We report large orthorhombic distortions in the ferrimagnetic tetragonal $(c>a)$ spinel ${\mathrm{Cu}}_x{\mathrm{Mn}}_{3-x}{\mathrm{O}}_4$ stabilized by a few percent of Cu doping. The orthorhombic strains of the ferrimagnetic phases increased linearly to the doping and reached up to $\epsilon \approx 8.2\times {10}^{-3}$ for $x=0.19$, which is three times larger than the saturated value under external magnetic fields. For high doping ($x\gtrsim 0.17$), the distortions first appeared in the paramagnetic phases and underwent further enhancement simultaneously with the onset of the noncollinear ferrimagnetic ordering. We present the rich magnetostructural phase diagram of ${\mathrm{Cu}}_x{\mathrm{Mn}}_{3-x}{\mathrm{O}}_4$, and argue that the diluted $t_2$ orbital degeneracy of ${\mathrm{Cu}}^{2+}$ under tetrahedral crystal field breaks the global symmetry and triggers the orthorhombic instability inherent in ${\mathrm{Mn}}_3{\mathrm{O}}_4$.

Electrical resistivity $\rho \left(T\right)$ of the $5f$-ferromagnet ${\mathrm{UGa}}_2$ was investigated on single-crystalline samples as a function of pressure and magnetic field. Under quasihydrostatic pressure the Curie temperature monotonously increases from $T_{\mathrm{C}}=124\mathrm{K}$ up to 154 K at $p=14.2\mathrm{GPa}$ after which it turns down steeply and reaches $T_{\mathrm{C}}=147\mathrm{K}$ at $p=15.2\mathrm{GPa}$. At 20 GPa the compound is already nonmagnetic. This dramatic variation is compatible with the exchange interactions mediated by the $5f$ hybridization with the non-$f$ states. The external pressure first enhances the exchange coupling of the $5f$ moment but eventually suppresses the magnetic order by washing out the $5f$ moments. Such a two-band model is adequate for the weakly delocalized $5f$ states. The external pressure gradually suppresses the spin-disorder resistivity, which reaches very high ρ values $\left(300\mu \Omega \mathrm{cm}\right)$ above $T_{\mathrm{C}}$ in ${\mathrm{UGa}}_2$. This leads to the crossover from the initial negative to the positive $d\rho /dT$ in the paramagnetic state.

The Kohn-Sham orbital kinetic energy density ${\tau }_{\sigma }\left(\mathbf{r}\right)={\sum }_i{w}_{i\sigma }{\left|\nabla {\psi }_{i\sigma }\left(\mathbf{r}\right)\right|}^2$ is one fundamental quantity for constructing metageneralized gradient approximations (meta-GGAs) for use by density functional theory. We present a computational scheme of ${\tau }_{\sigma }\left(\mathbf{r}\right)$ for the full-potential linearized augmented plane-wave (FLAPW) method. Our scheme is highly accurate and efficient and easy to implement with existing computer codes. To illustrate its performance, we construct the Becke-Johnson meta-GGA exchange potentials for Be, Ne, Mg, Ar, Ca, Zn, Kr, and Cd atoms, which are in very good agreement with the original results. For bulk solids, we construct the Tran-Blaha modified Becke-Johnson potential (mBJ) and confirm its capability to calculate band gaps with the reported bad convergence of the mBJ potential being substantially improved. The present computational scheme of ${\tau }_{\sigma }\left(\mathbf{r}\right)$ should also be valuable for developing other meta-GGAs in the FLAPW as well as in similar methods utilizing atom centered basis functions.

In the semiconductor double quantum dot singlet-triplet qubit architecture, the decoherence caused by the qubit's charge environment poses a serious obstacle towards large scale quantum computing. The effects of the charge decoherence can be mitigated by operating the qubit in the so-called sweet spot regions where it is insensitive to electrical noise. In this paper, we propose singlet-triplet qubits based on two quantum dots of different sizes. Such asymmetric double quantum dot systems allow the implementation of exchange gates with controllable exchange splitting $J$ operated in the doubly occupied charge region of the larger dot, where the qubit has high resilience to charge noise. In the larger dot, $J$ can be quenched to a value smaller than the intradot tunneling using magnetic fields, while the smaller dot and its larger splitting can be used in the projective readout of the qubit.

Nanowire double quantum dots occupied by an even number of electrons are investigated in the context of energy level structure revealed by electric dipole spin resonance measurements. We use a numerically exact configuration interaction approach up to six electrons for systems tuned to a Pauli spin blockade regime. We point out the differences between the spectra of systems with two and a greater number of electrons. For two electrons the unequal length of the dots results in a different effective $g$ factor in the dots as observed by the recent experiments. For an increased number of electrons the $g-\text{factor}$ difference between the dots appears already for symmetric systems and it is greatly amplified when the dots are of unequal length. We find that the energy splitting defining the resonant electric dipole spin frequency can be quite precisely described by the two electrons involved in the Pauli blockade with the lower-energy occupied states forming a frozen core.

