Recently, Haldane's conjecture about SU(2) chains has been generalized to $\mathrm{SU}\left(n\right)$ chains in the symmetric representations. For a rank-$p$ representation, a gapless phase is predicted when $p$ and $n$ are coprime; otherwise, a finite energy gap is present above the ground state. In this work we offer an intuitive explanation of this behavior based on fractional topological excitations, which are able to generate a mass gap except when $p$ and $n$ have no common divisor. This is a generalization of an older work in SU(2), which explains the generation of the Haldane gap in terms of merons in the O(3) nonlinear $\sigma$ model.

The carrier mobility and dynamics of nonequilibrium carriers in Eu-doped GaN have been determined by above-band-gap, optically excited, time-resolved terahertz spectroscopy and compared with that of undoped GaN. The carrier mobility of GaN:Eu was an order of magnitude smaller than that of a similarly grown undoped GaN layer. Two time constants were observed in the carrier relaxation dynamics for GaN:Eu. A fast one (2.2 ps), which we related to an efficient nonradiative carrier trap, and a slower one (∼85 ps) related to Eu-related traps. Ultrafast photoluminescence spectroscopy shows that Eu-related emission is observed within 100 ps after a pulsed excitation. These values set an upper value to the energy-transfer time from the Eu-related trap to $\mathrm{E}{\mathrm{u}}^{3+}$ ion and underlie the high efficiency of the excitation of $\mathrm{E}{\mathrm{u}}^{3+}$ ions in GaN.

We use scanning tunneling microscopy (STM) to study charge density wave (CDW) states in the rare-earth ditelluride, ${\mathrm{CeTe}}_2$. Our STM measurements surprisingly detect a unidirectional CDW with $q\sim 0.28a^{*}$, which differs from previous experimental and first-principles studies of the rare-earth ditellurides, and which is very close to what is found in experimental measurements of the related rare-earth tritellurides. Furthermore, in the vicinity of an extended subsurface defect, we find spatially-separated as well as spatially-coexisting unidirectional CDWs at the surface of ${\mathrm{CeTe}}_2$. We quantify the nanoscale strain and its variations induced by this defect, and establish a correlation between local lattice strain and the locally-established CDW states; this suggests that lattice strain plays an important role in determining the specific characteristics of the established CDW state. Our measurements probe the fundamental properties of a weakly-bound two-dimensional Te sheet, which experimental and theoretical work has previously established as the fundamental component driving much of the essential physics in both the rare-earth di- and tritelluride compounds.

Semiconductor quantum dots provide a spin-coupled system of an electron and nuclei via enhanced hyperfine interaction. We showed that the nuclear spin polarization in single quantum dots can have three stable branches under a longitudinal magnetic field. The states were accompanied by hysteresis loops around the boundaries of each branch with a change in the excitation condition. To explain these findings, we incorporated the electron spin relaxation caused by the nuclear spin fluctuation into the previously studied dynamic nuclear spin polarization mechanism. The model reproduces the features of nuclear spin polarization and the associated strong reduction in the observed electron spin polarization, and can refer to the tristability of nuclear spin polarization.

We investigate light transport in three-dimensional scattering systems generated according to subrandom sequences and demonstrate subdiffusive behavior typical of wave transport in disordered systems at the critical point for metal-insulator-transitions but in a wider range of parameters. Specifically, we solve the electromagnetic multiple scattering problem using the Green's matrix spectral theory for aperiodic systems based on Halton, Sobol, and stochastic Latin hypercube sequences. By studying the Thouless number and the level-spacing statistics of the electromagnetic resonances at different scattering densities, we demonstrate that light transport in deterministic Halton and Sobol structures exhibit multifractal behavior characterized by inverse power-law scaling of level-spacing statistics across a wide range of densities of dipolar scatterers. On the other hand, this scenario is absent in the stochastic Latin hypercube array, whose behavior instead resembles standard diffusion in uniform random media. Our findings establish a connection between subdiffusion and subrandom aperiodic order and provide a strategy to design three-dimensional structures with multifractal properties over a broad spectral range.

