We report the dynamics of soft phonon modes and their role toward various structural transformations in Aurivillius materials by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen $\mathrm{B}{\mathrm{i}}_4\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{12}$ as a model system and identified soft phonon modes associated with the paraelectric tetragonal to the ferroelectric monoclinic transition. Three soft phonon modes have been discovered that exhibit a strong temperature dependence. We have determined that the anharmonicity in Bi-O bonds plays a significant role in phonon softening, and that Bi cations play an important role in the emergence of ferroelectricity.

We investigate the self-compensation mechanism in phosphorus-doped CdTe. The formation energies, charge transition levels, and defect states of several P-related point defects susceptible to cause self-compensation are addressed by first-principles calculations. Moreover, we assess the influence of the spin-orbit coupling and supercell-size effects on the stability of AX centers, which are believed to be responsible for most of the self-compensation. We report an improved result for the lowest-energy configuration of the P interstitial $\left({\text{P}}_{\text{i}}\right)$ and find that the self-compensation mechanism is not due to the formation of AX centers. Under Te-rich growth conditions, $\left({\text{P}}_{\text{i}}\right)$ exhibits a formation energy lower than the substitutional acceptor $\left({\text{P}}_{\text{Te}}\right)$ when the Fermi level is near the valence band, acting as compensating donor, while, for Cd-rich growth conditions, our results suggest that p-type doping is limited by the formation of $({\text{P}}_{\text{Te}}-{\text{V}}_{\text{Te}})$ complexes.

We present results of electronic structure calculations of the bulk properties of CrAs. The crystalline structures of CrAs are investigated by the use of ab initio calculations with an unbiased swarm structural search under ambient and high pressure. Both the double helimagnetic and nonmagnetic phases in the neutron experiment are obtained and the pressure dependence of the magnetic phase transition is predicted. The $Pnma$ structure is the most stable structure below 33.1 GPa, meanwhile a magnetic phase transition of helimagnetic$\to$antiferromagnetic$\to$nonmagnetic is observed. The system tends to be antiferromagnetic after the double helimagnetic state near the optimal superconductivity regime without a crystal phase transition, which suggests that antiferromagnetic correlations between neighboring exchange interactions may be essential for the emergence of superconductivity in CrAs rather than a structural phase transition. Furthermore, the present results show that the $Pnma$ phase undergoes a pressure-induced phase transition to a cubic $P{2}_{1}3$ phase at 33.1 GPa. In addition, three low-enthalpy phases with a diverse magnetic order are identified as metastable states under ambient pressure. As the analytical results of the electronic structure, the three metastable structures, $P\overline{6}m2$ type A, $P\overline{6}m2$ type B, and $P4/nmm$, are antiferromagnetic, nonmagnetic, and ferromagnetic, respectively. An explanation for the interesting difference in the spin polarization is sought, in particular, for the covalent-ionic interactions between Cr and As atoms. The present results elucidate the structural and electronic properties of CrAs, and offer major implications as regards the diverse magnetic behaviors of CrAs.

Numerical simulations of magnetization reversal of a quantum uniaxial magnet under a swept magnetic field [Hatomura et al., Phys. Rev. Lett. 116, 037203 (2016)] are extended. In particular, how the “wave packet” describing the time evolution of the system is scattered in the successive avoided level crossings is investigated from the viewpoint of the distribution of the eigenstate populations. It is found that the peak of the distribution as a function of the magnetic field does not depend on spin-size $S$, which indicates that the delay of magnetization reversal due to the finite sweeping rate is the same in both the quantum and classical cases. The peculiar synchronized oscillations of all the spin components result in the beating of the spin length. Here, dissipative effects on this beating are studied by making use of the generalized Lindblad-type master equation. The corresponding experimental situations are also discussed in order to find conditions for experimental observations.

We theoretically study the levitation of a single magnetic domain nanosphere in an external static magnetic field. We show that, apart from the stability provided by the mechanical rotation of the nanomagnet (as in the classical Levitron), the quantum spin origin of its magnetization provides two additional mechanisms to stably levitate the system. Despite the Earnshaw theorem, such stable phases are present even in the absence of mechanical rotation. For large magnetic fields, the Larmor precession of the quantum magnetic moment stabilizes the system in full analogy with magnetic trapping of a neutral atom. For low magnetic fields, the magnetic anisotropy stabilizes the system via the Einstein–de Haas effect. These results are obtained with a linear stability analysis of a single magnetic domain rigid nanosphere with uniaxial anisotropy in a Ioffe-Pritchard magnetic field.

