Millikelvin magnetotransport studies are carried out on heavily $n$-doped wurtzite GaN:Si films grown on semi-insulating GaN:Mn buffer layers by metal-organic vapor phase epitaxy. The dependence of the conductivity on magnetic field and temperature is interpreted in terms of theories that take into account disorder-induced quantum interference of one-electron and many-electron self-crossing trajectories. The Rashba parameter ${\alpha }_{\text{R}}=(4.5\pm 1)$ meV Å is determined, and it is shown that in the previous studies of electrons adjacent to GaN/(Al,Ga)N interfaces, bulk inversion asymmetry was dominant over structural inversion asymmetry. The comparison of experimental and theoretical values of ${\alpha }_{\text{R}}$ across a series of wurtzite semiconductors is presented as a test of current relativistic ab initio computation schemes. Low-temperature decoherence is discussed in terms of disorder-modified electron-electron scattering.

We discuss the so-called hybrid improper ferroelectricity (HIF) appearing in $A^{\prime }B$O${}_3$/$A^{\prime \prime }B$O${}_3$ perovskite superlattices. By means of straightforward analytical derivations and/or associated graphical analysis, we demonstrate that two previously-proposed elemental interatomic couplings between the O${}_6$ octahedral rotations and the displacements of the $A$-site cations [Bellaiche and Íñiguez, Phys. Rev. B 88, 014104 (2013)] naturally reproduce and explain HIF in ($A^{\prime }B$O${}_3$)${}_1$/($A^{\prime \prime }B$O${}_3$)${}_1$ superlattices composed of layers that are only one unit cell thick. Further, we show that our approach permits an easy treatment of superlattices of arbitrary stacking direction and layer thickness. In particular, this allows us to predict (i) other previously overlooked types of HIF in ($A^{\prime }B$O${}_3$)${}_1$/($A^{\prime \prime }B$O${}_3$)${}_1$ superlattices and (ii) original and striking effects, such as the possibility of discretizing the magnitude of the spontaneous polarization in ($A^{\prime }B$O${}_3$)${}_m$/($A^{\prime \prime }B$O${}_3$)${}_n$ systems when varying the $m$ and $n$ layer thicknesses. We further show first-principles results confirming the latter predictions.

In the context of disordered systems with quenched Hamiltonians I address the problem of characterizing rare samples where the thermal average of a specific observable has a value different from the typical one. These rare samples can be selected through a variation of the replica trick which amounts to replicating the system and dividing the replicas intwo two groups containing, respectively, $M$ and $-M$ replicas. Replicas in the first (second) group experience a positive (negative) small field $O(1/M)$ conjugate to the observable considered and the $M\to \infty$ limit is to be taken in the end. Applications to the random-field Ising model and to the Sherrington-Kirkpatrick model are discussed.

We report a high-field electron spin resonance study in the sub-THz frequency domain of a single crystal of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ that has been recently proposed as a prototypical spin-orbital Mott insulator. In the antiferromagnetically ordered state with noncollinear spin structure that occurs in this material at $T_{\mathrm{N}}$ $\approx$ 240 K we observe both the “low” frequency mode due to the precession of weak ferromagnetic moments arising from a spin canting, and the “high” frequency modes due to the precession of the antiferromagnetic (AFM) sublattices. Surprisingly, the energy gap for the AFM excitations appears to be very small, amounting to only 0.83 meV. This suggests a rather isotropic Heisenberg dynamics of interacting Ir${}^{4+}$ effective spins despite the spin-orbital entanglement in the ground state.

The equal-interval splitting of quantum tunneling observed in simple-Ising-model systems of Ni${}_4$ (3D) and Mn${}_3$ (2D) single-molecule magnets (SMMs) is reported. The splitting is due to the identical exchange coupling in the SMMs, and is simply determined by the difference between the two numbers of the spin-down $n_{\downarrow }$ and spin-up $n_{\uparrow }$ molecules neighboring to the tunneling molecule. The splitting may be presented as $(n_{\downarrow }-n_{\uparrow })JS/g{\mu }_0{\mu }_B$, and the number of the splittings follows $n+1$ where $n=n_{\downarrow }+n_{\uparrow }$ is the coordination number. Besides, since the quantum tunneling is heavily dependent on local spin environment, the manipulation of quantum tunneling may become feasible for this kind of system, which may shed light on applications of SMMs.