We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far blue-detuned IX mode, with its subsequent adiabatic transformation to an indirect exciton condensate by lowering IX energy via applied electric field. The following allows for generation of a spatially localized cold exciton gas, on the contrary to currently used methods, where IX cloud appears due to diffusion of carriers from spatially separated electron- and hole-rich areas.

Structural phase transitions in epitaxial stoichiometric $\mathrm{VN}/\mathrm{MgO}(011)$ thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below $T_c=250\mathrm{K}$, XRD and SAED results reveal forbidden $\left(00l\right)$ reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior $I\propto {(T_c-T)}^{1/2}$. Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function ${\alpha }_{\mathrm{tr}}^{2}F\left(\hslash \omega \right)$, the product of the phonon density of states $F\left(\hslash \omega \right)$ and the transport electron/phonon coupling strength ${\alpha }_{\mathrm{tr}}^2\left(\hslash \omega \right)$, is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at $T>T_c$. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

We use ab initio and classical molecular dynamics (AIMD and CMD) based on the modified embedded-atom method (MEAM) potential to simulate diffusion of N vacancy and N self-interstitial point defects in $B1$ TiN. TiN MEAM parameters are optimized to obtain CMD nitrogen point-defect jump rates in agreement with AIMD predictions, as well as an excellent description of $\mathrm{Ti}{\mathrm{N}}_x(\sim 0.7<x\le 1)$ elastic, thermal, and structural properties. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, the MD simulations presented in this paper reveal an unanticipated atomistic process, which controls the spontaneous formation of N self-interstitial/N vacancy $({\mathrm{N}}^{\mathrm{I}}/{\mathrm{N}}^{\mathrm{V}})$ pairs (Frenkel pairs), in defect-free TiN. This entails that the N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair ${\mathrm{N}}^{\mathrm{I}}$ and ${\mathrm{N}}^{\mathrm{V}}$ recombine within a fraction of ns; ∼50% of these processes result in the exchange of two nitrogen lattice atoms $\left(\mathrm{N}-{\mathrm{N}}^{\mathrm{Exc}}\right)$. Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired ${\mathrm{N}}^{\mathrm{I}}$ and ${\mathrm{N}}^{\mathrm{V}}$ point defects.

We consider superconductivity in the presence of impurities in a two-band model suited for the description of iron-based superconductors. We analyze the effect of interband scattering processes on superconductivity, allowing for orbital, i.e., nonspin-magnetic but time-reversal symmetry-breaking impurities. Pair breaking in such systems is described by a nontrivial phase in an interband-scattering matrix element. We find that the transition temperature of conventional superconductors can be suppressed due to interband scattering, whereas unconventional superconductors may be unaffected. We also discuss the stability of density wave phases in the presence of impurities. As an example, we consider impurities associated with imaginary charge density waves that are of interest for iron-based superconductors.

There is evidence that the pseudogap phase in the cuprates breaks time-reversal symmetry. Here we show that pair density wave (PDW) states give rise to a translational invariant nonsuperconducting order parameter that breaks time-reversal and parity symmetries, but preserves their product. This secondary order parameter has a different origin, but shares the same symmetry properties as a magnetoelectric loop current order that has been proposed earlier in the context of the cuprates to explain the appearance of intracell magnetic order. We further show that, due to fluctuations, this secondary loop current order, which breaks only discrete symmetries, can preempt PDW order, which breaks both continuous and discrete symmetries. In such a phase, the emergent loop current order coexists with spatial short-range superconducting order and possibly short-range charge density wave (CDW) order. Finally, we propose a PDW phase that accounts for intracell magnetic order and the Kerr effect, has CDW order consistent with x-ray scattering and nuclear magnetic resonance observations, and quasiparticle properties consistent with angle-resolved photoemission scattering.

XY pyrochlore antiferromagnets are well-known to exhibit order-by-disorder through both quantum and thermal selection. In this paper, we consider the effect of substituting nonmagnetic ions onto the magnetic sites in a pyrochlore XY model with generally anisotropic exchange tuned by a single parameter $J^{\pm \pm }/J^{\pm }$. The physics is controlled by two points in this space of parameters $J^{\pm \pm }/J^{\pm }=\pm 2$ at which there are line modes in the ground state and hence an $O\left(L^2\right)$ ground-state degeneracy intermediate between that of a conventional magnet and a Coulomb phase. At each of these points, single vacancies seed pairs of line defects. Two line defects carrying incompatible spin configurations from different vacancies can cross leading to an effective one-dimensional description of the resulting spin texture. In the thermodynamic limit at finite density, we find that dilution selects a state “opposite” to the state selected by thermal and quantum disorder which is understood from the single vacancy limit. The latter finding hints at the possibility that ${\mathrm{Er}}_{2-x}{\mathrm{Y}}_x{\mathrm{Ti}}_2{\mathrm{O}}_7$ for small $x$ exhibits a second phase transition within the thermally selected ${\psi }_2$ state into a ${\psi }_3$ state selected by the quenched disorder.