We find that a conventional $s$-wave superconductor in proximity to a three-dimensional Dirac material (3DDM), to all orders of perturbation in tunneling, induces a combination of $s$- and $p$-wave pairing only. We show that the Lorentz invariance of the superconducting pairing prevents the formation of Cooper pairs with higher orbital angular momenta in the 3DDM. This no-go theorem acquires stronger form when the probability of tunneling from the conventional superconductor to positive and negative energy states of 3DDM are equal. In this case, all the $p$-wave contribution except for the lowest order, identically vanish and hence we obtain an exact result for the induced $p$-wave superconductivity in 3DDM. Fierz decomposing the superconducting matrix we find that the temporal component of the vector superconducting order and the spatial components of the pseudovector order have odd-frequency pairing symmetry. We find that the latter is odd with respect to exchange of position and chirality of the electrons in the Cooper pair and is a spin-triplet, which is necessary for NMR detection of such an exotic pseudovector pairing. Moreover, we show that the tensorial order breaks into a polar vector and an axial vector and both of them have conventional pairing symmetry except for being a spin triplet. According to our study, for gapless 3DDM, the tensorial superconducting order will be the only order that is odd with respect to the chemical potential $\mu$. Therefore we predict that a transverse $p-n$ junction binds Majorana fermions. This effect can be used to control the neutral Majorana fermions with electric fields.

We combine spin-polarized scanning tunneling microscopy (SP-STM) and first-principles calculations to demonstrate the control of the wavelength of helical spin textures in Fe nanoislands by varying their atomic structure. We make use of the complexity of submonolayer growth of Fe on Cu(111) to prepare nanoislands characterized by different thickness and in-plane atomic structure. SP-STM results reveal that the magnetic states of different nanoislands are spin helices. The wavelength of the spin helices varies strongly. Calculations performed for Fe films with different thickness and in-plane atomic structure explain the strong variation of the wavelength by a subtle balance in the competition between ferromagnetic and antiferromagnetic exchange interactions. We identify the crucial role of the effectively enhanced weak antiferromagnetic exchange interactions between distant atoms.

We investigate ultrafast demagnetization due to electron-phonon interaction in a model band ferromagnet. We show that the microscopic mechanism behind the spin dynamics due to electron-phonon interaction is the interplay of scattering and the precession around momentum-dependent effective internal spin-orbit magnetic fields. The resulting magnetization dynamics can only be mimicked by spin-flip transitions if the spin precession around the internal fields is sufficiently fast (compared to the scattering time) so that it averages out the transverse spin components.

Recently, $\mathrm{Pb}\left(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3\text{−}\mathrm{PbZr}{\mathrm{O}}_3\text{−}\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}\mathrm{PbTi}{\mathrm{O}}_3$ (PIN-PZ-PMN-PT) relaxor single crystals were demonstrated to possess improved temperature-insensitive properties, which would be desirable for high-power device applications. The relaxor character associated with the development of local random fields (RFs) and a high rhombohedral-tetragonal (R-T) ferroelectric transition temperature $(T_{\mathrm{R}-\mathrm{T}}>120{}^{\circ }\mathrm{C})$ would be critical for the excellent properties. A significant effect of the chemical substitution of $\mathrm{I}{\mathrm{n}}^{3+}$ and $\mathrm{Z}{\mathrm{r}}^{4+}$ in PMN-PT to give PIN-PZ-PMN-PT is the development of local strain heterogeneity, which acts to suppress the development of macroscopic shear strains without suppressing the development of local ferroelectric moments and contribute substantially to the RFs in PIN-PZ-PMN-PT. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show that PIN-PZ-PMN-PT crystal has a quite different form of elastic anomaly due to Vogel-Fulcher freezing, rather than the a discrete cubic-$T$ transition seen in a single crystal of PMN-28PT. It also has high acoustic loss of the relaxor phase down to $T_{\mathrm{R}-\mathrm{T}}$. Analysis of piezoresponse force microscopy phase images at different temperatures provides a quantitative insight into the extent to which the RFs influence the microdomain structure and the short-range order correlation length $\langle \xi \rangle$.

Spin pumping has been studied within Ta / Ag / ${\text{Ni}}_{81}{\mathrm{Fe}}_{19}$ (0–5 nm) / Ag (6 nm) / ${\mathrm{Co}}_2\mathrm{MnGe}$ (5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the ${\mathrm{Co}}_2\mathrm{MnGe}$ source layer damping in vector network analyzer ferromagnetic resonance (VNA-FMR) experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and superparamagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR linewidth that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.

