Semiconductor heteronanostructures are of interest for a vast field of applications in optoelectronics. Fast and reliable characterization of their structural properties is of high importance for industry and science. By the example of free-standing ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ quantum dots (QDs), we discuss the contributions of the strained epitaxial surface nanostructures and the substrate to the scattered x-ray intensity under grazing incidence conditions. In the frame of the distorted wave Born approximation, we take into account multiple scattering and coherent interactions of the x-ray beams involved in the diffraction process. Employing the finite-element method, we give a detailed analysis of the strain distribution inside the nanostructure and the substrate. Further, analyzing the experimental and simulated x-ray diffraction patterns in x-ray grazing incidence diffraction, we demonstrate the importance of accounting for strain in the substrate and interference effects between the surface and the QDs for strain analysis of the nano-objects. The advantages of our approach are demonstrated by comparison with the standard isostrain approach that is commonly used for strain analysis of the nanostructures. We show that neglecting the substrate contribution to the x-ray scattering causes errors in the standard isostrain approach. Finally, the substrate contribution itself may yield information sufficient for a novel and fast approach for the structural characterization of strained semiconductor heteronanostructures.

A recent experimental breakthrough has enabled probing the electronic parametric resonance of a single magnetic atom in a scanning tunneling microscopy setup. The results present intriguing features, such as an asymmetric line shape and an unusually large ratio of the decoherence and decay rates, which defy standard approaches using the conventional Bloch equations. To address these issues we employ generalized Bloch equations, together with proper microscopic modeling of the magnetic adatom, and show how all the experimental features can be naturally accounted for. The proposed approach may also be useful in treating any future similar experiments, as well as next generation hybrid quantum devices.

We report a study of physical properties of two quasi-low-dimensional metals ${\mathrm{YNiC}}_2$ and ${\mathrm{LuNiC}}_2$ including the investigation of transport, magnetotransport, galvanomagnetic, and specific heat properties. In ${\mathrm{YNiC}}_2$ we reveal two subsequent transitions associated with the formation of weakly coupled charge density wave at $T_{\mathrm{CDW}}=318\mathrm{K}$ and its locking in with the lattice at $T_1=275\mathrm{K}$. These characteristic temperatures follow the previously proposed linear scaling with the unit cell volume, demonstrating its validity extended beyond the lanthanide-based $R{\mathrm{NiC}}_2$. We also find that, in the absence of magnetic ordering able to interrupt the development of charge density wave, the Fermi surface nesting leads to opening of small pockets, containing high-mobility carriers. This effect gives rise to substantial enhancement of magnetoresistance, reaching 470% for ${\mathrm{YNiC}}_2$ and 50% for ${\mathrm{LuNiC}}_2$ at $T=1.9\mathrm{K}$ and $B=9\mathrm{T}$.

We study a ground-state ansatz for the single-hole-doped $t\text{−}J$ model in two dimensions via a variational Monte Carlo method. Such a single-hole wave function possesses finite angular momenta generated by hidden spin currents, which give rise to a novel ground-state degeneracy in agreement with recent exact diagonalization (ED) and density matrix renormalization group (DMRG) results. We further show that the wave function can be decomposed into a quasiparticle component and an incoherent momentum distribution in excellent agreement with the DMRG results up to an $8\times 8$ lattice. Such a two-component structure indicates the breakdown of Landau's one-to-one correspondence principle, and in particular, the quasiparticle spectral weight vanishes by a power law in the large sample size limit. By contrast, turning off the phase string induced by the hole hopping in the so-called $\sigma ·t\text{−}J$ model, a conventional Bloch-wave wave function with a finite quasiparticle spectral weight can be recovered, also in agreement with the ED and DMRG results. The present study shows that a singular effect already takes place in the single-hole-doped Mott insulator, by which the bare hole is turned into a non-Landau quasiparticle with translational-symmetry breaking. Generalizations to pairing and finite doping are briefly discussed.