Layered misfit cobaltate ${\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]}_{0.62}\left[{\mathrm{CoO}}_2\right]$, which emerged as an important thermoelectric material [A. C. Masset et al. Phys. Rev. B 62, 166 (2000)], has been explored extensively in the last decade for the exact mechanism behind its high Seebeck coefficient. Its complex crystal and electronic structures have inhibited consensus among such investigations. This situation has arisen mainly due to difficulties in accurate identification of the chemical state, spin state, and site symmetries in its two subsystems (rocksalt $\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]$ and triangular $\left[{\mathrm{CoO}}_2\right]$). By employing resonant photoemission spectroscopy and x-ray absorption spectroscopy along with charge transfer multiplet simulations (at the Co ions), we have successfully identified the site symmetries, valencies, and spin states of the Co in both layers. Our site-symmetry observations explain the experimental value of the high Seebeck coefficient and also confirm that the carriers hop within the rocksalt layer, which is in contrast to earlier reports where hopping within triangular ${\mathrm{CoO}}_2$ layer has been held responsible for the large Seebeck coefficient.

The chaotic dynamics describes how a small change of initial conditions can result in a large difference in a deterministic nonlinear system, i.e., the “butterfly effect.” Through a combination of experimental and theoretical analysis, here we showed unambiguously that the deformation of metallic glasses (MGs) exhibits such an effect where the experimentally observed plasticity displays a large plasticity fluctuation under the normally same conditions. The “butterfly effect” for the plasticity of MGs is related to the chaotic dynamics of a shear band, evidenced by the existence of a torus destroyed phase diagram, a positive Lyapunov exponent, and a fractional Lyapunov dimension. Physically, the chaotic shear-band dynamics arises from the interplay between structural disordering and temperature rise within the shear band, which could lead to an uncertainty in the appearance of the critical condition for runaway shear banding events. Our results provide a perspective on the plasticity of MGs from the viewpoint of complex dynamics and are also important for evaluating the plastic deformation properties of MGs in practical applications.

We investigate thermodynamic properties of FeSe under in-plane magnetic fields using torque magnetometry, specific heat, and magnetocaloric measurements. Below the upper critical field $H_{c2}$, we observed the field induced anomalies at $H_1\sim 15$ T and $H_2\sim 22$ T near $H\parallel ab$ and below a characteristic temperature $T^{*}\sim$ 2 K. The transition magnetic fields $H_1$ and $H_2$ exhibit negligible dependence on both temperature and field orientation. This contrasts to the strong temperature and angle dependence of $H_{c2}$, suggesting that these anomalies are attributed to the field induced phase transitions, originating from the inherent spin-density-wave instability of quasipaticles near the superconducting gap minima or possible Flude-Ferrell-Larkin-Ovchinnikov state in the highly spin-polarized Fermi surfaces. Our observations imply that FeSe, an atypical multiband superconductor with extremely small Fermi energies, represents a unique model system for stabilizing unusual superconducting orders beyond the Pauli limit.

The noncentrosymmetric superconductor $\alpha$-PdBi is a candidate material for the realization of topological superconductivity. Here, we present a detailed de Haas–van Alphen (dHvA) study together with band-structure calculations within the framework of density functional theory. The rich dHvA spectra are a manifestation of the 13 bands that cross the Fermi energy $E_F$. We find excellent agreement between calculated and experimentally observed dHvA frequencies with moderately enhanced effective masses. One of the bands crossing $E_F$, the so-called $\alpha$ band, exhibits topological character with Weyl nodes lying 43 meV below $E_F$.

We investigate the influence of a static, uncorrelated distribution of charged impurities on the spectrum of bound excitons in the copper oxide ${\mathrm{Cu}}_2\mathrm{O}$. We show that the statistical distribution of Stark shifts and ionization rates leads to the vanishing of Rydberg resonances into an apparent continuum. The appearance of additional absorption lines due to the broken rotational symmetry, together with spatially inhomogeneous Stark shifts, leads to a modification of the observed line shapes that agrees qualitatively with the changes observed in the experiment.

