Symmetry-protected topological (SPT) phases are gapped phases of quantum matter protected by global symmetries that cannot be adiabatically deformed to a trivial phase without breaking symmetry. In this work, we show that, for several SPT phases that are short-range entangled (SRE), enlarging symmetries may effectively achieve the consequences of explicitly breaking symmetries. In other words, we demonstrate that nontrivial SPT phases can be unwound to trivial ones by symmetry extension—through a path where the Hilbert space is enlarged and the Hamiltonian is invariant under an extended symmetry group applying the idea of Wang, Wen, and Witten, Phys. Rev. X 8, 031048 (2018). We show examples of both bosonic and fermionic SPT phases in 1+1 dimensions (1+1 D), including Haldane's bosonic spin chain and layers of Kitaev's fermionic Majorana chains. By adding degrees of freedom into the boundary/bulk, we can lift the zero mode degeneracy, or unwind the whole system. Furthermore, based on properties of Schur cover, we sketch a general picture of unwinding applicable to any (1+1)-D bosonic SPT phase protected by on-site finite symmetry. Altogether we show that SRE states can be unwound by symmetry breaking, inversion, and symmetry extension.

Visibility of singlet-triplet qubit readout is reduced to almost zero in large magnetic field gradients due to relaxation processes. Here we present a new readout technique that is robust against relaxation and allows for measurement when previously studied methods fail. This technique maps the qubit onto spin states that are immune to relaxation using a spin-dependent electron tunneling process between the qubit and the lead. We probe this readout's performance as a function of magnetic field gradient and applied magnetic field, and optimize the pulse applied to the qubit through experiment and simulation.

We present a theoretical description of energy transfer processes between two noncontact quasi-two-dimensional crystals separated by distance $a$, oscillating with frequency ${\omega }_0$ and amplitude ${\rho }_0$, and compare it with the case of two quasi-two-dimensional crystals in uniform parallel motion. We apply the theory to calculate van der Waals energy and dissipated energy in two oscillating slabs where each slab consists of a graphene monolayer deposited on ${\mathrm{SiO}}_2$ substrate. The graphene dielectric response is determined from first principles, and ${\mathrm{SiO}}_2$ surface response is described using empirical local dielectric function. We studied the modification of vdW attraction as a function of the driving frequency and graphene doping. We propose the idea of controlling the binding energy between two slabs by tuning the graphene dopings $E_{Fi}$ and driving frequency ${\omega }_0$. We found simple ${\rho }_0^2$ dependence of vdW and dissipated energy. As the Dirac plasmons of frequency ${\omega }_p$ are the dominant channels through which the energy between slabs can be transferred, the dissipated power in equally doped $E_{F1}=E_{F2}\ne 0$ graphenes shows strong ${\omega }_0=2{\omega }_p$ peak. This peak is substantially reduced when graphenes are deposited on the ${\mathrm{SiO}}_2$ substrate. If only one graphene is pristine ($E_{Fi}=0$) the $2{\omega }_p$ peak disappears. For larger separations $a$ the phononic losses also become important and the doping causes shifts, appearance, and disappearance of many peaks originating from resonant coupling between hybridized electronic/phononic excitations in graphene/substrate slabs.

Two intriguing unresolved issues of iridate physics are the avoided metallization under applied pressure of undoped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and related materials, and the apparent absence of superconductivity under electron doping despite the similarity of the fermiology of these materials with respect to cuprates. Here, we investigate the crystal structure and lattice vibrations of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to 66 GPa and room temperature. Density functional theory (DFT) and ab initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an 8% collapse of the $c$-axis is observed at high pressures, with phase coexistence being observed between $\sim 40$ and 55 GPa. At lower pressures and still within the high-symmetry tetragonal phase, a number of lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of $P_1=17$ GPa is associated with the following anomalies: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal $c/a$ ratio take place above $P_1$; (ii) a fourfold symmetry-breaking lattice strain associated with lattice disorder is observed above $P_1$; (iii) two strong Raman-active modes at ambient conditions (at $\sim 180$ and $\sim 260{\mathrm{cm}}^{-1}$) are washed out at $P_1$; and (iv) an asymmetric Fano line shape is observed for the $\sim 390{\mathrm{cm}}^{-1}$ mode above $P_1$, revealing a coupling of this phonon with electronic excitations. DFT indicates that the ${\mathrm{Ir}}^{4+}$ in-plane canted magnetic moment is unstable against volumetric compression, indicating that the phase above $P_1$ is most likely nonmagnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at $P_1$, providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at $P_1$ are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at $P_2=30$ and $P_3=40$ GPa, indicating that other competing phases are favored at high pressures.