We identify fingerprints of a proximate quantum spin liquid (QSL), observable by finite-temperature dynamical thermal transport within a minimal version of the idealized Kitaev model on a two-leg ladder if subjected to inevitably present Heisenberg couplings. Using exact diagonalization and quantum typicality, we uncover (i) an insulator-conductor crossover induced by recombination of fractionalized excitations at small Heisenberg couplings, (ii) low- and high-energy signatures of fractionalized excitations, which survive far off the pure QSL point, and (iii) a nonmonotonous current lifetime versus Heisenberg couplings. Guided by perturbation theory, we find (iv) a Kitaev-exchange-induced “one-magnon” contribution to the dynamical heat transport in the strong Heisenberg rung limit.

Resonant inelastic x-ray scattering (RIXS) has become an important tool for studying elementary excitations in correlated materials. Here, we present a systematic theoretical investigation of the Cu $L$-edge RIXS spectra of undoped and doped cuprate two-leg spin-ladders in both the non-spin-conserving (NSC) and spin-conserving (SC) channels. The spectra are rich and host many exotic excitations. In the NSC channel of the undoped case, we identify one-triplon and bound triplet two-triplon excitations in the strong rung coupling limit, as well as confined spinons in the weak rung coupling limit. In the doped case, we observe a quasiparticle excitation formed from a bound charge and spin-$1/2$ in the strong rung coupling limit. In the SC channel, we also identify several new features, including bound singlet two-triplon excitations and confined spinons in the undoped ladders in the strong and weak rung coupling limits, respectively. Conversely, in the doped case, the SC channel primarily probes both gapless and gapped charge excitations. Finally, we revisit the available data for the ladder compound ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ in the context of our results.

Atomically precise placement of dopants in Si permits creating substitutional P nanowires by design. High-resolution images show that these wires are a few atoms wide with some positioning disorder with respect to the substitutional Si structure sites. Disorder is expected to lead to electronic localization in one-dimensional (1D)-like structures. Experiments, however, report good transport properties in quasi-1D P nanoribbons. We investigate theoretically their electronic properties using an effective single-particle approach based on a linear combination of donor orbitals (LCDO), with a basis of six orbitals per donor site, thus keeping the ground state donor orbitals' oscillatory behavior due to interference among the states at the Si conduction band minima. Our model for the P positioning errors accounts for the presently achievable placement precision allowing us to study the localization crossover. In addition, we show that a gatelike potential may control its conductance and localization length, suggesting the possible use of Si:P nanostructures as elements of quantum devices, such as nanoswitches and nanowires.

The hyperpolarization of nuclear spins can enable powerful imaging and sensing techniques provided the hyperpolarization is sufficiently long lived. Recent experiments on nanodiamond ${}^{13}\mathrm{C}$ nuclear spins demonstrate that relaxation times can be extended by three orders of magnitude by building up dynamic nuclear polarization (DNP) through the driving of electron-nuclear flip-flop processes at defect centers. This finding raises the question of whether the nuclear spin coherence times are also impacted by this hyperpolarization process. Here, we theoretically examine the effect of DNP on the nuclear spin coherence times as a function of the hyperpolarization drive time. We do this by developing a microscopic theory of DNP in a nuclear spin ensemble coupled to microwave-driven defect centers in solids and subject to spin diffusion mediated by internuclear dipolar interactions. We find that, similarly to relaxation times, the nuclear spin coherence times can be increased substantially by a few orders of magnitude depending on the driving time. Our theoretical model and results will be useful for current and future experiments on enhancing nuclear spin coherence times via DNP.

