${\mathrm{PrV}}_2{\mathrm{Al}}_{20}$ is a heavy-fermion superconductor based on the cubic ${\mathrm{\Gamma }}_3$ doublet that exhibits nonmagnetic quadrupolar ordering below $\sim 0.6$ K. Our magnetotransport study on ${\mathrm{PrV}}_2{\mathrm{Al}}_{20}$ reveals field-induced quadrupolar quantum criticality at ${\mu }_0{H}_{\mathrm{c}}\sim 11$ T applied along the [111] direction. Near the critical field ${\mu }_0{H}_{\mathrm{c}}$ required to suppress the quadrupolar state, we find a marked enhancement of the resistivity $\rho (H,T)$, a divergent quasiparticle effective mass and concomitant non-Fermi-liquid (NFL) behavior [i.e., $\rho \left(T\right)\propto T^n$ with $n\le 0.5$]. We also observe the Shubnikov–de Haas effect above ${\mu }_0{H}_{\mathrm{c}}$, indicating effective mass enhancement or $m^{*}/m_0\sim 10$. This reveals the competition between the nonmagnetic Kondo effect and the intersite quadrupolar coupling which leads to pronounced NFL behavior in an extensive region of $T$ and ${\mu }_{0}H$ emerging from the quantum-critical point.

We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including ${\mathrm{C}}_{60}, {\mathrm{C}}_{70}$, [6,6]-phenyl-${\mathrm{C}}_{61}$-butyric-acid-methyl-ester ($\left[{\mathrm{C}}_{60}\right]\text{PCBM}$), and bis-$\left[{\mathrm{C}}_{60}\right]\text{PCBM}$ using many-body perturbation theory within the Hedin's $G_0{W}_0$ approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ agree very well with experimental values measured in the gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin films, and they exhibit a close agreement—a rigid energy-gap renormalization owing to intermolecular interactions in the thin films. In addition, it is shown that the low-lying unoccupied states of $\left[{\mathrm{C}}_{60}\right]\text{PCBM}$ are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene ${\mathrm{C}}_{60}$. The functional side group in $\left[{\mathrm{C}}_{60}\right]\text{PCBM}$ introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene ${\mathrm{C}}_{60}$ with $\left[{\mathrm{C}}_{60}\right]\text{PCBM}$ as the electron acceptor in bulk heterojunction polymer solar cells.

We study numerically the influence of disorder and localization effects on the local spectroscopic characteristics and infrared optical properties of ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}$. We treat the band structure and disorder effects at an equal level by using an exact diagonalization supercell simulation method. This method accurately describes the low-doping limit and gives a clear picture of the transition to higher dopings, which captures the localization effects inaccessible to other theoretical methods commonly used. Our simulations capture the rich in-gap localized states observed in scanning tunneling microscopy studies and reproduce the observed features of the infrared optical absorption experiments. We show clear evidence of a disordered-valence-band model for metallic samples in which (i) there is no impurity band detached from the valence band, (ii) the disorder tends to localize and pull states near the top of the valence band into the gap region, and (iii) the Fermi energy is located deep in the delocalized region away from the mobility edge. We identify localized states deep in the gap region by visualizing the probability distribution of the quasiparticles and connecting it to their respective participation ratios. The analysis of the infrared optical absorption data indicates that it does not have a direct relation to the nature of the states at the Fermi energy.

We study the physical properties of ZnX (X = O, S, Se, Te) and CdX (X = O, S, Se, Te) in the zinc-blende, rock-salt, and wurtzite structures using the recently developed fully ab initio pseudohybrid Hubbard density functional ACBN0. We find that both the electronic and vibrational properties of these wide band-gap semiconductors are systematically improved over the PBE values and reproduce closely the experimental measurements. Similar accuracy is found for the structural parameters, especially the bulk modulus. ACBN0 results compare well with hybrid functional calculations at a fraction of the computational cost.

One dimensional systems strongly enhance the quantum character of electron transport. Such systems can be realized in $5d$ transition metals Au, Pt, and Ir, in the form of suspended monatomic chains between bulk leads. Atomic chains between ferromagnetic leads would open up many perspectives in the context of spin-dependent transport and spintronics, but the evidence suggests that for pure metals only the mentioned three $5d$ metals are susceptible to chain formation. It has been argued that the stability of atomic chains made up from ferromagnetic metals is compromised by the same exchange interaction that produces the local moments. Here we demonstrate that magnetic atomic chains can be induced to form in break junctions under the influence of light molecules. Explicitly, we find deuterium assisted chain formation in the $3d$ ferromagnetic transition metals Fe and Ni. Chain lengths up to eight atoms are formed upon stretching the ferromagnetic atomic contact in deuterium atmosphere at cryogenic temperatures. From differential conductance spectra vibronic states of ${\mathrm{D}}_2$ can be identified, confirming the presence of deuterium in the atomic chains. Shot noise spectroscopy indicates the presence of weakly spin polarized transmission channels.