The Heisenberg model on a triangular lattice is a prime example of a geometrically frustrated spin system. However most experimentally accessible compounds have spatially anisotropic exchange interactions. As a function of this anisotropy, ground states with different magnetic properties can be realized. Motivated by recent experimental findings on ${\mathrm{Cs}}_2{\mathrm{CuCl}}_{4-x}{\mathrm{Br}}_x$, we discuss the full phase diagram of the anisotropic model with two exchange constants $J_1$ and $J_2$, including possible ferromagnetic exchange. Furthermore a comparison with the related square lattice model is carried out. We discuss the zero-temperature phase diagram, ordering vector, ground-state energy, and ordered moment on a classical level and investigate the effect of quantum fluctuations within the framework of spin-wave theory. The field dependence of the ordered moment is shown to be nonmonotonic with field and control parameter.

We present a numerical method for calculating piecewise smooth spectral functions of correlated quantum systems in the thermodynamic limit from the spectra of finite systems computed using the dynamical or correction-vector density-matrix renormalization group method. The key idea is to consider this problem as a blind deconvolution with an unknown kernel, which causes both a broadening and finite-size corrections of the spectrum. In practice, the method reduces to a least-square optimization under nonlinear constraints which enforce the positivity and piecewise smoothness of spectral functions. The method is demonstrated on the single-particle density of states of one-dimensional paramagnetic Mott insulators represented by the half-filled Hubbard model on an open chain. Our results confirm that the density of states has a steplike shape but no square-root singularity at the spectrum onset.

Electron spin diffusion is investigated in monolayer ${\mathrm{MoS}}_2$ in the absence of external electric and magnetic fields. The electron-impurity scattering, which is shown to play a negligible role in spin relaxation in time domain in this material, has a marked effect on the in-plane spin diffusion due to the anisotropic spin precession frequency in the spatial domain. With the electron-impurity and intervalley electron-phonon scatterings separately included in the scattering term, we study the intra- and intervalley diffusion processes of the in-plane spins by analytically solving the kinetic spin Bloch equations. The intravalley process is found to be dominant in the in-plane spin diffusion, in contrast to the case of spin relaxation in time domain, where the intervalley process can be comparable to or even more important than the intravalley one. For the intravalley process, we find that the in-plane spin diffusion is suppressed with the increase of impurity density but effectively enhanced by increasing electron density in both the degenerate and nondegenerate limits. We also take into account the electron-electron Coulomb scattering in the intravalley process. Interestingly, we find that in the nondegenerate limit, the intravalley spin-diffusion length presents an opposite trend in the electron-density dependence compared to the one with only electron-impurity scattering.

We study the Boltzmann transport equation for the Bose-Hubbard chain in the kinetic regime. The time-dependent Wigner function is matrix-valued with odd dimension due to integer spin. For nearest neighbor hopping only, there are infinitely many additional conservation laws and nonthermal stationary states. Adding longer-range hopping amplitudes entails exclusively thermal equilibrium states. Especially for small next-nearest neighbor hopping amplitudes, we observe prethermalization with two time scales, which can be related to the relative strength of the nearest and next-nearest hopping. We provide a derivation of the Boltzmann equation based on the Hubbard Hamiltonian, including general interactions beyond on-site, and illustrate the results by numerical simulations.