We report first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon $\mathrm{\Gamma }$ point, electrons with energies less than 200 meV are scattered predominantly by ${\mathrm{LA}}^{\prime }$ and ${\mathrm{TA}}^{\prime }$ modes while higher-energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters into $K$-point phonon optical modes due to their strong coupling with electrons and their high energies. A Drude model is used to calculate photoconductivity for bilayer graphene with different doping levels. Good agreement with prior experimental trends for both the real and imaginary components of photoconductivity confirms the model's applicability. The effects of doping levels and electron-phonon scattering on photoconductivity are analyzed. We also extract acoustic and optical deformation potentials from average scattering rates obtained from density functional theory (DFT) calculations and compare associated photoconductivity calculations with DFT results. The comparison indicates that momentum-dependent electron-phonon scattering potentials are required to provide accurate predictions.

Finite-temperature density functional theory has become a standard tool for first-principles calculations of the properties of warm dense matter (WDM) relevant to high-energy-density physics (HEDP) applications. Here we present theoretical grounds of thermal hybrid exchange-correlation (XC) functionals within the generalized Mermin-Kohn-Sham scheme for an improved description of WDM. Building on the previously developed KSDT (Karasiev-Sjostrom-Dufty-Trickey) [Karasiev et al., Phys. Rev. Lett. 112, 076403 (2014)] local density approximation (LDA) and the KDT16 (Karasiev-Dufty-Trickey 2016) [Karasiev et al., Phys. Rev. Lett. 120, 076401 (2018)] generalized-gradient approximation (GGA) XC free-energy density functionals, we construct a novel thermal hybrid XC functional, referred to here as KDT0. The KDT0 model at low temperature reduces to the popular ground-state PBE0 hybrid due to properties of the used KDT16 density functional approximation. Application to static calculations of electronic band gap and band structure at a wide range of temperatures for various systems of interest to HEDP show that KDT0 provides a significant improvement to the lower LDA and GGA rung XC functionals and to the ground-state PBE0 hybrid.

Multilayers composed of aluminum (Al) and platinum (Pt) exhibit a nonmonotonic trend in thermal resistance with bilayer thickness as measured by time domain thermoreflectance. The thermal resistance initially increases with reduced bilayer thickness only to reach a maximum and then decrease with further shrinking of the multilayer period. These observations are attributed to the evolving impact of an intermixed amorphous complexion approximately 10 nm in thickness, which forms at each boundary between Al- and Pt-rich layers. Scanning transmission electron microscopy combined with energy dispersive x-ray spectroscopy find that the elemental composition of the complexion varies based on bilayer periodicity as does the fraction of the multilayer composed of this interlayer. These variations in complexion mitigate boundary scattering within the multilayers as shown by electronic transport calculations employing density-functional theory and nonequilibrium Green's functions on amorphous structures obtained via finite temperature molecular dynamics. The lessening of boundary scattering reduces the total resistance to thermal transport leading to the observed nonmonotonic trend thereby highlighting the central role of complexion on thermal transport within reactive metal multilayers.

The active harnessing of quantum resources in engineered quantum devices poses unprecedented requirements on device control. Besides the residual interaction with the environment, causing environment-induced decoherence, uncontrolled parameters in the system itself—disorder—remain as a substantial factor limiting the precision and thus the performance of devices. These perturbations may arise, for instance, due to imperfect sample production, stray fields, or finite accuracy of control electronics. Disorder-dressed quantum evolution means a unifying framework, based on quantum master equations, to analyze how these detrimental influences cause deviations from the desired system dynamics. This description may thus contribute to unveiling and mitigating disorder effects towards robust schemes. To demonstrate the broad scope of this framework, we evaluate two distinct scenarios: a central spin immersed in an isotropic spin bath and a random mass Dirac particle. In the former scenario, we demonstrate how the disorder average reflects purity oscillations, indicating the time- and state-dependent severity of the disorder impact. In the latter scenario, we discuss disorder-induced backscattering and disorder-induced Zitterbewegung as consequences of the breakup of spin-momentum locking.

