We study the role of long-range dipolar interactions on antiferromagnetic spin chains, from the classical $S\to \infty$ limit to the deep quantum case $S=1/2$, including a transverse magnetic field. To this end, we combine different techniques such as classical energy minima, classical Monte Carlo, linear spin waves, bosonization, and density matrix renormalization group (DMRG). We find a phase transition from the already reported dipolar ferromagnetic region to an antiferromagnetic region for high enough antiferromagnetic exchange. Thermal and quantum fluctuations destabilize the classical order before reaching magnetic saturation in both phases, and also close to zero field in the antiferromagnetic phase. In the extreme quantum limit $S=1/2$, extensive DMRG computations show that the main phases remain present with transition lines to saturation significatively shifted to lower fields, in agreement with the bosonization analysis. The overall picture maintains a close analogy with the phase diagram of the anisotropic XXZ spin chain in a transverse field.

The microstructure of magnetic domain walls in epitaxial, single-layer cobalt films on Pt(111) with a pseudomorphic, atomically flat interface is studied by means of scanning electron microscopy with polarization analysis. Uncapped, thermally evaporated cobalt on a clean platinum single-crystal surface is imaged in situ in ultrahigh vacuum. For a cobalt thickness of 1.4 nm we observe Néel-like domain walls that show a fixed, counterclockwise sense of rotation. This is indicating a strong Dzyaloshinskii-Moriya interaction (DMI) that originates from the single Co/Pt interface. The width of the domain walls has been determined as $d_{\mathrm{w}}=(27\pm 2)\mathrm{nm}$. From the observation of a pure Néel-like rotation, we derive a lower bound for the DMI strength of $0.5\times {10}^{-3}{\mathrm{J}/\mathrm{m}}^2$, which gives a DMI energy per interface atom larger than 0.8 meV. An upper bound for the DMI energy of 4.3 meV per interface atom is derived from the observation of stable domains at the onset of ferromagnetism at 0.3-nm Co thickness, corresponding to an average Co coverage of 1.5 monolayers.

Since the progress in the fabrication techniques of thin films of exotic materials such as strongly correlated heavy-fermion compounds, microscopic studies of the magnetic and electronic properties inside the films have been needed. Herein, we report the observation of ${}^{115}\mathrm{In}$ nuclear quadrupole resonance (NQR) in an epitaxial film of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$, for which the microscopic field gradient within the unit cell as well as magnetic and superconducting properties at zero field are evaluated. We find that the nuclear spin-lattice relaxation rate in the film is in excellent agreement with that of bulk crystals, whereas the NQR spectra show noticeable shifts and significant broadening indicating a change in the electric-field distribution inside the film. The analysis implies a displacement of In layers in the film, which, however, does not affect the magnetic fluctuations and superconducting pairing. This implies that inhomogeneity of the electronic field gradient in the film sample causes no pair-breaking effect.

We study the field-induced magnetization processes of extended Heisenberg-Kitaev models on the honeycomb lattice, taking into account off-diagonal and longer-range exchange interactions, using a combination of Monte Carlo simulations, classical energy minimization, and spin-wave theory. We consider a number of different parameter sets, previously proposed to describe the magnetic behavior of $\alpha \text{−}{\mathrm{RuCl}}_3$ and ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ with their antiferromagnetic zigzag ground states. By classifying these parameter sets, we reveal the existence of three distinct mechanisms to stabilize zigzag states, which differ in the sign of the nearest-neighbor Kitaev interaction, the role of longer-range interactions, and the magnitude of the off-diagonal ${\mathrm{\Gamma }}_1$ interaction. While experimentally hardly distinguishable at zero field, we find that the three different scenarios lead to significantly different magnetization processes in applied magnetic fields. In particular, we show that a sizable off-diagonal interaction ${\mathrm{\Gamma }}_1>0$ naturally explains the strongly anisotropic field responses observed in $\alpha \text{−}{\mathrm{RuCl}}_3$ without the need for a strong anisotropy in the effective $g$ tensor. Moreover, for a generic field direction, it leads to a high-field state with a finite transversal magnetization, which should be observable in $\alpha \text{−}{\mathrm{RuCl}}_3$.

Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet ${\mathrm{CuCrO}}_2$ below its Néel temperature $T_N$, we investigate its magnetic and ferroelectric properties using ab initio calculations and Monte Carlo simulations. Exchange interactions up to the third nearest neighbors in the $ab$ plane, interlayer interaction, and single ion anisotropy constants in ${\mathrm{CuCrO}}_2$ are estimated by a series of density functional theory calculations. In particular, our results evidence a hard axis along the [110] direction due to the lattice distortion that takes place along this direction below $T_N$. Our Monte Carlo simulations indicate that the system possesses a Néel temperature $T_N\approx 27$ K very close to the ones reported experimentally ($T_N=24–26$ K). Also we show that the ground state is a proper-screw magnetic configuration with an incommensurate propagation vector pointing along the [110] direction. Moreover, our work reports the emergence of spin helicity below $T_N$ which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model. We confirm the electric control of spin helicity by simulating $P\text{−}E$ hysteresis loops at various temperatures.

We report on the growth and characterization of ultrathin ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have also been successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films and critical current densities, which are limited by only vortex dynamics. The 10-nm-thick YBCO nanowires without the Au capping present hysteretic current-voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. We associate such bistability to the presence of localized normal domains within the superconductor. The presence of the voltage switch in ultrathin YBCO nanostructures, characterized by high sheet resistance values and high critical current values, makes our nanowires very attractive devices to engineer single-photon detectors.

Optical lattice experiments with ultracold fermion atoms and quantum gas microscopy have recently realized direct measurements of magnetic correlations at the site-resolved level. We calculate the short-range spin-correlation functions in the ground state of the two-dimensional repulsive Hubbard model with the auxiliary-field quantum Monte Carlo (AFQMC) method. The results are numerically exact at half filling where the fermion sign problem is absent. Away from half filling, we employ the constrained path AFQMC approach to eliminate the exponential computational scaling from the sign problem. The constraint employs unrestricted Hartree-Fock trial wave functions with an effective interaction strength $U$, which is optimized self-consistently within AFQMC. Large supercells are studied, with twist averaged boundary conditions as needed, to reach the thermodynamic limit. We find that the nearest-neighbor spin correlation always increases with the interaction strength $U$, contrary to the finite-temperature behavior where a maximum is reached at a finite $U$ value. We also observe a change of sign in the next-nearest-neighbor spin correlation with increasing density, which is a consequence of the buildup of the long-range antiferromagnetic correlation. We expect the results presented in this paper to serve as a benchmark as lower temperatures are reached in ultracold atom experiments.

Control of spin relaxation is an important prerequisite for the successful implementation of spintronic devices in a materials system. We realized two directly coupled two-dimensional (2D) electron gases (2DEG-2DEG) in a $\mathrm{LaAl}{\mathrm{O}}_3\text{−}\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure system and observed a modification of the spin relaxation mechanism by varying the coupling strength. A strong enhancement of the carrier density for separation distances below a critical thickness of 6 unit cells was revealed. Electric-field-dependent analysis demonstrated tuning from positive to negative magnetoresistance for large separation distances of 10 unit cells indicating Rashba-type spin-orbit coupling, while for small separation distances of only 1 unit cell the magnetoresistance always remained positive. Analysis of the spin-orbit relaxation time and elastic scattering time revealed a modification of the spin relaxation mechanism between Elliott-Yafet and D’yakonov-Perel’ for separation distances of 1 and 10 unit cells, respectively. The tunable spin relaxation fits very well with the presence (or absence) of structural inversion symmetry in our coupled 2DEGs system for different separation distances.

$\beta \text{−}{\mathrm{Ga}}_2{\mathrm{Ox}}_3$ is a wide-band-gap semiconductor with promising applications in transparent electronics and in power devices. $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ has monoclinic crystal symmetry and does not display a layered structured characteristic of 2D materials in the bulk; nevertheless, monolayer-thin ${\mathrm{Ga}}_2{\mathrm{O}}_3$ layers can be created. We used first-principles techniques to investigate the structural and electronic properties of these nanolayers. Surprisingly, freestanding films do not exhibit any signs of quantum confinement and exhibit the same electronic structure as bulk material. A detailed examination reveals that this can be attributed to the presence of states that are strongly confined near the surface. When the ${\mathrm{Ga}}_2{\mathrm{O}}_3$ layers are embedded in a wider band-gap material such as ${\mathrm{Al}}_2{\mathrm{O}}_3$, the expected effects of quantum confinement can be observed. The effective mass of electrons in all the nanolayers is small, indicating promising device applications.