The influence of oxygen vacancies on the relaxors with perovskite structure was considered in the framework of Landau-Ginzburg-Devonshire phenomenological theory. We focused on the PZN-PLZT relaxor, where an earlier experimental investigation of the influence of oxygen vacancies on the polar properties was performed and evidence of oxygen vacancy induced ferroelectricity was obtained. Since the oxygen vacancies are known to be elastic dipoles, they affect the elastic and electric fields due to Vegard and flexoelectric couplings. We have shown that a negative Curie temperature ${T_C}^{*}$ of a relaxor is renormalized by the elastic dipoles due to the electrostriction coupling and could become positive at some large enough concentration of the vacancies. A positive renormalized temperature $T_C^R=T_C^{*}+\mathrm{\Delta }T$ is characteristic for the ferroelectric state. At $T<T_C^R$, all the polar properties could be calculated in the conventional way for ferroelectrics, but the obtained experimental data favor the coexistence of the ferroelectric phase with a relaxor state, i.e., the presence of a morphotropic region in PZN-PLZT relaxor. At $T>T_C^R$, the random field characteristic for relaxors is preserved, but since the mean square deviation of the polarization is nonzero, the coexistence with a dipole glass state is not excluded. For the case $T>T_C^R$, we calculated the local polarization and electric field induced by the flexochemical coupling with oxygen vacancies.

Honeycomb iridate $\mathrm{N}{\mathrm{a}}_2\mathrm{Ir}{\mathrm{O}}_3$, a $J_{\mathrm{eff}}=1/2$ magnet, is a potential platform for realizing quantum spin liquid. Many experiments have shown that its magnetic ground state is a zigzag antiferromagnetic order. However, there is still a lack of consensus on the theoretical model explaining such order, since its second-nearest-neighbor (NN) and long-range third-NN magnetic interactions are highly unclear. By properly considering the orbital moments achieved through constraining their directions in first-principles calculations, we obtain that the relative angle between orbital and spin moments is fairly small and in the order of several degrees, which thus validates the $J_{\mathrm{eff}}=1/2$ state in $\mathrm{N}{\mathrm{a}}_2\mathrm{Ir}{\mathrm{O}}_3$. Surprisingly, we find that the long-range third-NN Heisenberg interactions are sizable, whereas the second-NN magnetic interactions are negligible. Furthermore, we show that sizable long-range third-NN Heisenberg interactions closely correlate with the appreciable distribution of Wannier orbitals of $J_{\mathrm{eff}}=1/2$ states over the three NN Ir atoms. Based on our study, we propose a minimal $J_1\text{−}{K}_1\text{−}{\mathrm{\Gamma }}_1\text{−}{J}_3$ model in which the magnetic excitations have an intensity peak at 5.6 meV, consistent with the inelastic neutron-scattering experiment [Phys. Rev. Lett. 108, 127204 (2012)]. The present work demonstrates again that constraining orbital moments in first-principles calculations is a powerful way to investigate the intriguing magnetism in $J_{\mathrm{eff}}=1/2$ magnets, and it paves the way toward gaining a deep insight into the novel magnetism discovered in the honeycomb $J_{\mathrm{eff}}=1/2$ magnets.

We present a new model compound with the $S=1/2$ frustrated square lattice composed of the charge-transfer salt ($o$-MePy-V)${\mathrm{PF}}_6$. $Abinitio$ calculations indicate the formation of an $S=1/2$ square lattice, in which six types of nearest-neighbor ferromagnetic and antiferromagnetic interactions cause frustration. By applying a magnetic field, we observe an unusually gradual increase of magnetization and a subsequent 1/2-plateau-like behavior. A numerical analysis using the tensor network method qualitatively demonstrates such behaviors and suggests a collinear ordered state and a field-enhanced quantum fluctuation. Furthermore, the local magnetization and $T_1^{-1}$ probed by nuclear magnetic resonance measurements support these findings.

The effects of finite temperature, Meissner screening, and surface roughness on the spontaneous edge current for higher chirality quasi-two-dimensional superconductors are studied in the continuum limit using the quasiclassical Eilenberger equations. We find that the total spontaneous current is nonzero at finite temperature $T$ and maximized near $T=T_c/2$, where $T_c$ is the transition temperature, although it vanishes at $T=0$. In the presence of surface roughness, we observe a surface current inversion in the chiral $d$-wave case that can be understood in terms of a disorder-induced $s$-wave pairing component in the rough surface regime. This conclusion is supported by a Ginzburg-Landau analysis. However, this current inversion is nonuniversal beyond the continuum limit, as demonstrated by self-consistent lattice Bogoliubov-de Gennes calculations.

