Our magnetotransport measurements of quantum Hall stripes in a high-quality GaAs quantum well in a slightly tilted magnetic field reveal that the orientation of stripes can be changed by temperature. Field-cooling and field-warming measurements, as well as observation of hysteresis at intermediate temperatures allow us to conclude that the observed temperature-induced reorientation of stripes is owing to the existence of two distinct minima in the symmetry-breaking potential. We also find that the native symmetry-breaking mechanism does not depend on temperature and that low-temperature magnetotransport data should be treated with caution as they do not necessarily reveal the true ground state, even in the absence of hysteresis.

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in $\mathrm{NbNiT}{\mathrm{e}}_2$, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the $\mathrm{NbNiT}{\mathrm{e}}_2$ structure in space-group 53 (Pmna)—having an inversion center arising from two glide planes and one mirror plane with a twofold rotation and screw axis—a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space-group $28\left(Pma2\right)$, eliminating one glide plane along $c$, the associated rotation and screw axis, and the inversion center. As a result, 20 Weyl points emerge, including four type-II Weyl points in the Γ-$X$ direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.

Twin growth in hexagonal close-packed zirconium is investigated at the atomic scale by modeling the various disconnections that can exist on twin boundaries. Thanks to a coupling with elasticity theory, core energies are extracted from atomistic simulations, and the formation energy of isolated disconnection dipoles is defined. For twin systems where several disconnections can exist, because of this core contribution, the most stable disconnection is not always the one with the smallest Burgers vector. Crystallographic parameters of the disconnection with the lowest formation energy correlate well with twin modes observed experimentally. On the other hand, disconnection migration, characterized here by computing their migration energy and Peierls stress, does not appear critical for twin-mode selection.

The absorption peaks at $\sim 3200-3300\mathrm{c}{\mathrm{m}}^{-1}$ in $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ crystal that were previously observed and assigned to hydrogen interstitial were reinvestigated by first-principles local vibration mode calculations. Based on our calculations, we found that the vibration of hydrogen interstitial is not consistent with the observed polarized IR absorption peaks at $\sim 3200-3300\mathrm{c}{\mathrm{m}}^{-1}$ for two reasons: (1) the orientation of the O-H bond is not aligned with the direction of the oscillator observed and (2) the calculated vibrational frequency is much lower than the observed values. The calculated formation energies showed that interstitial hydrogen can strongly bind with aluminum vacancy, forming $\left[{\mathrm{H}}_n\text{−}{V}_{\mathrm{Al}}\right]$ complex defects. The complexes have the O-H bond in the direction aligned with the direction of the oscillator. The vibration frequencies of the complex defects are also close to the observed value. Therefore, we assign the experimentally observed $\sim 3200-3300\mathrm{c}{\mathrm{m}}^{-1}$ modes to $\left[{\mathrm{H}}_n\text{−}{V}_{\mathrm{Al}}\right]$ complex defects.

We study the nonequilibrium dynamics of second-order classical phase transitions in a simplified Ginzburg-Landau model using the Fokker-Planck formalism. In particular, we focus on deriving the Kibble-Zurek scaling laws that dictate the dependence of spatial correlations on the quench rate. In the limiting cases of overdamped and underdamped dynamics, the Fokker-Planck method confirms the theoretical predictions of the Kibble-Zurek scaling theory. The developed framework is computationally efficient, enables the prediction of finite-size scaling functions, and is applicable to microscopic models as well as their hydrodynamic approximations. We demonstrate this extended range of applicability by analyzing the nonequilibrium linear to zigzag structural phase transition in ion Coulomb crystals confined in a trap with periodic boundary conditions.

Small angle neutron scattering measurements have been performed to study the thermodynamic stability of skyrmion-lattice phases in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. We found that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be stabilized through different thermal histories; by cooling from the paramagnetic phase under finite magnetic field, the SkX(2) phase is selected. On the other hand, the ${30}^{\circ }$-rotated SkX(1) phase becomes dominant by heating the sample from the ordered conical phase under finite field. This difference in stabilization is surprisingly similar to the irreversibility observed in spin glasses. The zero-field cooling results in the coexistence of the two phases. It is further found that once one of the skyrmion-lattice phases is formed, it is hardly destabilized. This indicates unusual thermal stability of the two skyrmion-lattice phases originating from an unexpectedly large energy barrier between them.

