinger equation for the itinerant electrons inside magnetic domain walls. Due to the presence of the Rashba spin-orbit coupling induced by the broken inversion symmetry, the spin transfer torque is chiral and the nonadiabaticity, which is defined to measure the relative importance of the nonadiabatic fieldlike torque to the adiabatic dampinglike torque, exhibits an inverse power-law decay as the domain wall width is increased. This algebraic decay is much slower than the exponential decay observed for systems without the Rashba spin-orbit coupling, and may find applications in innovative design of spintronic devices utilising magnetic topological textures such as magnetic domain walls and skyrmions.