We study field-driven magnetic domain wall dynamics in garnet strips by large-scale three-dimensional micromagnetic simulations. The domain wall propagation velocity as a function of the applied field exhibits a low-field linear part terminated by a sudden velocity drop at a threshold field magnitude, related to the onset of excitations of internal degrees of freedom of the domain wall magnetization. By considering a wide range of strip thicknesses from 30 nm to $1.89\mu \mathrm{m}$, we find a nonmonotonic thickness dependence of the threshold field for the onset of this instability, proceeding via nucleation and propagation of Bloch lines within the domain wall. We identify a critical strip thickness above which the velocity drop is due to nucleation of horizontal Bloch lines, while for thinner strips and depending on the boundary conditions employed, either generation of vertical Bloch lines, or close-to-uniform precession of the domain wall internal magnetization takes place. For strips of intermediate thicknesses, the vertical Bloch lines assume a deformed structure due to demagnetizing fields at the strip surfaces, breaking the symmetry between the top and bottom faces of the strip, and resulting in circulating Bloch line dynamics along the perimeter of the domain wall.

We systematically investigate the magnetic properties and local structure of ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this $5d^4$ compound. The experimental investigation involved comparison of the magnetic properties and atomic imaging of (1) a slow-cooled crystal, (2) a crystal quenched from $900{}^{\circ }\mathrm{C}$ after growth, and (3) a crystal grown using a faster cooling rate during growth than the slow-cooled one. Atomic-scale imaging by scanning transmission electron microscopy (STEM) shows that quenching from $900{}^{\circ }\mathrm{C}$ introduces Ir-rich antiphase boundaries in the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. Compared to the slow-cooled crystals, ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment, while quenched crystals with Ir-rich antiphase boundary show a slightly suppressed moment. Our DFT and model magnetic Hamiltonian calculations suggest magnetic condensation is unlikely, as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local nonmagnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased orbital overlap and (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ and other $J=0$ systems.

In frustrated spinel oxide $\mathrm{Co}{\mathrm{V}}_2{\mathrm{O}}_4$, the knowledge of orbital, lattice, and spin structures has been fragmentary thus far. To investigate the structural and magnetic phase transitions in $\mathrm{Co}{\mathrm{V}}_2{\mathrm{O}}_4$, we performed high-resolution neutron powder diffraction, single crystal synchrotron radiation diffraction, and magnetization and specific heat measurements for high-quality samples. Extremely small crystal distortion from cubic to tetragonal phase ($1-c/a<0.06\%$) was observed using the high-resolution neutron diffraction measurement below $T^{*}\sim 95\mathrm{K}$, where a cusp was observed in the temperature dependence of magnetization. The single crystal diffraction experiment revealed that the structural phase transition accompanied by a change in the space group from $I{4}_1/amd$ to $I{4}_1/a$ occurred at $T_2=59\mathrm{K}$, where the phase transition was observed in the specific heat measurement. Crystal and magnetic structure analysis was carried out with the neutron data, and the results suggest that the magnetic phase transition from collinear to noncollinear magnetic ordering occurs at $T^{*}$ and the antiferro-orbital ordering occurs below $T_2$. We discuss the observed small crystal distortion and orbital characteristics in the light of the boundary between localization and delocalization.

Yb exhibits unusual physical behavior in ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{6-x}{\mathrm{Sn}}_x$ with coexisting intermediate valent state and magnetism. We complete the $(x,T)$ magnetic phase diagram of this system by using dc magnetization and powder neutron-diffraction experiments in a composition range ($4.60\le x\le 5.30$) where the alloys were previously reported to two-phase separate. In these compounds, Mn ferromagnetically orders near room temperature while the intermediate valent Yb sublattice ($\upsilon \approx 2.8$) obeys a Doniach-like behavior. Upon increasing the $4f$ conduction electron hybridization strength through Sn for Ge substitution, the Yb magnetic moment continuously reduces as a result of the enhancement in the Kondo screening. Meanwhile the Yb magnetic ordering temperature reaches its maximum $T_{\mathrm{Yb}}\sim 125$ K in the alloy with $x=4.65$, before dropping upon further Sn doping. The Yb magnetic instability is located near $x\sim 5.23$, in the vicinity of which the nature of the quantum phase transition remains to be elucidated.

We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy measurements on in situ cleaved and ex situ polished ${\mathrm{SmB}}_6$ single crystals. Using the multiplet structure in the Sm $3d$ core level spectra, we determined reliably that the valence of Sm in bulk ${\mathrm{SmB}}_6$ is close to 2.55 at $\sim 5$ K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly observe that not only the $J=0$ but also the $J=1$ state of the Sm $4f^6$ configuration becomes occupied at elevated temperatures. Making use of the polarization dependence, we were able to identify and extract the Sm $4f$ spectral weight of the bulk material. Finally, we revealed that the oxidized or chemically damaged surface region of the ex situ polished ${\mathrm{SmB}}_6$ single crystal is surprisingly thin, about 1 nm only.