We report the magnetotransport properties of ferromagnet (FM)/antiferromagnet (AFM) ${\mathrm{Fe}}_2\mathrm{CrSi}/{\mathrm{Ru}}_2\mathrm{MnGe}$ epitaxial bilayers using current-in-plane configurations. Above the critical thickness of the ${\mathrm{Ru}}_2\mathrm{MnGe}$ layer to induce exchange bias, symmetric and asymmetric curves were observed in response to the direction of FM magnetocrystalline anisotropy. Because each magnetoresistance curve showed full and partial AFM rotation, the magnetoresistance curves imply the impact of the ${\mathrm{Fe}}_2\mathrm{CrSi}$ magnetocrystalline anisotropy to govern the AFM rotation. The maximum magnitude of the angular-dependent resistance-change ratio of the bilayers is more than an order of magnitude larger than that of single-layer ${\mathrm{Fe}}_2\mathrm{CrSi}$ films, resulting from the reorientation of AFM spins via the FM rotation. These results highlight the essential role of controlling the AFM rotation and reveal a facile approach to detect the AFM moment even in current-in-plane configurations in FM/AFM bilayers.

Following the idea that hydrogen-rich compounds might be high-$T_c$ superconductors at high pressures, and the very recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high $T_c=203\mathrm{K}$, an ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero-point energy). Our calculations indicate that the C2/m phase of germane is a superconductor with $T_c=67\mathrm{K}$ at 280 GPa. Germane is found to become thermodynamically unstable to decomposition to hydrogen and the compound ${\mathrm{Ge}}_3{\mathrm{H}}_{11}$ at pressures above 300 GPa. ${\mathrm{Ge}}_3{\mathrm{H}}_{11}$ with space group $I\overline{4}m2$ is found to become stable at above 285 GPa with $T_c=43\mathrm{K}$. We find that the pressure-induced phase stability of germanium hydrides is distinct from analogous isoelectronic systems, e.g., Si hydrides and Sn hydrides. Superconductivity stems from large electron-phonon coupling associated with the wagging, bending, and stretching intermediate-frequency modes derived mainly from hydrogen.

Synchrotron x-ray diffraction and Raman spectroscopy have been used to study the chemical reactions of molecular hydrogen $\left({\mathrm{H}}_2\right)$ with sulfur (S) at high pressures. We find theoretically predicted Cccm and $Im\overline{3}m$ ${\mathrm{H}}_3\mathrm{S}$ to be the reaction products at 50 and 140 GPa, respectively. $Im\overline{3}m$ ${\mathrm{H}}_3\mathrm{S}$ is a stable crystalline phase above 140 GPa and it transforms to $R3m{\mathrm{H}}_3\mathrm{S}$ on pressure release below 140 GPa. The latter phase is (meta)stable down to at least 70 GPa where it transforms to Cccm ${\mathrm{H}}_3\mathrm{S}$ upon annealing ($T<1300$ K) to overcome the kinetic hindrance. Cccm ${\mathrm{H}}_3\mathrm{S}$ has an extended structure with symmetric hydrogen bonds at 50 GPa, and upon decompression it experiences a transformation to a molecular mixed ${\mathrm{H}}_2\mathrm{S}\text{−}{\mathrm{H}}_2$ structure below 40 GPa without any apparent change in the crystal symmetry.

Electrical resistivities can be different for charge currents traveling parallel or perpendicular to the magnetization in magnetically ordered conductors or semiconductors, resulting in the well-known planar Hall effect and anisotropic magnetoresistance. Here we study the analogous anisotropic magnetotransport behavior for magnons in a magnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Electrical and thermal magnon injection, and electrical detection methods, are used at room temperature with transverse and longitudinal geometries to measure the magnon planar Hall effect and anisotropic magnetoresistance, respectively. We observe that the relative difference between magnon current conductivities parallel and perpendicular to the magnetization, with respect to the average magnon conductivity, i.e., $|({\sigma }_{\parallel }^{\mathrm{m}}-{\sigma }_{\perp }^{\mathrm{m}})/{\sigma }_0^{\mathrm{m}}|$, is approximately 5% with the majority of the measured devices showing ${\sigma }_{\perp }^{\mathrm{m}}>{\sigma }_{\parallel }^{\mathrm{m}}$.