By means of first-principles calculations, we have predicted the existence of phononic Weyl points (WPs) along the high-symmetric K($\frac{1}{3},\frac{1}{3},0$)-H($\frac{1}{3},\frac{1}{3},\frac{1}{2}$) line in the Brillouin zone for a series of noncentrosymmetric hexagonal WC-type materials (HfS, HfSe, IrB, MoC, MoN, MoP, NbN, NbS, TaN, TaS, TiO, TiS, WN, ZrS, ZrSe, and ZrTe) and these WPs carry nonzero topological charges. In terms of symmetry analysis, we have further derived two-band $\mathbf{k}·\mathbf{p}$ models to describe these WPs. For most WPs a first-order theory is sufficient to reproduce well the phonon spectra around these WPs. However, for some WPs a second-order theory has to be required. Particularly, when the second-order term plays a leading role, a topological charge $\pm 1\mathrm{WP}$ on the K-H line is accompanied by three nearby WPs with the opposite charges ($\mp 1$) off the K-H line. These four spatially close (in the reciprocal space) WPs have a total topological charge of $\mp 2$. On the crystal ($10\overline{1}0$) and ($01\overline{1}0$) surfaces, we have observed clear one-way topologically protected nontrivial phononic surface arc states connecting two WPs with opposite chirality. Our results pave the way for future experimental studies of the topological phonons for those WC-type materials.

A Heisenberg mean-field model is used to study the effect of size and shape on the Curie temperature of magnetic nanoparticles. Simple cubic, body-centered cubic, and magnetite nanoparticles are modelled as spheres, cubes, and needlelike particles. The Curie temperatures of particles of different shape, but with the same crystal structure and smallest dimension $d$, are found to differ. The range in the value of the Curie temperature between particles of different shape, $\mathrm{\Delta }{T}_C$, is found to be $\sim 20\%$ of the bulk value of $T_C$ in particles where $d<10$ atoms. As particle size increases, the value of $\mathrm{\Delta }{T}_C$ reduces rapidly and becomes negligible above a threshold size. This threshold size differs between systems and is controlled predominantly by crystal structure. All systems were fit to the finite-size scaling equation, with values of the scaling exponent $\nu$ found to lie between 0.46 and 0.55, in good agreement with the expected value of $\nu =0.5$. No trend in the value of $\nu$ due to shape was found.

Cesium halide has a simple cubic crystal structure and hosts low thermal conductivity, but its microscopic mechanism has not been fully understood. In the present work, we took cesium iodide (CsI) single crystal as an example, to investigate the lattice dynamics and thermal conductivity by performing inelastic neutron scattering (INS), heat transport measurements, and first-principles calculations. The temperature dependent phonon dispersions of CsI were obtained from INS and the low temperature anharmonicity of transverse optic $\left(o\right)$ and transverse acoustic $\left(a\right)$ phonon modes in CsI was observed. By performing the thermal conductivity measurement and first-principles calculations, it is shown that the low thermal conductivity of CsI originates from the combined effect of the small phonon group velocities and the large phonon scattering rates, which is dominated by the ($a,a,a$) and ($a,a,o$) phonon scattering processes. This work highlights the importance of phonon anharmonicity in lattice dynamics, which sheds light on the design of materials with low thermal conductivity.

The FeIr/MgO multilayer system was found to exhibit large interfacial perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy effects. We herein report the x-ray magnetic circular dichroism spectroscopy of both the Fe and Ir absorption edges in the FeIr/MgO system. Although Ir insertion at the Fe/MgO interface increased the PMA energy in the system, the orbital magnetic-moment anisotropy of Fe remained approximately constant. We found significant anisotropy in the orbital magnetic moment but not in the magnetic-dipole $T_{\mathrm{z}}$ term in the Ir atoms. Our experimental and theoretical results demonstrated that the large PMA of FeIr/MgO originates from second-order perturbation of the spin-orbit interaction energy of Ir. Moreover, the energy mainly originates from the spin-conserved virtual excitation process terms from the majority-spin bands of the Ir atoms located in the second atomic layer from MgO.