In this paper, we present an exactly solvable model for two-dimensional topological superconductors with helical Majorana edge modes protected by time-reversal symmetry. Our construction is based on the idea of decorated domain walls and makes use of the Kasteleyn orientation on a two-dimensional lattice, which was used for the construction of the symmetry protected fermion phase with $Z_2$ symmetry by Tarantino et al. and Ware et al. By decorating the time-reversal domain walls with spinful Majorana chains, we are able to construct a commuting projector Hamiltonian with zero correlation length ground state wave function that realizes a strongly interacting version of the two-dimensional topological superconductor. From our construction, it can be seen that the $T^2=-1$ transformation rule for the fermions is crucial for the existence of such a nontrivial phase; with $T^2=1$, our construction does not work.

A massive Goldstone (MG) mode, often referred to as a Higgs amplitude mode, is a collective excitation that arises in a system involving spontaneous breaking of a continuous symmetry, along with a gapless Nambu-Goldstone mode. It has been known in the previous studies that a pure amplitude MG mode emerges in superconductors if the dispersion of fermions exhibits the particle-hole (p-h) symmetry. However, clear understanding of the relation between the symmetry of the Hamiltonian and the MG modes has not been reached. Here we reveal the fundamental connection between the discrete symmetry of the Hamiltonian and the emergence of pure amplitude MG modes. To this end, we introduce nontrivial charge-conjugation ($\mathcal{C}$), parity ($\mathcal{P}$), and time-reversal ($\mathcal{T}$) operations that involve the swapping of pairs of wave vectors symmetrical with respect to the Fermi surface. The product of $\mathcal{CPT}$ (or its permutations) represents an exact symmetry analogous to the CPT theorem in the relativistic field theory. It is shown that a fermionic Hamiltonian with a p-h symmetric dispersion exhibits the discrete symmetries under $\mathcal{C},\mathcal{P},\mathcal{T}$, and $\mathcal{CPT}$. We find that in the superconducting ground state, $\mathcal{T}$ and $\mathcal{P}$ are spontaneously broken simultaneously with the U(1) symmetry. Moreover, we rigorously show that amplitude and phase fluctuations of the gap function are uncoupled due to the unbroken $\mathcal{C}$. In the normal phase, the MG and NG modes become degenerate, and they have opposite parity under $\mathcal{T}$. Therefore, we conclude that the lifting of the degeneracy in the superconducting phase and the resulting emergence of the pure amplitude MG mode can be identified as a consequence of the spontaneous breaking of $\mathcal{T}$ symmetry but not of $\mathcal{P}$ or U(1).

We investigated transport and magnetic properties of single crystal ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ under hydrostatic pressures up to 50 GPa. The ambient pressure superconductivity at $T_c\sim$ 45.4 K was fully suppressed at $P_c\sim 21$ GPa. Upon a further increase of pressure, ferromagnetism associated with the order of the rare-earth subsystem was induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M(H$) loops and the strong increase in the field cooled data $M(T$) toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi liquid to non-Fermi liquid to Fermi liquid. The Hall measurements suggest that the multiband electronic structures have changed with pressure, which should also affect the resistivity behavior. These results give access to the high-pressure side of the superconducting phase diagram in the 1111 type of materials.

We numerically investigate the phase diagram of the giant-piezoelectric $(1-x)\mathrm{Ba}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3-x\left({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}\right){\mathrm{TiO}}_3$ system, treating the ions either as classical objects (via classical Monte Carlo or CMC simulations) or quantum mechanically (via path-integral quantum Monte Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram, but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that “only” yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.

Liquid atomic metallic hydrogen is the simplest, lightest, and most abundant of all liquid metals. The importance of nucleon motions or ion dynamics can be studied by comparing the metallization phase lines of the dissociative insulator-metal transitions of hydrogen and deuterium. We use static compression to study the optical properties of dense liquid metallic deuterium in the pressure region of 1.2–1.7 Mbars and measured temperatures up to ∼3000 K. At the same pressure (density) for the two isotopes, the phase lines of this transition reveal a prominent isotopic shift of ∼700 K. Our paper shows the importance of quantum nuclear effects in the metallization transition and conduction properties in dense hydrogen isotopes at conditions of giant planetary interiors and provides an invaluable benchmark for ab initio calculations.