(100)-epitaxial ${\mathrm{SrTiO}}_3$ thin films having biaxial compressive strains up to $-1.60\%$ were grown on lattice mismatched substrates. Two phase transitions induced by the coupled instabilities (antiferrodistortive and ferroelectric) in ${\mathrm{SrTiO}}_3$ were revealed in a common set of samples investigated at temperatures from 8 to 600 K by $\mathrm{CuK}\alpha$ and synchrotron x-ray diffractions and by UV-Raman spectroscopy. It is shown that the in-plane compressive strain in ${\mathrm{SrTiO}}_3$ significantly increases the transition temperature between cubic and tetragonal-antiferrodistortive phases and furthermore precipitates a ferroelectric polar phase at lower temperatures. The strain-temperature phase diagram based on the measurements shows a cubic-to-antiferrodistortive transition temperature that is substantially higher than predicted within a phenomenological Landau-Ginsburg-Devonshire treatment. In contrast the transition to the ferroelectric polar phase is found to be close to the temperature predicted, with the consequence that there is no crossing of the two phase transitions for temperatures up to 600 K.

The random-field Ising model shows an extreme critical slowdown that has been described by activated dynamic scaling: The characteristic time of relaxation toward equilibrium diverges exponentially with the correlation length $ln\tau \sim {\xi }^{\psi }/T$, with $\psi$ an a priori unknown barrier exponent. Through a nonperturbative functional renormalization group, we show that for spatial dimensions $d$ less than a critical value $d_{\text{DR}}\simeq 5.1$, also associated with dimensional-reduction breakdown, $\psi =\theta$ with $\theta$ the temperature exponent near the zero-temperature fixed point that controls the critical behavior. For $d>d_{\text{DR}}$, on the other hand, $\psi =\theta -2\lambda$, where $\theta =2$ and $\lambda >0$ an additional exponent. At the upper critical dimension $d=6,\lambda =1$, so that $\psi =0$, and activated scaling gives way to conventional scaling. We give a physical interpretation of the results in terms of collective events in real space, avalanches, and droplets. We also propose a way to check the two regimes by computer simulations of long-range one-dimensional systems.

Within the wave-packet semiclassical approach, the Bloch electron energy is derived to second order in the magnetic field and classified into gauge-invariant terms with clear physical meaning, yielding a fresh understanding of the complex behavior of orbital magnetic susceptibility. The Berry curvature and quantum metric of the Bloch states give rise to a geometrical magnetic susceptibility, which can be dominant when bands are filled up to a small energy gap. There is also an energy polarization term, which can compete with the Peierls-Landau and Pauli magnetism on a Fermi surface. All these, and an additional Langevin susceptibility, can be calculated from each single band, leaving the Van Vleck susceptibility as the only term truly from interband coupling.

The effects of electron-electron correlations on the low-energy electronic structure and their relationship with unconventional superconductivity are central aspects in the research on iron-based pnictide superconductors. Here we use soft x-ray angle-resolved photoemission spectroscopy to study how electronic correlations evolve in different chemically substituted iron pnictides. We find that correlations are intrinsically related to the effective filling of the correlated orbitals, rather than to the filling obtained by valence counting. Combined density functional theory and dynamical mean-field theory calculations capture these effects, reproducing the experimentally observed trend in the correlation strength. The occupation-driven trend in the electronic correlation reported in our paper supports and extends the recently proposed connection between cuprate and pnictide phase diagrams.

Thermalization of a parametrically driven magnon gas leading to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum was studied by time- and wave-vector-resolved Brillouin light scattering spectroscopy. It has been found that the condensation is preceded by the conversion of initially pumped magnons into a second group of frequency-degenerated magnons, which appear due to parametrically stimulated scattering of the initial magnons to a short-wavelength spectral region. In contrast to the first magnon group, in which wave vectors are orthogonal to the wave vectors of the magnons at the lowest energy states, the secondary magnons can effectively scatter to the bottom of the spectrum and condense there.