In ${\mathrm{Sr}}_2{\mathrm{NiIrO}}_6$, a metastable compound, the magnetic structure could be elucidated. The magnetic susceptibility data of the compound suggests a three-dimensional Heisenberg behavior. Moreover, neutron diffraction reveals a novel incommensurate magnetic order, requiring exchange interactions beyond second neighbor to stabilize this phase. An incommensurate propagation vector of $\mathbf{k}=(0,k,k)$ with $k=0.356$ was observed in such a pseudocubic system. This can be an example of how to induce an incommensurate ground state and a possible type-II multiferroicity in double perovskites.

Effective non-Hermitian Hamiltonians are obtained to describe coherent perfect absorbing and lasing boundary conditions. $\mathcal{PT}$ symmetry of the Hamiltonians enables us to design configurations which perfectly absorb at multiple frequencies. Broadened and flat perfect absorption is predicted at the exceptional point of $\mathcal{PT}$-symmetry breaking while, for a particular case, absorption is enhanced with the use of gain. The aforementioned phenomena are illustrated for acoustic scattering through Helmholtz resonators revealing how tailoring the non-Hermiticity of acoustic metamaterials leads to novel mechanisms for enhanced absorption.

We explore routes to realize electrically driven Bose-Einstein condensation of magnons in insulating antiferromagnets. Even in insulating antiferromagnets, the localized spins can strongly couple to itinerant spins in adjacent metals via spin-transfer torque and spin pumping. We describe the formation of steady-state magnon condensates controlled by a spin accumulation polarized along the staggered field in an adjacent normal metal. Two types of magnons, which carry opposite magnetic moments, exist in antiferromagnets. Consequently, and in contrast to ferromagnets, Bose-Einstein condensation can occur for either sign of the spin accumulation. This condensation may occur even at room temperature when the interaction with the normal metal is fast compared to the relaxation processes within the antiferromagnet. In antiferromagnets, the operating frequencies of the condensate are orders of magnitude higher than in ferromagnets.

An oxyborate $\mathrm{Co}{}_2{\mathrm{AlBO}}_5$ belonging to the ludwigite family is investigated using structural, thermodynamic, dielectric, and magnetic measurements. Magnetic measurements indicate that this system is seen to exhibit long-range magnetic ordering at $T{}_N=42\mathrm{K}$, signatures of which are also seen in the specific heat, dielectric susceptibility, and lattice parameters. The absence of a structural phase transition down to the lowest measured temperatures distinguishes it from the more extensively investigated Fe-based ludwigites. At low temperatures, the system is seen to stabilize in a reentrant superspin glass phase at $T{}_G=10.6\mathrm{K}$ from within the magnetically ordered state. This ground state is also characterized by magnetic-field-induced metamagnetic transitions, which at the lowest measured temperatures exhibit a number of sharp magnetization steps, reminiscent of that observed in the mixed valent manganites.

Lorentz-invariance-violating Weyl and Dirac fermions have recently attracted intensive interests as new types of particles beyond high-energy physics, and they demonstrate novel physical phenomena such as angle-dependent chiral anomaly and topological Lifshitz transitions. Here, we predict the existence of Lorentz-invariance-violating Dirac fermions in the ${\mathrm{YPd}}_2\mathrm{Sn}$ class of Heusler alloys that emerge at the boundary between the electronlike and holelike pockets in the Brillouin zone, based on first-principles electronic structure calculations. In combination with the fact that this class of materials was reported to be superconductors, the ${\mathrm{YPd}}_2\mathrm{Sn}$ class provides an appropriate platform for studying exotic physical properties distinguished from conventional Dirac fermions, especially for realizing possible topological superconductivity.