Defect engineering in wide-gap semiconductors is important in controlling the performance of single-photon emitter devices. The effective incorporation of defects depends strongly on the ability to control their formation and location, as well as to mitigate attendant damage to the material. In this study, we combine density functional theory, molecular dyamics (MD), and kinetic Monte Carlo (KMC) simulations to study the energetics and kinetics of the silicon monovacancy $V_{\mathrm{Si}}$ and related defects in $4H$-SiC. We obtain the defect formation energy for $V_{\mathrm{Si}}$ in various charge states and use MD simulations to model the ion implantation process for creating defects. We also study the effects of high-temperature annealing on defect position and stability using KMC and analytical models. Using a larger (480-atom) supercell than previous studies, we obtain the temperature-dependent diffusivity of $V_{\mathrm{Si}}$ in various charge states and find significantly lower barriers to diffusion than previous estimates. In addition, we examine the recombination with interstitial Si and conversion of $V_{\mathrm{Si}}$ into ${\mathrm{C}}_{\mathrm{Si}}{V}_{\mathrm{C}}$ during annealing and propose methods for using strain to reduce changes in defect concentrations. Our results provide guidance for experimental efforts to control the position and density of $V_{\mathrm{Si}}$ defects within devices, helping to realize their potential as solid-state qubits.

In order to identify and quantitatively analyze the anharmonicity of phonons relevant to the anomalous thermal expansion in the $\mathrm{I}h$ phase of ice, we performed neutron inelastic scattering measurements to study the phonon spectrum as a function of pressure up to 1 kbar at 225 K in deuterated ice (${\mathrm{D}}_2\mathrm{O}$), and as a function of temperature over 10–225 K at ambient pressure in both ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ ice. We also performed density functional theory calculations of the lattice dynamics. The anomalous expansion is quantitatively reproduced from the analysis of the neutron data as well as from the ab initio calculations. Further, the ab initio calculations are used to visualize the nature of anharmonic phonons across a large part of the Brillouin zone. We find that the negative thermal expansion below 60 K in the hexagonal plane is due to anharmonic librational motion of the hexagonal rings of the ice molecules, and that along the hexagonal axis originates from the transverse vibrations of the hexagonal layers.

The unconventional magnetic orderings found in Kitaev spin liquid candidates suggest frustration of spin interactions and raise a possibility of nearby spin liquid phases. In particular, counter-rotating spiral ordering in three-dimensional (3D) iridates is striking, and understanding the microscopic mechanism of such ordering may provide routes to 3D Kitaev spin liquids. We study a minimal 3D model including Kitaev $K$ and a symmetric off-diagonal bond-dependent $\mathrm{\Gamma }$ interaction on the hyperhoneycomb lattice by using exact diagonalization (ED). An ED cluster was chosen to satisfy a hidden SU(2) symmetry when $K=\mathrm{\Gamma }$ via a 12-site unitary transformation. The magnetic ordering in the transformed basis then generically maps to the counter-rotating noncoplanar spiral order of the original spin when the moment is pinned along a certain direction. When $K$ and $\mathrm{\Gamma }$ are negative, a small positive Heisenberg interaction favors the (1,1,0) direction consistent with the reported spiral order in $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$. Two other ordered phases close to the spiral order and spin liquids are identified. Our findings offer a relevant set of microscopic parameters, which in turn guides a way to approach possible Kitaev spin liquids.

We present an analytical theory to describe three-dimensional magnetic textures in perpendicularly magnetized magnetic multilayers that arise in the presence of magnetostatic interactions and the Dzyaloshinskii-Moriya interaction (DMI). We demonstrate that domain walls in multilayers develop a complex twisted structure, which persists even for films with strong DMI. The origin of this twist is surface-volume stray field interactions that manifest as a depth-dependent effective field whose form mimics the DMI effective field. We find that the wall twist has a minor impact on the equilibrium skyrmion or domain size but can significantly affect current-driven dynamics. Our conclusions are based on the derived analytical expressions for the magnetostatic energy and confirmed by micromagnetic simulations.

Manipulation of the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque requires high spin Hall conductivity of the heavy metal. We measure inverse spin Hall voltage using a coplanar waveguide based broadband ferromagnetic resonance setup in the Py/Ta system with a varying crystalline phase of Ta. We demonstrate a strong correlation between the measured spin mixing conductance and spin Hall conductivity with the crystalline phase of Ta thin films. We found a large spin Hall conductivity of $-2439(\hslash /e){\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ for low-resistivity (68 $\mu \mathrm{\Omega }$ cm) Ta film having mixed crystalline phase, which we attribute to an extrinsic mechanism of the spin Hall effect.