Doping $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{o}}_x\mathrm{W}{\mathrm{O}}_4$ with Co produces a rich magnetic phase diagram with a large number of ferroelectric phases. We present resonant x-ray magnetic scattering experiments in the known collinear AF4 phase of multiferroic $\mathrm{M}{\mathrm{n}}_{0.85}\mathrm{C}{\mathrm{o}}_{0.15}\mathrm{W}{\mathrm{O}}_4$ showing that Mn and Co spins point to different directions despite they randomly occupy the same crystallographic site. The resultant noncollinear exotic spin configuration is determined by a competition of $\mathrm{M}{\mathrm{n}}^{2+}$ and $\mathrm{C}{\mathrm{o}}^{2+}$ magnetic anisotropies and demands a reexamination of the ferroelectric phase diagram of this model family beyond the previous average description of its magnetic orders.

We present time-of-flight inelastic neutron scattering measurements of ${\mathrm{La}}_{1.965}{\mathrm{Ba}}_{0.035}{\mathrm{CuO}}_4$ (LBCO), a lightly doped member of the high temperature superconducting La-based cuprate family. By using time-of-flight neutron instrumentation coupled with single crystal sample rotation we obtain a four-dimensional data set (three Q and one energy) that is both comprehensive and spans a large region of reciprocal space. Our measurements identify rich structure in the energy dependence of the highly dispersive spin excitations, which are centered at equivalent ($\frac{1}{2},\frac{1}{2},L$) wave vectors. These structures correlate strongly with several crossings of the spin excitations with the lightly dispersive phonons found in this system. These effects are significant and account for on the order of $25\%$ of the total inelastic scattering for energies between $\approx 5$ and 40 meV at low $\left|\mathbf{Q}\right|$. Interestingly, this scattering also presents little or no $L$ dependence. As the phonons and dispersive spin excitations centered at equivalent ($\frac{1}{2},\frac{1}{2},L$) wave vectors are common to all members of La-based 214 copper oxides, we conclude such strong quasi-two-dimensional scattering enhancements are likely to occur in all such 214 families of materials, including those concentrations corresponding to superconducting ground states. Such a phenomenon appears to be a fundamental characteristic of these materials and is potentially related to superconducting pairing.

There have been increasing efforts in realizing topological metallic phases with nontrivial surface states. It was suggested that orthorhombic perovskite iridates are classified as a topological crystalline metal (TCM) with flat surface states protected by lattice symmetries. Here we perform first-principles electronic structure calculations for epitaxially stabilized orthorhombic perovskite iridates. Remarkably, two different types of topological surface states are found depending on surface directions. On side surfaces, flat surface states protected by lattice symmetries emerge, manifesting the topological crystalline character. On the top surface, on the other hand, an unexpected Dirac cone appears, indicating surface states protected by a time-reversal symmetry, which is confirmed by the presence of a nontrivial topological ${\mathbb{Z}}_2$ index. These results suggest that the orthorhombic iridates are unique systems exhibiting both lattice- and global-symmetry-protected topological phases and surface states. Transitions to weak and strong topological insulators and implications of surface states in light of angle resolved photoemission spectroscopy are also discussed.

The magnetoresistance (MR) $\mathrm{\Delta }\rho /\rho$ of the cage-glass compound ${\mathrm{Ho}}_x{\mathrm{Lu}}_{1-x}{\mathrm{B}}_{12}$ with various concentrations of magnetic holmium ions $\left(x\le 0.5\right)$ has been studied in detail concurrently with magnetization $M\left(T\right)$ and Hall effect investigations on high-quality single crystals at temperatures 1.9–120 K and in magnetic field up to 80 kOe. The undertaken analysis of $\mathrm{\Delta }\rho /\rho$ allows us to conclude that the large negative magnetoresistance (nMR) observed in the vicinity of the Néel temperature is caused by scattering of charge carriers on magnetic clusters of ${\mathrm{Ho}}^{3+}$ ions, and that these nanosize regions with antiferromagnetic (AF) exchange inside may be considered as short-range-order AF domains. It was shown that the Yosida relation $-\mathrm{\Delta }\rho /\rho \sim M^2$ provides an adequate description of the nMR effect for the case of Langevin-type behavior of magnetization. Moreover, a reduction of Ho-ion effective magnetic moments in the range 3–9 ${\mu }_B$ was found to develop both with temperature lowering and under the increase of holmium content. A phenomenological description of the large positive quadratic contribution $\mathrm{\Delta }\rho /\rho \sim {\mu }_D^2{H}^2$ which dominates in ${\mathrm{Ho}}_x{\mathrm{Lu}}_{1-x}{\mathrm{B}}_{12}$ in the intermediate temperature range 20–120 K allows us to estimate the drift mobility exponential changes ${\mu }_D\sim T^{-\alpha }$ with $\alpha =1.3$–1.6 depending on Ho concentration. An even more comprehensive behavior of magnetoresistance has been found in the AF state of ${\mathrm{Ho}}_x{\mathrm{Lu}}_{1-x}{\mathrm{B}}_{12}$ where an additional linear positive component was observed and attributed to charge-carrier scattering on the spin density wave (SDW). High-precision measurements of $\mathrm{\Delta }\rho /\rho =f(H,T)$ have allowed us also to reconstruct the magnetic H-T phase diagram of ${\mathrm{Ho}}_{0.5}{\mathrm{Lu}}_{0.5}{\mathrm{B}}_{12}$ and to resolve its magnetic structure as a superposition of $4f$ (based on localized moments) and $5d$ (based on SDW) components.

