The fine-structure splitting of quantum confined In${}_x$Ga${}_{1-x}$N excitons is investigated using polarization-sensitive photoluminescence spectroscopy. The majority of the studied emission lines exhibits mutually orthogonal fine-structure components split by 100–340 μeV, as measured from the cleaved edge of the sample. The exciton and the biexciton reveal identical magnitudes but reversed sign of the energy splitting.

We study an exactly solvable quantum spin model of Kitaev type on the kagome lattice. We find a rich phase diagram which includes a topological (gapped) chiral spin liquid with gapless chiral edge states, and a gapless chiral spin liquid phase with a spin Fermi surface. The ground state of the current model contains an odd number of electrons per unit cell, which qualitatively distinguishes it from previously studied exactly solvable models with a spin Fermi surface. Moreover, we show that the spin Fermi surface is stable against weak perturbations.

We analyze a manifestation of the partial ordering transition of adatoms on graphene in resistivity measurements. We find that the Kekulé mosaic ordering of adatoms increases the sheet resistance of graphene due to a gap opening in its spectrum and that critical fluctuations of the order parameter lead to a nonmonotonic temperature dependence of resistivity with a cusp-like minimum at $T=T_c$.

This study compares cross-sectional scanning tunneling microscopy and atom probe tomography. We use epitaxially grown self-assembled InAs quantum dots (QDs) in GaAs as an exemplary material with which to compare these two nanostructural analysis techniques. We studied the composition of the wetting layer and the QDs, and performed quantitative comparisons of the indium concentration profiles measured by each method. We show that computational models of the wetting layer and the QDs, based on experimental data, are consistent with both analytical approaches. This establishes a link between the two techniques and shows their complimentary behavior, an advantage which we exploit in order to highlight unique features of the examined QD material.

Graphene stacked in a Bernal configuration (${60}^{°}$ relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle-dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material.

Strain glass (STG) in Ni-rich binary Ti-Ni possesses an $R$-like (rhombohedral) local strain order, but it transforms into $B$19' martensite under stress. It remains a puzzle why the local strain order in STG yields a different long-range strain order. Here, we systematically investigated a ternary Ti${}_{50}$Ni${}_{44.5}$Fe${}_{5.5}$ STG, which exhibited the same STG features as the binary STG, and the local strain order is also an $R$-like one. Different from the binary STG, under stress this ternary STG transforms into a normal R phase rather than $B$19'. By considering that both systems have bi-instability with respect to both $R$ and $B$19' martensites in the schematic free-energy landscape, we provide a unified explanation for the different products of the stress-induced STG to martensite (STG-M) transition between a Ti-Ni binary system and the present ternary system. We show that the differences stem from the competing thermodynamic stability between $R$-phase and $B$19' martensites.

To illustrate the emergence of Coulomb blockade from coherent quantum phase-slip processes in thin superconducting wires, we propose and theoretically investigate two elementary setups, or devices. The setups are derived from the Cooper-pair box and Cooper-pair transistor, so we refer to them as the QPS box and QPS transistor, respectively. We demonstrate that the devices exhibit sensitivity to a charge induced by a gate electrode, this being the main signature of Coulomb blockade. Experimental realization of these devices will unambiguously prove the Coulomb blockade as an effect of coherence of phase-slip processes. We analyze the emergence of discrete charging in the limit of strong phase slips. We have found and investigated six distinct regimes that are realized depending on the relation between three characteristic energy scales: inductive energy, charging energy, and phase-slip amplitude. For completeness, we include a brief discussion of dual Josephson-junction devices.

Using a combination of high-resolution and high-sensitivity neutron and x-ray diffraction measurements, it is shown that BaPb${}_{1-x}$Bi${}_x$O${}_3$ is dimorphic for the critical superconducting composition regime of $x$ between 0.2 and 0.3. The system is not chemically phase separated; the simultaneous existence of tetragonal and orthorhombic polymorphs of the same composition is demonstrated. The polymorphs differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a second-order phase transition. It is established that the tetragonal symmetry polymorph is the superconducting phase.

We report the unambiguous observation of a fractional quantum Hall state in the Landau level of a two-dimensional-hole sample at the filling factor $\nu =8/3$. We identified this state by a quantized Hall resistance and an activated temperature dependence of the longitudinal resistance and found an energy gap of 40 mK. Notably, the particle-hole conjugate state at filling factor $\nu =7/3$ in our sample did not develop down to 6.9 mK. This observation is contrary to that in electron samples, in which the 7/3 state is typically more stable than the 8/3 state. We present evidence that the asymmetry between the 7/3 and 8/3 states in our hole sample is due to Landau-level mixing.