Use of electron vortex beams (EVB), that is, convergent electron beams carrying an orbital angular momentum (OAM), is a novel development in the field of transmission electron microscopy. They should allow measurement of element-specific magnetic properties of thin crystals using electron magnetic circular dichroism (EMCD)—a phenomenon similar to the x-ray magnetic circular dichroism. Recently, it has been shown computationally that EVBs can detect magnetic signal in a scanning mode only at atomic resolution. In this follow-up work, we explore in detail the elastic and inelastic scattering properties of EVBs on crystals, as a function of beam diameter, initial OAM, acceleration voltage, and beam displacement from an atomic column. We suggest that for a 10-nm layer of bcc iron oriented along (001) zone axis, an optimal configuration for a detection of EMCD is an EVB with OAM of $1\hslash$ and a full width at half maximum diffraction-limited beam diameter of 1.6 Å, acceleration voltage 200 kV, and an annular detector with inner and outer radii of 7 and 44 mrad, respectively.

We have performed polarized Raman scattering measurements on the newly discovered superconductor ${\mathrm{Ba}}_2$${\mathrm{Ti}}_2$${\mathrm{Fe}}_2$${\mathrm{As}}_4$O ($T_c=21$ K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principles calculations. The phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling and vary only slightly with temperature, except for one ${\mathrm{E}}_g$ mode associated with large displacements of As atoms near the ${\mathrm{Ti}}_2$O planes. We also identify a small anomaly around 125 K in the linewidth of an ${\mathrm{A}}_{1g}$ mode involving the same As atoms. Our results suggest that the transition at 125 K is most likely driven by electronic interactions taking place in the ${\mathrm{Ti}}_2$O planes.

The proximity coupling of a thin superconducting layer and an inhomogeneous ferromagnet can lead to a significant reduction of the critical temperature due to the generation of spin-polarized triplet Cooper pairs. We report critical temperature measurements of Co/Cu/NiFe(Py)/Cu/Nb superconducting pseudo spin valves (PSVs) in which the magnetization of the soft layer (Py) can be independently rotated in-plane with a magnetic field to create an angle (θ) between it and the magnetization of Co. Here we observe results consistent with spin-triplet theory and demonstrate large changes in $\Delta T_C$ up to −120 mK as the Py layer is rotated from 0° (Co and Py are parallel) to 90° (Co and Py are orthogonal), which offers the potential for active control of the superconducting state. The key to this achievement is engineered magnetic anisotropy in Py, which enables well-defined control over the magnetization configuration of the PSV.

As one kind of many body entangled states, spin squeezed states can be used to implement the high precise measurement beyond the standard quantum limit. Inspired by the novel spin squeezing scheme based on phonon-induced spin-spin interactions [S. D. Bennett et al., Phys. Rev. Lett. 110, 156402 (2013)], we reexamine the steady-state behaviors for the spin ensemble in diamond nanostructures by exerting a controllable microwave field. By using the phase-space approach we calculate analytically fluctuations of collective spin operators. We find that there is bistability and spin squeezing for the steady-state spin ensemble, despite the mechanical damping considered. Moreover, our work shows that bistability and spin squeezing can be controlled by microwave field and Zeeman splitting. The present scheme can be used to increase the stability of spin clocks, magnetometers, and other measurements based on spin-spin interaction in diamond nanostructures.

The effects of octahedral tilting of Rb$A$Nb${}_2$O${}_7$ ($A$ = Bi, Nd) compounds was studied using density-functional theory. In this compound, the structural phase transition was correlated with two octahedral tilting modes ($a^{-}$$a^{-}$$c^0$ tilting and $a^0$$a^0$$c^{+}$ tilting), and the magnitude of the octahedral tilting mode was analyzed in the optimized structure. The theoretical results correlated well with the recent experimental results on the ferroelectricity of RbBiNb${}_2$O${}_7$. The hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes and off-center displacement mode was analyzed by group theory and symmetry mode analysis. The detailed relationship of the tilting modes to the structural phase transition and the detailed physical properties of ferroelectricity are also presented.

We propose a scaling theory for the universal imaginary-time quantum critical dynamics for both short and long times. We discover that there exists a universal critical initial slip related to a small initial order parameter $M_0$. In this stage, the order parameter $M$ increases with the imaginary time $\tau$ as $M\propto M_0{\tau }^{\theta }$ with a universal initial-slip exponent $\theta$. For the one-dimensional transverse-field Ising model, we estimate $\theta$ to be $0.373$, which is markedly distinct from its classical counterpart. Apart from the local order parameter, we also show that the entanglement entropy exhibits universal behavior in the short-time region. As the critical exponents in the early stage and in equilibrium are identical, we apply the short-time dynamics method to determine quantum critical properties. The method is generally applicable in both the Landau-Ginzburg-Wilson paradigm and topological phase transitions.