We consider two spin-$1/2$ fermions in a two-dimensional magnetic system that is close to the $O\left(3\right)$ magnetic quantum critical point (QCP) which separates magnetically ordered and disordered phases. Focusing on the disordered phase in the vicinity of the QCP, we demonstrate that the criticality results in a strong long-range attraction between the fermions, with potential $V\left(r\right)\propto -1/r^{\alpha }, \alpha \approx 0.75$, where $r$ is the separation between the fermions. The mechanism of the enhanced attraction is similar to the Casimir effect and corresponds to multimagnon exchange processes between the fermions. While we consider a model system, the problem is originally motivated by the recent establishment of magnetic QCP in hole-doped cuprates under the superconducting dome at doping of about 10%. We suggest a mechanism of magnetic critical enhancement of pairing in cuprates.

The need for prediction of accurate electronic binding energies has led to the development of different schemes for combining density functional calculations, typically at the level of the generalized gradient approximation (GGA), with experimental information. We analyze one such scheme by Stevanović  et al. [Phys. Rev. B 85, 115104 (2012)] for predictions of compound enthalpies of formation using fitted elemental-phase reference energies. We show that different versions of GGA with or without +U and a meta-GGA (TPSS) lead to comparable accuracy after fitting the reference energies. Our results also show that the recently developed mBEEF, a Bayesian error estimation functional, gives comparable accuracy with the other functionals even without the fitting. The mBEEF functional furthermore supplies an ensemble estimate of the prediction errors in reasonable agreement with the actual errors. We also show that using the fitting scheme on the mBEEF ensemble leads to improved accuracy including realistic error estimation.

We studied how the surface reconstruction and passivation influence the antiferromagnetic and electronic structures of NiO(111) surface using first-principles electronic structure calculations. These features lead to a surprisingly wide variety of different surface electronic structures, and some surfaces are even metallic. Different reconstructions and surface passivation were also found to qualitatively alter the charge-transfer band gap type of bulk NiO. At the same time, the antiferromagnetic character of bulk NiO in the ⟨111⟩ direction is retained even near the surface, and the magnetic moment quickly converges to the bulk value within a few surface layers.

We have investigated the many-body quantum states in single InAs quantum dots (QDs) by simultaneously obtaining the terahertz (THz) intersublevel transition and single electron tunneling spectra. It is found that the intersublevel transition energies measured in the few-electron region are systematically larger than the excited state (ES) energies determined from the transport measurements. We show that tunneling and THz spectroscopy probe the same many-body excited states in the QDs, but their sensitivities depend on their selection rules. In the many-electron region, we observe THz peaks whose energies coincide with the tunneling ESs.

Using density-functional theory (DFT), we study the growth of pristine and functionalized tin monolayers (Sn-MLs) on three different substrates, CdTe, InSb, and Si(111), and the impact these substrates have on the topological insulating properties of the electronic band structure. The presence of the substrate leads to strain and electronic charge transfer, which cause significant changes in the stability and electronic properties of the supported Sn-ML. Growth of pristine Sn-MLs on Si(111) leads to metallic behavior resembling that of the high-buckled Sn-ML phase; pristine Sn-MLs grown on InSb do not maintain a gap throughout the entire Brillouin zone; and pristine Sn-MLs grown on CdTe are unlikely to exhibit an experimentally observable gap. Provided the charge transfer from the substrate can be compensated, halogen-functionalized Sn-MLs grown on CdTe and InSb are topological insulators, albeit with a reduced band gap compared to their free-standing counterparts (from 0.34 eV for Sn-ML-I to 0.17 eV for InSb-Sn-ML-I). We employ ab initio thermodynamics calculations to study the thermodynamic stability of the halogenated InSb-Sn-MLs and CdTe-Sn-MLs surfaces for a temperature range of 100–1000 K under two extreme environments: ultrahigh vacuum (used in most of the laboratory characterization techniques) and rich-halogen conditions (10% vol. halogen environment). Our results indicate that it is possible to obtain stable topologically insulating Sn-MLs grown epitaxially on lattice-matched substrates.