We compute the phase diagram of HgTe/CdTe quantum wells in the three-dimensional parameter space of Dirac mass, Fermi level, and disorder strength. The phase diagram reveals the quantum spin-Hall, the metallic, and the normal insulating phases. The phase boundary of the quantum spin-Hall state is shown to be strongly deformed by the disorder. Taking specific cuts into this three-dimensional phase diagram, we recover the so-called topological Anderson insulator (TAI) phase, but now we can demonstrate explicitly that the TAI is not a distinct phase; instead, it is part of the quantum spin-Hall phase. Calculations are performed with $S_z$-conserving and $S_z$-nonconserving Hamiltonians.

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves, forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy or interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a “flower” pattern in scanning tunneling microscopy studies of epitaxial graphene grown on SiC(0001). We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations.

Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of $~1.3\hspace{0.5em}\mathrm{ns}$, a $100\times 300\hspace{0.5em}{\mathrm{nm}}^2$ ellipsoidal device switches in $~1\hspace{0.5em}\mathrm{ns}$ via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods.

We show that recursively generated Chebyshev expansions offer numerically efficient representations for calculating zero-temperature spectral functions of one-dimensional lattice models using matrix product state (MPS) methods. The main features of this Chebyshev matrix product state (CheMPS) approach are as follows: (i) it achieves uniform resolution over the spectral function’s entire spectral width; (ii) it can exploit the fact that the latter can be much smaller than the model’s many-body bandwidth; (iii) it offers a well-controlled broadening scheme that allows finite-size effects to be either resolved or smeared out, as desired; (iv) it is based on using MPS tools to recursively calculate a succession of Chebyshev vectors $|t_n\rangle$, (v) the entanglement entropies of which were found to remain bounded with increasing recursion order $n$ for all cases analyzed here; and (vi) it distributes the total entanglement entropy that accumulates with increasing $n$ over the set of Chebyshev vectors $|t_n\rangle$, which need not be combined into a single vector. In this way, the growth in entanglement entropy that usually limits density matrix renormalization group (DMRG) approaches is packaged into conveniently manageable units. We present zero-temperature CheMPS results for the structure factor of spin-$\frac{1}{2}$ antiferromagnetic Heisenberg chains and perform a detailed finite-size analysis. Making comparisons to three benchmark methods, we find that CheMPS (a) yields results comparable in quality to those of correction-vector DMRG, at dramatically reduced numerical cost; (b) agrees well with Bethe ansatz results for an infinite system, within the limitations expected for numerics on finite systems; and (c) can also be applied in the time domain, where it has potential to serve as a viable alternative to time-dependent DMRG (in particular, at finite temperatures). Finally, we present a detailed error analysis of CheMPS for the case of the noninteracting resonant level model.

${}^{75}\mathrm{As}$ and ${}^{139}\mathrm{La}$ NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO${}_{0.85}$ have been performed to microscopically investigate the Zn-impurity effects. Although superconductivity in LaFeAsO${}_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong nonmagnetic pair-breaking effect to superconductivity.

We consider some classical and frustrated lattice spin models with global $\mathrm{O}(3)$ spin symmetry. No general analytical method to find a ground state exists when the spin dependence of the Hamiltonian is more than quadratic (i.e., beyond the Heisenberg model) and/or when the lattice has more than one site per unit cell. To deal with these cases, we introduce a family of variational spin configurations, dubbed “regular magnetic orders” (RMO’s), which respect all the lattice symmetries modulo global $O(3)$ spin transformations (rotations and/or spin flips). The construction of these states is made explicit through a group-theoretical approach, and all the RMO’s on the square, triangular, honeycomb, and kagome lattices are listed. Their equal-time structure factors and powder averages are shown for comparison with experiments. Well known Néel states with one, two, or three sublattices on various lattices are RMO’s, but the RMO’s also encompass exotic nonplanar states with cubic, tetrahedral, or cuboctahedral geometry of the $T=0$ order parameter. Regardless of the details of the Hamiltonian (with the same symmetry group), a large fraction of these RMO’s are energetically stationary with respect to small deviations of the spins. In fact, these RMO’s appear as exact ground states in large domains of parameter space of simple models that we have considered. As examples, we display the variational phase diagrams of the $J_1$-$J_2$-$J_3$ Heisenberg model on all the previous lattices as well as that of the $J_1$-$J_2$-$K$ ring-exchange model on square and triangular lattices.

