We report results of ab initio calculations on Y-doped anion Si${}_n$ clusters with $n$ $=$ 4–20. Our results suggest two growth behaviors in the intermediate range of $n$ $=$ 8 and 20: (1) There is the formation of linked clusters in which a metal atom links two subclusters and (2) where silicon atoms form a cage structure and the metal atom is inside the cage to produce endohedral cages of silicon clusters. The cluster structures have been identified by comparing the calculated spectra of the electronic states with the photoemission spectra on anion clusters. Our results suggest that in some cases a higher energy isomer may be present in experiments. We report the calculations of the infrared and Raman spectra as well as the dipole moments, electron affinity, and polarizability that could provide other ways of identifying the growth behavior in these clusters.

We study the entanglement entropy of a region of length $2L$ with the remainder of an infinite one-dimensional gapless quantum system in the case where the region is centered on a quantum impurity. The coupling to this impurity is not scale invariant, and the physics involves a crossover between weak- and strong-coupling regimes. While the impurity contribution to the entanglement has been computed numerically in the past, little is known analytically about it, since in particular the methods of conformal invariance cannot be applied because of the presence of a crossover length. We show in this paper that the small coupling expansion of the entanglement entropy in this problem is quite generally plagued by strong infrared divergences, implying a nonperturbative dependence on the coupling. The large coupling expansion turns out to be better behaved, thanks to powerful results from the boundary CFT formulation and, in some cases, the underlying integrability of the problem. However, it is clear that this expansion does not capture well the crossover physics. In the integrable case—which includes problems such as an $XXZ$ chain with a modified link, the interacting resonant level model or the anisotropic Kondo model—a nonperturbative approach is in principle possible using form factors. We adapt in this paper the ideas of Cardy et al. [J. Stat. Phys. 130, 129 (2008)] and Castro-Alvaredo and Doyon [J. Stat. Phys. 134, 105 (2009)] to the gapless case and show that, in the rather simple case of the resonant level model, and after some additional renormalizations, the form-factors approach yields remarkably accurate results for the entanglement all the way from short to large distances. This is confirmed by detailed comparison with numerical simulations. Both our form factor and numerical results are compatible with a nonperturbative form at short distance.

We probe the effects of different initial conditions on the process of aging in spin glasses arising from alternative temperature quenching protocols for the canonical and representative Cu:Mn (6 at.%) dilute magnetic alloy. The effects of these changes are observed through the decay of the thermoremanent magnetization, $M_{\mathrm{TRM}}\left(t\right)$. We find significant changes in the initial state as a consequence of the different cooling processes, reflected through an effective waiting time, $t_w^{\mathrm{eff}}$. The effective waiting time shortens as the time spent above the measuring temperature in the cooling protocol is reduced, reaching a minimum when the temperature is not allowed to rise above the final measurement temperature after the initial quench. This behavior is consistent with the picture of a rapid temperature dependence of the barrier heights separating metastable states.

We report on the influence of correlated substrate roughness on the two-magnon scattering in 30 nm Ni${}_{81}$Fe${}_{19}$ thin films. Using ion beam erosion, periodically modulated substrates (ripple) were produced with wavelengths between 20 and $432\mathrm{nm}$. This surface corrugation is adopted by magnetic layers grown on top yielding dipolar stray fields if magnetization and ripple ridges are aligned perpendicular to each other. In case of $\lambda \ge 222\mathrm{nm}$, the evolving periodic field pattern triggers two-magnon scattering, which depends strongly on the direction of magnetization with respect to the ripple pattern. In-plane broadband ferromagnetic resonance reveals prominent peaks in the frequency-dependent linewidth measured perpendicularly to the ripple ridges. These peaks can be switched off if the magnetization is aligned along the ripple ridges. Our results are compared to predictions obtained from recent theory on spin waves in periodically perturbed films.