We consider the capacitive interaction between a charge qubit and a sensor quantum dot (SQD) perturbatively to the second order of their coupling constant at zero temperature by utilizing the method of nonequilibrium Green's functions together with infinite-$U$ Lacroix approximation and employing Majorana fermion representation for qubit isospin operators. The effect of back-actions on dynamics of the system is taken into account by calculating the self-energies and the Green's functions in a self-consistent manner. To demonstrate the applicability of the method, we investigate relevant physical quantities of the system at zero and finite bias voltages. In the regime of weak SQD-qubit coupling, we find a linear relation between the stationary-state expectation values of the third component of the qubit isospin vector, $〈{\tau }_3〉$, and the differential conductance of the SQD. Furthermore, our numerical results predict that the effect of SQD-qubit coupling on differential conductance of the SQD should be maximized at zero bias voltage. Moreover, we obtain an analytical expression to describe the behavior of the differential conductance of the SQD with respect to the qubit parameters. Our results at zero bias voltage are consistent with the results of numerical renormalization group method.

Determination and characterization of criticality in two-dimensional (2D) quantum many-body systems is one of the most important challenges and problems in modern physics. Based on the scaling theory in one-dimensional (1D) systems and tensor networks, we propose an efficient scheme to access the criticality of 2D quantum states. We demonstrate that the boundary state of a critical infinite projected entangled pair state can be described by 1D conformal field theory, which provides a solid indicator that allows one to identify and characterize the criticality of 2D states. Our scheme is verified on the resonating valence bond (RVB) states on kagome and square lattices, where the boundary state of the honeycomb RVB is found to be described by a $c=1$ conformal field theory. We apply our scheme also to the ground state of the spin-1/2 XXZ model on honeycomb lattice, illustrating the difficulties of standard variational tensor network approaches to study the critical ground states. Our scheme is of high versatility and flexibility, and can be further applied to investigate the quantum criticality in many other phenomena, such as finite-temperature and topological phase transitions.

We study the spin-orbit interaction (SOI) in InAs/GaSb and InAs quantum wells. We show through temperature- and gate-dependent magnetotransport measurements of weak antilocalization that the dominant spin-orbit relaxation mechanism in our low-mobility heterostructures is Elliott-Yafet and not Dyakonov-Perel in the form of the Rashba or Dresselhaus SOI as previously suggested. We compare our findings with recent work on this material system and show that the SOI length lies within the same range. The SOI length may be controlled using an electrostatic gate, opening up prospects for developing spintronic applications.

Titanium dioxide is one of the most promising semiconductors for photocatalytic splitting of water for hydrogen. The mixed rutile/anatase system shows even more favorable photocatalytic properties than the pristine ones. Band offset is a key factor that determines the photocatalytic activity of the mixed phase. However, the type of band alignment and the value of the band offset are still under debate both experimentally and theoretically. The difficulty of determining the band offset by commonly used core-level alignment calculation lies in the different symmetry and large lattice mismatch between the two phases. Here, we adopt our recently developed three-step method, which can overcome the lattice mismatch problem, to study the band offset with high accuracy. In the calculation, we used an intermediate phase $\mathrm{Ti}{\mathrm{O}}_2\mathrm{II}$ to build superlattice models of rutile(101)//$\mathrm{Ti}{\mathrm{O}}_2\mathrm{II}\left(001\right)$ and $\mathrm{Ti}{\mathrm{O}}_2\mathrm{II}\left(100\right)$//anatase(112) to determine the core-level alignment. Our studies show a type-II, staggered band alignment, with the valence band maximum (VBM) of rutile 0.80 eV above that of anatase, in agreement with recent experimental results. We further analyzed the electronic structure of the two phases, and found that the band offsets of the VBM originate from both the electrostatic interaction and electronic hybridization in rutile and anatase, which contribute 0.36 eV and 0.44 eV, respectively.

We report a theoretical study of electron transport along helical organic molecules subject to an external electric field which is perpendicular to molecular helix axis. Our results reveal that topological states can appear in single-helical molecules as well as double-stranded DNA under the perpendicular electric field. In particular, a topological charge pumping can be realized by rotating the electric field in the transverse plane, where during each pumping cycle, an integer number of electrons can transport across the helical molecules at zero bias voltage, with pumped current being quantized. The quantized current constitutes multiple plateaus by scanning the Fermi energy as well as the bias voltage, and holds for various model parameters, since the edge states are topologically protected. These results could pave the way to explore topological states and quantized current in the biological systems and the helical molecules, and help in designing stable molecular devices.