A quasi-one-dimensional magnetism was discovered in the post-perovskite CaRuO${}_3$ (Ru${}^{4+}$: $4d^4$, Cmcm), which is isocompositional with the perovskite CaRuO${}_3$ (Pbnm). An antiferromagnetic (AFM) spin-chain function with −$J$/$k_{\mathrm{B}}$ $=$ 350 K reproduces the experimental curve of the magnetic susceptibility vs temperature well, suggesting long-range AFM correlations. The anisotropic magnetism is probably owed to the $d_{\mathit{yz}}$-$2p_{\mathrm{\pi }}$-$d_{\mathit{zx}}$ and $d_{\mathit{zx}}$-$2p_{\mathrm{\pi }}$-$d_{\mathit{yz}}$ superexchange bonds along the $a$ axis. The Sommerfeld coefficient of the specific heat is fairly small, 0.16(2) mJ mol${}^{-1}$ ${\mathrm{K}}^{-2}$, indicating that the magnetism reflects the localized nature of the 4$d$ electrons. This is an observation of an integer ($S$ $=$ 1) spin-chain AFM in the 4$d$ electron system.

The magnetization of two spin-$\frac{1}{2}$ kagome antiferromagnets, volborthite and vesignieite, has been measured in pulsed magnetic fields up to 68 T. A magnetization plateau is observed for each compound near the highest magnetic field. Magnetizations at saturation are approximately equal to $0.40M_{\mathrm{s}}$ for both compounds, where $M_{\mathrm{s}}$ is the fully saturated magnetization, irrespective of a difference in the distortion of the kagome lattice between the two compounds. It should be noted that these values of magnetizations are significantly larger than $M_{\mathrm{s}}$/3 predicted theoretically for the one-third magnetization plateau in the spin-$\frac{1}{2}$ kagome antiferromagnet. The excess magnetization over $M_{\mathrm{s}}$/3 is nearly equal to the sum of the magnetizations gained at the second and third magnetization steps in volborthite, suggesting that there is a common origin for the excess magnetization and the magnetization steps.

Far-infrared spectra of magnetodielectric Dy${}_3$Fe${}_5$O${}_{12}$ garnet were studied using a combination of transmittance, reflectivity, and rotating analyzer ellipsometry. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Using $4\times 4$ matrix formalism for materials with $\mu (\omega )\ne 1$, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition $\mu ({\omega }_h)S_e=\varepsilon ({\omega }_h)S_m$ for the oscillator strengths $S_{e(m)}$ explains the observed vanishing of certain hybrid modes at ${\omega }_h$ in reflectivity.

The paper is partly motivated by recent pump-probe experiments with ultrashort laser pulses on antiferromagnetic FeRh that have shown the generation of magnetization within a subpicosecond time scale. On the other hand, the physical mechanism of the thermal antiferromagnetic-ferromagnetic (AFM-FM) phase transition in FeRh, known for many decades, remains a topic of controversial discussions. The selection of the magnetic degrees of freedom as well as the treatment of the magnetic excited states differ strongly in recent models by different authors. We report a density functional theory (DFT) investigation of FeRh. For the study of excited states, DFT calculations with constraints imposed on the directions and values of the atomic moments are employed. We show that the formation of the Rh moment as a consequence of the AFM-FM phase transition cannot be described within the Stoner picture. Instead, an implicit spin splitting of the Rh states takes place in the AFM phase, resulting in the intra-atomic spin polarization of the Rh atoms. This property is a consequence of the strong hybridization between Rh and Fe states. The Fe-Rh hybridization is an important factor in the physics of FeRh. We demonstrate that the ferromagnetic Fe-Rh exchange interaction is robust with respect to the crystal volume variation, whereas the antiferromagnetic Fe-Fe exchange interaction is strongly volume dependent. These different volume dependencies of the competing exchange interactions lead to their strong compensation at certain crystal volume. We perform Monte Carlo simulations and show that the calculated thermodynamics depends on the way the magnetic degrees of freedom are selected. We argue that the excited states resulting from the variation of the value of the Rh moment treated as degree of freedom are important for both the equilibrium thermodynamics of FeRh and the femtomagnetic phenomena in this system. We also study the spin mixing caused by spin-orbit coupling. The obtained value of the Elliott-Yafet spin-mixing parameter is comparable with earlier calculations for the ferromagnetic $3d$ metals. We draw the conclusion that the Elliott-Yafet mechanism of the angular-momentum transfer between electrons and lattice plays an important role in the femtomagnetic properties of FeRh.

CaBaFe${}_4$O${}_7$ is a mixed-valent transition metal oxide having both Fe${}^{2+}$ and Fe${}^{3+}$ ions in tetrahedral coordination. Here we characterize its magnetic properties by magnetization measurements and investigate its local electronic structure using soft x-ray absorption spectroscopy at the Fe $L_{2,3}$ edges, in combination with multiplet cluster simulations and band structure calculations. We found that the Fe${}^{2+}$ ion in the unusual tetrahedral coordination is Jahn-Teller active with the high-spin $e_{\uparrow }^2{t}_{2\uparrow }^3{e}_{\downarrow }^1$ configuration having a minority-spin electron with an $x^2$-$y^2$ character. We deduce that there is an appreciable orbital moment of $L_z\approx 0.36$ caused by multiplet interactions, thereby explaining the observed magnetic anisotropy.