The electronic transport properties in magnetically doped ultrathin films of topological insulators are investigated by using Landauer-Buttiker formalism. The chiral selective tunneling is addressed in such systems which leads to transport gap and as a consequence current blocking. This quantum blocking of transport occurs when the magnetic states with opposite chirality are aligned energetically. This can be observed when an electron tunnels through a barrier or well of magnetic potential induced by the exchange field. It is proved and demonstrated that this chiral transition rule fails when structural inversion asymmetric potential or an in-plane magnetization is turning on. This finding is useful to interpret quantum transport through topological-insulator thin films especially to shed light on longitudinal conductance behavior of quantum anomalous Hall effect. Besides, one can design electronic devices by means of magnetic topological-insulator thin films based on the chiral selective tunneling leading to negative differential resistance.

We study the emergent band topology of subgap Andreev bound states in the three-terminal Josephson junctions. We scrutinize the symmetry constraints of the scattering matrix in the normal region connecting superconducting leads that enable the topological nodal points in the spectrum of Andreev states. When the scattering matrix possesses time-reversal symmetry, the gap closing occurs at special stationary points that are topologically trivial as they carry vanishing Berry fluxes. In contrast, for the time-reversal broken case we find topological monopoles of the Berry curvature and corresponding phase transition between states with different Chern numbers. The latter is controlled by the structure of the scattering matrix that can be tuned by a magnetic flux piercing through the junction area in a three-terminal geometry. The topological regime of the system can be identified by nonlocal conductance quantization that we compute explicitly for a particular parametrization of the scattering matrix in the case where each reservoir is connected by a single channel.

We study the eigenstate phases of disordered spin chains with on-site finite non-Abelian symmetry. We develop a general formalism based on standard group theory to construct local spin Hamiltonians invariant under any on-site symmetry. We then specialize to the case of the simplest non-Abelian group, $S_3$, and numerically study a particular two-parameter spin-1 Hamiltonian. We observe a thermal phase and a many-body localized phase with a spontaneous symmetry breaking (SSB) from $S_3$ to ${\mathbb{Z}}_3$ in our model Hamiltonian. We diagnose these phases using full entanglement distributions and level statistics. We also use a spin-glass diagnostic specialized to detect spontaneous breaking of the $S_3$ symmetry down to ${\mathbb{Z}}_3$. Our observed phases are consistent with the possibilities outlined by A. C. Potter and R. Vasseur [Phys. Rev. B 94, 224206 (2016)], namely thermal/ergodic and many-body-localized spin-glass (MBL-SG) phases. We also speculate on the nature of an intermediate region between the thermal and MBL + SSB regions where full $S_3$ symmetry exists.

Despite the efforts to implement the idea of a deep level impurity intermediate band (IB) into bulk solar cell materials, a breakthrough in efficiency increase has not yet been achieved. Taking Sn-doped ${\mathrm{CuGaS}}_2$ as an example, we investigate the problem here from the perspective of defect physics, considering all possible charge states of the dopant and its interaction with native defects. Using an optimized hybrid functional, we find that ${\mathrm{Sn}}_{\mathrm{Ga}}$ has not only a donor-type (+/0), but also an acceptor-type ($0/-$) charge transition level. We estimate the probability of the optical transition of an electron from/to the neutral defect to/from the conduction-band edge to be about equal, therefore, the lifetimes of the excited carriers are probably quite short, limiting the enhancement of the photocurrent. In addition, we find that doping with ${\mathrm{Sn}}_{\mathrm{Ga}}$ leads to the spontaneous formation of the intrinsic acceptor ${\mathrm{Cu}}_{\mathrm{Ga}}$ defects which passivate the donor ${\mathrm{Sn}}_{\mathrm{Ga}}$ and pin the Fermi level to a position (1.4 eV above the valence-band edge) where both defects are ionized. As a result, the possibility of absorption in the middle of the visible range gets lost. These two recombination and passivation mechanisms appear to be quite likely the case for other donors and other similar host materials as well, explaining some of the experimental bottlenecks with IB solar cells based on deep level impurities.

We calculate the dispersion of spinor exciton-polaritons in a planar microcavity with its active region containing a single transitional metal dichalcogenide monolayer, taking into account excitonic and photonic spin-orbit coupling. We consider the radial propagation of polaritons in the presence of disorder. We show that the reduction of the disorder scattering induced by the formation of polariton states allows us to observe an optical valley Hall effect, namely the coherent precession of the locked valley and polarization pseudospins leading to the formation of spatial valley-polarized domains.