The low-energy physics of graphene is described by relativistic Dirac fermions with spin and valley degrees of freedom. Mechanical strain can be used to create a pseudomagnetic field pointing to opposite directions in the two valleys. We study interacting electrons in graphene exposed to both an external real magnetic field and a strain-induced pseudomagnetic field. For a certain ratio between these two fields, it is proposed that a fermionic symmetry-protected topological state can be realized. The state is characterized in detail using model wave functions, Chern-Simons field theory, and numerical calculations. Our paper suggests that graphene with artificial gauge fields may host a rich set of topological states.

We use an empirical tight-binding approach to calculate electron and hole states in [111]-grown PbSe nanowires. We show that the valley-orbit and spin-orbit splittings are very sensitive to the atomic arrangement within the nanowire elementary cell and differ for [111] nanowires with microscopic $D_{3d},C_{2h}$, and $D_3$ symmetries. For the nanowire diameter below 4 nm the valley-orbit splittings become comparable with the confinement energies and the $\mathit{k}·\mathit{p}$ method is inapplicable. Nanowires with the $D_3$ point symmetry having no inversion center exhibit giant spin splitting $E=\alpha k_z$, linear in one-dimensional wave vector $k_z$, with the constant $\alpha$ up to $1\mathrm{eV}\AA$.

We argue that quantum fluctuations of the phase of the order parameter may strongly affect the electron density of states (DOS) in ultrathin superconducting wires. We demonstrate that the effect of such fluctuations is equivalent to that of a quantum dissipative environment formed by soundlike plasma modes propagating along the wire. We derive a nonperturbative expression for the local electron DOS in superconducting nanowires which fully accounts for quantum phase fluctuations. At any nonzero temperature these fluctuations smear out the square-root singularity in DOS near the superconducting gap and generate quasiparticle states at subgap energies. Furthermore, at sufficiently large values of the wire impedance this singularity is suppressed down to $T=0$ in which case DOS tends to zero at subgap energies and exhibits the power-law behavior above the gap. Our predictions can be directly tested in tunneling experiments with superconducting nanowires.

We study the electronic and optical properties of single-layer phosphorene quantum dots with various shapes, sizes, and edge types (including disordered edges) subjected to an external electric field normal to the structure plane. Compared to graphene quantum dots, in phosphorene clusters of similar shape and size there is a set of edge states with energies dispersed at around the Fermi level. These states make the majority of phosphorene quantum dots metallic and enrich the phosphorene absorption gap with low-energy absorption peaks tunable by the electric field. The presence of the edge states dispersed around the Fermi level is a characteristic feature that is independent of the edge morphology and roughness.

Ferroelectric perovskite oxides possess a large electrocaloric (EC) effect, but usually at high temperatures near the ferroelectric/paraelectric phase transition temperature, which limits their potential application as next-generation solid-state cooling devices. We use classical molecular dynamics to study the electric-field-induced phase transitions and EC effect in PMN-PT $(\mathrm{PbM}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}{\mathrm{O}}_3-\mathrm{PbTi}{\mathrm{O}}_3)$. We find that the maximum EC strength of PMN-PT occurs within the morphotropic phase boundary (MPB) region at 300 K. The large adiabatic temperature change is caused by easy rotation of polarization within the MPB region.