We use series expansion methods to investigate the phase diagram of a spin-1/2 Heisenberg antiferromagnet on the triangular lattice with first- and second-neighbor interactions. We find regions of ${120}^{\circ }$ and collinear “stripe” order, separated by an intermediate nonmagnetic phase, in agreement with results obtained previously using other methods. Magnon excitation spectra in the ordered phases are also obtained, and discussed.

Based on first-principles calculations, we investigated how the intrinsic anomalous Hall conductivity (IAHC) can be tailored in ${\mathrm{Mn}}_3\mathrm{GaN}$ with a noncollinear magnetic ground state. It is observed that IAHC displays a strong dependence with respect to the in-plane magnetization direction between the ${\mathrm{\Gamma }}_{5g}$ and ${\mathrm{\Gamma }}_{4g}$ magnetic configurations, and it is also susceptible to the biaxial strains, where symmetry plays an essential role. Particularly, we demonstrated that significant enhancement of IAHC can be achieved by tuning the relative positions of Weyl nodes with respect to the Fermi energy, resulting in large anomalous Nernst effect.

We thoroughly study the zero temperature properties of monoaxial chiral spin systems under a tilted magnetic field. The magnetic phase diagram includes two kinds of continuous phase transitions and one discontinuous phase transition. We clarify the properties of the phase transition in terms of the helical wave picture and the particle (soliton) picture and also in terms of the interaction between solitons. The interacting-soliton picture properly describes most of the discontinuous phase transition. In addition, we investigate several instabilities of the modulated structures such as an isolated soliton and the surface modulation, since their instabilities should be important to the changes of the magnetic properties. For this purpose, we perform the analyses based on the energy landscape as well as the excitation spectrum; the former approach gives an intuitive interpretation. We clarify the mechanisms of the instabilities through these analyses and draw the stability lines of solitons in the magnetic phase diagram.

We perform a numerical simulation of a three-band Hubbard model with two ${\mathrm{CuO}}_2$ planes and a single CuO chain layer for YBCO cuprates. The spin-fluctuation mediated pairing interaction is computed within the multiband random-phase approximation, and its pairing eigenvalues and eigenfunctions are solved as a function of chain state filling factor $n_c$. We find that for the intrinsic value of $n_c$ in YBCO samples, one obtains the usual $d$-wave pairing symmetry. However, if we dope the chain layers with holes, while keeping the plane states doping fixed, the leading pairing symmetry solution becomes an unconventional $f$-wave symmetry. The mechanism behind the $f$-wave pairing is the competition between the plane states antiferromagnetic nesting and chain states' uniaxial nesting. We also find that the pairing strength is strongly augmented when the flat band bottom of the chain state passes the Fermi level for a fixed plane states doping. The $f$-wave pairing symmetry can be realized in YBCO cuprates in future experiments where the self-doping mechanism between the chain and plane states can be minimized so that only chain state can be selectively hole doped.

In a typical experiment in magnonics, thin films are magnetized in plane and spin waves only carry angular momentum along their spatial propagation direction. Motivated by the experiments of Bozhko et al. [Phys. Rev. Research 2, 023324 (2020)], we show theoretically that for obliquely magnetized thin films, exchange-dipolar spin waves are accompanied by a transverse spin current. We propose an experiment to electrically detect this transverse spin current with Pt strips on top of an yttrium iron garnet film, by comparing the induced spin current for spin waves with opposite momenta. We predict the relative difference to be of the order ${10}^{-4}$, for magnetic fields tilted at least ${30}^{\circ }$ out of plane. This transverse spin current is the result of the long-range dipole-dipole interaction and the inversion symmetry breaking of the interface.