Shell ferromagnetism is a new functional property of certain Heusler alloys which was recently observed in ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{45}{\mathrm{In}}_5$. We report the results of a comparative study of the magnetic microstructure of bulk ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{45}{\mathrm{In}}_5$ Heusler alloys using magnetometry, synchrotron x-ray diffraction, and magnetic small-angle neutron scattering (SANS). By combining unpolarized and spin-polarized SANS (so-called POLARIS) we demonstrate that a number of important conclusions regarding the mesoscopic spin structure can be made. In particular, the analysis of the magnetic neutron data suggests that nanoprecipitates with an effective ferromagnetic component form in an antiferromagnetic matrix on field annealing at $700\mathrm{K}$. These particles represent sources of perturbation, which seem to give rise to magnetically disordered regions in the vicinity of the particle-matrix interface. Analysis of the spin-flip SANS cross section via the computation of the correlation function yields a value of $\sim 55$ nm for the particle size and $\sim 20$ nm for the size of the spin-canted region.

Acentric and nonpolar $\mathrm{P}{\mathrm{b}}_2\mathrm{Mn}{\mathrm{O}}_4$ was predicted to exhibit unique multipiezo induced magnetoelectric (ME) phenomena. In this paper, we present the results of magnetization as well as dielectric properties as a function of temperature (T), magnetic field (H), pressure (P), and electric field (E) primarily to address the ME coupling and identify the underlying mechanism behind this phenomenon. Magnetization and specific-heat measurements reveal the antiferromagnetic ordering of $\mathrm{M}{\mathrm{n}}^{4+}$ spins at temperature $T_N=17\mathrm{K}$. Metamagnetic transitions at three critical magnetic fields ($H_{c1}$, $H_{c2}$, and $H_{c3}$) are observed for T < $T_N$ and H > 3.5 T. Further, the influences of pressure and magnetic field on $H_{c1}$ and $H_{c2}$ are investigated. The $T_N$, $H_{c1}$, and $H_{c2}$ all decrease with increasing external pressure. The dielectric anomaly observed at $T_N$ is influenced by applying a magnetic field of H > 3.5 T. However, the electric field has minimal influence on the metamagnetic transition. The scaling between dielectric constant and magnetization meaningfully resolves the existence of magnetic-field-induced higher-order ME coupling in $\mathrm{P}{\mathrm{b}}_2\mathrm{Mn}{\mathrm{O}}_4$ at T < $T_N$ and H > 3.5 T.

Producing high band and valley degeneracy through aligning of conducting electronic bands is an effective strategy to improve the thermoelectric performance of complex band-structure materials. Half-Heuslers, an emerging thermoelectric material group, has complex band structures with multiple bands that can be aligned through band engineering approaches, giving us an opportunity to improve their power factor. Theoretical calculations to identify the outcome of band engineering usually employ detailed density functional theory for band-structure calculations, but the transport calculations are kept simplistic using the constant relaxation time approximation due to the complications involved with detailed scattering physics. In this work, going beyond the constant relaxation time approximation, we perform an investigation of the benefits of band alignment in improving the thermoelectric power factor under different density of states dependent scattering scenarios. As a test case we consider the Co-based $p$-type half-Heuslers TiCoSb, NbCoSn, and ZrCoSb. First, using simplified effective mass models combined with Boltzmann transport, we investigate the conditions of band alignment that are beneficial to the thermoelectric power factor under three different carrier scattering scenarios: (i) the usual constant relaxation time approximation, (ii) intraband scattering restricted to the current valley with the scattering rates proportional to the density of states as dictated by Fermi's golden rule, and (iii) both intra- and interband scattering across all available valleys, with the rates determined by the total density of states at the relevant energies. We demonstrate that the band-alignment outcome differs significantly depending on the scattering details. Next, using the density functional theory calculated band structures of the half-Heuslers we study their power factor behavior under strain induced band alignment. We show that strain can improve the power factor of half-Heuslers, but the outcome heavily depends on the curvatures of the bands involved, the specifics of the carrier scattering mechanisms, and the initial band separation. Importantly, we also demonstrate that band alignment is not always beneficial to the power factor. In addition, we show that the band structure itself can undergo changes as the bands are aligned in practice, which further affect the band alignment optimization. Our work illustrates the importance of going beyond the constant relaxation time approximation, as well as understanding how the band structure of each material behaves when considering band alignment.