The vibrational states of hydrogen atoms in bulk and nanocrystalline palladium were examined in a wide energy region $0\le \hslash \omega \le 300$ meV using neutron spectroscopy. In bulk ${\mathrm{PdH}}_{0.73}$, the vibrational excitations of H atoms were roughly reproduced by the quantum harmonic oscillator (QHO) model. In ${\mathrm{PdH}}_{0.42}$ nanocrystals with a diameter of 8 nm, however, additional vibrational excitations were found at energies above 80 meV. The energies and intensities of the additional states were not explained by the QHO but reasonably described as vibrations in a highly anharmonic trumpetlike potential. The additional excitations are attributed to the vibrations of H atoms at tetrahedral sites in the subsurface region stabilized by surface effects. Thus, this is an experimental work which clearly detects hydrogen vibration inside metal nanoparticles.

We investigate the ground state properties of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ based on numerical calculations of the recently proposed ab initio Hamiltonian represented by Kitaev and extended Heisenberg interactions. To overcome the limitation posed by small tractable system sizes in the exact diagonalization study employed in a previous study [Y. Yamaji et al., Phys. Rev. Lett. 113, 107201 (2014)], we apply a two-dimensional density matrix renormalization group and an infinite-size tensor-network method. By calculating at much larger system sizes, we critically test the validity of the exact diagonalization results. The results consistently indicate that the ground state of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is a magnetically ordered state with zigzag configuration in agreement with experimental observations and the previous diagonalization study. Applications of the two independent methods in addition to the exact diagonalization study further uncover a consistent and rich phase diagram near the zigzag phase beyond the accessibility of the exact diagonalization. For example, in the parameter space away from the ab initio value of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ controlled by the trigonal distortion, we find three phases: (i) an ordered phase with the magnetic moment aligned mutually in 120 degrees orientation on every third hexagon, (ii) a magnetically ordered phase with a 16-site unit cell, and (iii) an ordered phase with presumably incommensurate periodicity of the moment. It suggests that potentially rich magnetic structures may appear in $A_2{\mathrm{IrO}}_3$ compounds for $A$ other than Na. The present results also serve to establish the accuracy of the first-principles approach in reproducing the available experimental results thereby further contributing to finding a route to realize the Kitaev spin liquid.

In the spirit of searching for Gd-based, frustrated, rare earth magnets, we have found antiferomagnetism (AF) in GdPtPb, which crystallizes in the ZrNiAl-type structure that has a distorted kagome lattice of Gd triangles. Single crystals were grown and investigated using structural, magnetic, transport, and thermodynamic measurements. GdPtPb orders antiferromagnetically at 15.5 K, arguably with a planar, noncollinear structure. The high temperature magnetic susceptibility data reveal an “anti-frustration” behavior having a frustration parameter, $\left|f\right|=\left|\mathrm{\Theta }\right|/T_N=0.25$, which can be explained by mean field theory within a two-sublattice model. Study of the magnetic phase diagram down to $T=1.8\mathrm{K}$ reveals a change of magnetic structure through a metamagnetic transition at around 20 kOe and the disappearance of the AF ordering near 140 kOe. In total, our work indicates that GdPtPb can serve as an example of a planar, noncollinear AF with a distorted kagome magnetic sublattice.

Recent experimental work has demonstrated optical control of spin wave emission by tuning the shape of the optical pulse [Satoh et al., Nat. Photon. 6, 662 (2012)]. We reproduce these results and extend the scope of the control by investigating nonlinear effects for large amplitude excitations. We observe an accumulation of spin wave power at the center of the initial excitation combined with short-wavelength spin waves. These kinds of nonlinear effects have not been observed in earlier work on nonlinearities of spin waves. Our observations pave the way for the manipulation of magnetic structures at a smaller scale than the beam focus, for instance in devices with all-optical control of magnetism.

We show that the effective coupling between the spin-1/2 edge states of a spin-1 chain of finite length can be continuously tuned by frustration. For the $J_1-J_2$ model with nearest- and next-nearest-neighbor antiferromagnetic interactions, we show that the effective coupling in a chain of length $L$ changes sign $N\simeq 0.38L$ times in the window $0.28\lesssim J_2/J_1\lesssim 0.75$ where the short-range correlations are incommensurate. This implies that there are $N$ zero modes where the singlet and the triplet are strictly degenerate, i.e., $N$ values of $J_2/J_1$ where the spin-1/2 edge states are completely decoupled. We argue that this effect must be generic for all incommensurate phases with localized edge states, and we briefly discuss a few experimental implications.