We report a detailed study of the magnetic properties of a monoatomic layer of Mn on W(110). By comparing multiscale numerical calculations with measurements we evaluate the magnetic ground state of the system and its temperature-dependent evolution. We find that the ground state consists of a cycloidal spin spiral (CSS) that persists up to the Néel temperature with a temperature-independent wavelength. However, by continuously increasing the temperature, that CSS becomes thermally depinned. This results in a time-averaged absence of magnetic order, a process that can be viewed as the antiferromagnetic analog of superparamagnetism.

On a set of 12 bulk and highly pure [residual resistance ratio (RRR) up to 6000] iron single crystals with different crystallographic orientation and on polycrystalline material the “hidden” basic resistivity $\rho (T,B\to 0)$ and magnetoresistive effects (anisotropic magnetoresistance, longitudinal and transverse Lorentz resistance) are quantitatively disentangled. The temperature-dependent basic resistivity for $T\le 30\mathrm{K}$ follows a $\Delta \rho (T,B\to 0)=a{T}^2+b{T}^5$ law, proving that electron-electron and electron-phonon scattering (with a resistive Debye temperature ${\Theta }_{\mathrm{R}}\cong 450\mathrm{K}$) are dominant also in iron, similar to nonferromagnetic transition metals. Here the crystal-orientation dependence of the intrinsic longitudinal magnetoresistance in the single-domain state could be quantitatively evaluated in accordance with the symmetry of the Fermi surface. The transverse magnetoresistance (TMR) turns out to be the sum of the so-called “two-band conduction term” and the unboundedly growing TMR term in compensated metals. It is proven that the second term is not restricted to the high-field limit (as usually discussed up to now) but acts down to lowest TMR or ${\omega }_{\mathrm{C}}\tau$ values. This conclusion is verified by TMR measurements on highly pure molybdenum single crystals (RRR ≅ 100 000). Supported by Kerr microscopy, a small positive domain wall resistance (DWR) could be isolated from the dominating negative DWR in the multidomain state of iron, resulting in a relation of larger than 5:1 for the electrons to pass a domain wall with spin tracking compared to scattering with spin conservation.

We have studied the local properties of single Pt atoms adsorbed on hydroxylated ${\mathrm{TiO}}_2\left(110\right)$-($1\times 1$) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the ${\mathrm{TiO}}_2\left(110\right)$ surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated ${\mathrm{TiO}}_2$ nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.

Double- and triple-walled carbon nanotubes are studied in detail by laser energy-dependent Raman spectroscopy in order to get a deeper understanding about the second-order ${\mathrm{G}}^{\prime }$ band Raman process, general nanotube properties, such as electronic and vibrational properties, and the growth method itself. In this work, the inner nanotubes from the double- and triple-walled carbon nanotubes are produced through the encapsulation of fullerene peapods with high-temperature thermal treatments. We find that the spectral features of the ${\mathrm{G}}^{\prime }$ band, such as the intensity, frequency, linewidth, and line shape are highly sensitive to the annealing temperature variations. We also discuss the triple-peak structure of the ${\mathrm{G}}^{\prime }$ band observed in an individual triple-walled carbon nanotube taken at several laser energies connecting its Raman spectra with that for the ${\mathrm{G}}^{\prime }$ band spectra obtained for bundled triple-walled carbon nanotubes.

We consider theoretically the retarded van der Waals interaction of a small gas bubble in water with a porous ${\mathrm{SiO}}_2$ surface. We predict a possible transition from repulsion to attraction as the surface is made more porous. It highlights that bubbles will interact differently with surface regions with different porosity (i.e., with different optical properties).

The cerium $\alpha -\gamma$ phase transition is characterized by means of a many-body Jastrow-correlated wave function, which minimizes the variational energy of the first-principles scalar-relativistic Hamiltonian, and includes correlation effects in a nonperturbative way. Our variational ansatz accurately reproduces the structural properties of the two phases, and proves that even at temperature $T=0\mathrm{K}$ the system undergoes a first-order transition, with ab initio parameters which are seamlessly connected to the ones measured by experiment at finite $T$. We show that the transition is related to a complex rearrangement of the electronic structure, with a key role played by the $p-f$ hybridization. The underlying mechanism unveiled by this work can hold in many Ce-bearing compounds, and more generally in other $f$-electron systems.

X-ray photoelectron spectroscopy combined with first-principles modeling is a powerful tool for determining the chemical composition and electronic structure of novel materials. Of these, graphene is an especially important model system for understanding the properties of other carbon nanomaterials. Here, we calculate the carbon $1s$ core level binding energy of pristine graphene using two methods based on density functional theory total energy differences: a calculation with an explicit core-hole, and an all-electron extension of the delta self-consistent field $\left(\Delta \mathrm{SCF}\right)$ method. We study systematically their convergence and computational workload, and the dependence of the energies on the chosen exchange-correlation functional. The $\Delta \mathrm{SCF}$ method is computationally more expensive, but gives consistently higher C $1s$ energies. Although there is a significant functional dependence, the binding energy calculated using the PBE functional is found to be remarkably close to what has been measured for graphite.