Dynamic control over the full $g$-tensor in individual InGaAs/GaAs self-assembled quantum dots is achieved by inducing external strain via a piezoelectric actuator. The full $g$-tensor is obtained by measuring in different geometries with different angles between an externally applied magnetic field and the quantum dot growth axes. A large decrease in the out-of-plane hole $g$-factor with strain is observed, whereas the other components are found to be less sensitive. To further investigate this, a numerical model based on eight-band k.p-theory is used and an excellent agreement with the experimental results is established, both qualitatively and quantitatively. Furthermore, the calculations reveal the origin of the observed large change in the out-of-plane hole $g$-factor to be the increase in heavy-hole light-hole splitting under compressive stress.

Wave transport through disordered materials is of broad interest in science and engineering. It has been generally believed that long-range transport cannot exist in low-dimensional disordered systems due to the Anderson localization of all states. However, in this work we report the anomalous transparency of phonon transport induced by cooperative effects of mass and force-constant disorders in completely random harmonic wires. In contrast to the full localization in mass-disordered wire, an anomalous transparent phonon state can emerge due to the intrinsic cooperation of disordered mass and force constants. We obtain the frequency of the transparent state, tuned by the disordered force constants and masses. Consequently, the phonon transport in the completely disordered system falls into three different regimes: the full localization, resonant, and sub-ballistic regimes. While there exists a single nonzero transparent frequency in the resonant regime, transparent states in a wide range of frequencies can be present in the sub-ballistic regime, which significantly enhances the thermal conductivity of disordered wires. These findings may open new doors to modulate thermal transport and design thermal devices, including thermal filter and high-thermal-conductivity materials.

Color centers in diamond have attracted much interest as candidates for optically active, solid-state quantum bits. Of particular interest are inversion-symmetric color centers based on group-IV impurities in diamond because they emit strongly into their zero-phonon lines and are insensitive to electric field noise to first order. Early studies of the negatively charged tin-vacancy (${\mathrm{SnV}}^{-}$) center in diamond have found the ${\mathrm{SnV}}^{-}$ to be a promising candidate: it has high quantum efficiency, emits strongly into its zero-phonon lines, and is expected to have a long $T_2$ spin coherence time at 4 K. To develop the ${\mathrm{SnV}}^{-}$ into a spin qubit requires further characterization, especially of the spin and optical properties of individual ${\mathrm{SnV}}^{-}$ in nanofabricated structures. In this work, we isolate single ${\mathrm{SnV}}^{-}$ centers in diamond nanopillars and characterize their emission properties and their spin response to a magnetic field. We observe narrow emission linewidths $<250$ MHz, as well as a strong polarization dependence of each transition. We also find the Zeeman splitting under a magnetic field to be in good agreement with theoretical prediction. Our results pave the way toward future employment of single ${\mathrm{SnV}}^{-}$ centers as optically accessible quantum memories.

Combining classical electrodynamics and density functional theory (DFT) calculations, we develop a general and rigorous theoretical framework that describes the energetics of metal surfaces under high electric fields. We show that the behavior of a surface atom in the presence of an electric field can be described by the polarization characteristics of the permanent and field-induced charges in its vicinity. We use DFT calculations for the case of a W adatom on a W{110} surface to confirm the predictions of our theory and quantify its system-specific parameters. Our quantitative predictions for the diffusion of W-on-W{110} under field are in good agreement with experimental measurements. This work is a crucial step towards developing atomistic computational models of such systems for long-term simulations.