The temperature-dependent interplane resistivity ${\rho }_c\left(T\right)$ was measured in the hole-doped iron arsenide superconductor (Ba${}_{1-x}$K${}_x$)Fe${}_2$As${}_2$ over a doping range from parent compound to optimal doping at $T_c\approx 38$ K, $0\le x\le 0.34$. The measurements were undertaken on high-quality single crystals grown from FeAs flux. The coupled magnetic/structural transition at $T_{SM}$ leads to a clear accelerated decrease of ${\rho }_c\left(T\right)$ on cooling in samples with $T_c<26$ K ($x<0.25$). This decrease in the hole-doped material is in notable contrast to the increase in ${\rho }_c\left(T\right)$ in the electron-doped Ba(Fe${}_{1-x}$Co${}_x$)Fe ${}_2$As${}_2$ and isoelectron-substituted BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. $T_{SM}$ decreases very sharply with doping, dropping from $T_s=71$ K to zero on increase of $T_c$ from approximately 25 to 27 K. ${\rho }_c\left(T\right)$ becomes linear in $T$ close to optimal doping. The broad crossover maximum in ${\rho }_c\left(T\right)$, found in the parent BaFe${}_2$As${}_2$ at around $T_{\max }\sim 200$ K, shifts to higher temperature $\sim$250 K with doping of $x=0.34$. The maximum shows clear correlation with the broad crossover feature found in the temperature-dependent in-plane resistivity ${\rho }_a\left(T\right)$. The evolution with doping of $T_{\max }$ in (Ba${}_{1-x}$K${}_x$)Fe${}_2$As${}_2$ is in notable contrast with both the rapid suppression of $T_{\max }$ found in Ba(Fe${}_{1-x}{T}_x$)${}_2$As${}_2$ ($T=\text{Co}$,Rh,Ni,Pd) and its rapid increase in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. This observation suggests that pseudogap features are much stronger in hole-doped than in electron-doped iron-based superconductors, revealing significant electron-hole doping asymmetry similar to that in the cuprates.

We study the phase diagram of the extended Hubbard model on the kagome lattice at 1/3 filling. By combining a configuration interaction approach to an unrestricted Hartree-Fock, we construct an effective Hamiltonian which takes the correlations back on top of the mean-field solution. We obtain a rich phase diagram with, in particular, the presence of two original phases. The first one consists of local polarized droplets of metal standing on the hexagons of the lattice and an enlarged kagome charge order, inversely polarized, on the remaining sites. The second, obeying a local ice-rule type constraint on the triangles of the kagome lattice, is driven by an antiferromagnetic coupling of spins and is constituted of disconnected six-spin singlet rings. The nature and stability of these phases at large interactions are studied via trial wave functions and perturbation theory.

We study the superfluid and insulating phases of interacting bosons on the triangular lattice with an inverted dispersion, corresponding to frustrated hopping between sites. The resulting single-particle dispersion has multiple minima at nonzero wave vectors in momentum space, in contrast to the unique zero-wave-vector minimum of the unfrustrated problem. As a consequence, the superfluid phase is unstable against developing additional chiral order that breaks time-reversal ($\mathcal{T}$) and parity ($\mathcal{P}$) symmetries by forming a condensate at nonzero wave vector. We demonstrate that the loss of superfluidity can lead to an even more exotic phase, the chiral Mott insulator, with nontrivial current order that breaks $\mathcal{T},\mathcal{P}$. These results are obtained via variational estimates, as well as a combination of bosonization and density-matrix renormalization group of triangular ladders, which, taken together, permit a fairly complete characterization of the phase diagram. We discuss the relevance of these phases to optical lattice experiments, as well as signatures of chiral symmetry breaking in time-of-flight images.