We report a study of the thermal conductivity $\kappa$ of the spin-ice material $\mathrm{Dy}{}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. From the anisotropic magnetic-field dependence of $\kappa$ and by additional measurements on the phononic reference compounds $\mathrm{Y}{}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\left(\mathrm{Dy}{}_{0.5}\mathrm{Y}{}_{0.5}\right)}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, we are able to separate the phononic and the magnetic contributions to the total heat transport, i.e., ${\kappa }_{\mathrm{ph}}$ and ${\kappa }_{\mathrm{mag}}$, respectively, which both depend on the magnetic field. The field dependence of ${\kappa }_{\mathrm{ph}}$ arises from the noncollinear magnetic moments of the Dy ions, which tend to align along the field direction. For ${\kappa }_{\mathrm{mag}}$, we observe a highly anisotropic magnetic-field dependence, which correlates with the corresponding magnetization data reflecting the different magnetic-field induced spin-ice ground states. The magnitude of ${\kappa }_{\mathrm{mag}}$ increases with the degree of the ground-state degeneracy. This anisotropic field dependence as well as various hysteresis effects suggest that ${\kappa }_{\mathrm{mag}}$ is essentially determined by the mobility of the magnetic monopole excitations in spin ice.

Tunneling anisotropic magnetoresistance (TAMR) has been characterized in junctions comprised of face-centered cubic (fcc) Co (111) ferromagnetic electrodes grown epitaxially on sapphire substrates, amorphous AlO${}_x$ tunnel barriers, and nonmagnetic Al counterelectrodes. Large TAMR ratios have been found, up to ∼7.5$\%$ and ∼11$\%$ (at 5 K), for the in-plane and out-of-plane magnetization geometry, respectively. Such large TAMR values were not expected a priori, given the weak anisotropy of the (bulk) Co bands due to spin-orbit interaction, and the absence of Co (111) surface states that cross the Fermi energy. Both the in-plane and out-of-plane TAMR effects exhibit a predominantly twofold symmetry, and a strong bias dependence. The in-plane TAMR shows a maximum along the (twofold) magnetic hard axis, suggesting a relation between magnetic anisotropy and TAMR. We propose that uniaxial strain in combination with Bychkov-Rashba spin-orbit interaction, producing an interfacial tunneling DOS that depends on the magnetization direction, is responsible for the TAMR effect. The importance of the interfacial Co/AlO${}_x$ (electronic) structure for the TAMR effect is underlined by measurements on junctions with overoxidized AlO${}_x$ barriers, which show markedly different bias and angle dependence.

We show that carbon nanotubes (CNTs) are good candidates for one-dimensional topological superconductivity with Majorana fermions (MFs) localized at the tube ends. Such states can potentially be useful for topological quantum computation using a nanotube network. The physics behind topological superconductivity in CNTs is novel and mediated by a recently reported curvature-induced spin-orbit coupling which itself has a topological origin. In addition to the spin-orbit coupling, an important new requirement for a robust topological state is broken chirality symmetry. We use topological arguments, calculations of the topological gap, and explicit numerical solutions of the Bogoliubov–de Gennes equations to show that, for recently reported strengths of spin-orbit coupling and broken chirality symmetry, MFs and a robust topological gap $\sim$500 mK are achievable in chiral carbon nanotubes.

The magnetization of three high-quality single crystals of YBa${}_2$Cu${}_3$O${}_{6+x}$, from slightly overdoped to heavily underdoped, has been measured using torque magnetometry. Striking effects in the angular dependence of the torque for the two underdoped crystals, a few degrees above the superconducting transition temperature ($T_c$), are described well by the theory of Gaussian superconducting fluctuations using a single adjustable parameter. The data at higher temperatures ($T$) are consistent with a strong cutoff in the fluctuations for $T\gtrsim 1.1T_c$. Numerical estimates suggest that inelastic scattering could be responsible for this cutoff.

Magnetoelectric hexaferrite Ba${}_{0.5}$Sr${}_{1.5}$Zn${}_2$Fe${}_{12}$O${}_{22}$ (BSZFO) and Ba${}_{0.5}$Sr${}_{1.5}$Zn${}_2$(Fe${}_{0.92}$Al${}_{0.08}$)${}_{12}$O${}_{22}$ (BSZFAO) were investigated by ${}^{57}$Fe nuclear magnetic resonance (NMR). The spin-canting angles of all NMR peaks were measured to assign each peak to corresponding Fe${}^{3+}$ sites. It was revealed that the spin-canting angle of Fe${}^{3+}$ ions at off-centered octahedra is fixed and Al${}^{3+}$ ions substitute for Fe${}^{3+}$ ions at those sites. The temperature dependence of the NMR frequency indicates that the low-temperature excitation is spin wave in ferromagnetic phase.

We report inelastic neutron scattering results on a rare-earth-based single-molecule magnet, the Tb-Cu dinuclear complex. By means of a high-resolution neutron chopper spectrometer, the details of the excitations were clarified. The magnetic excitations are clearly observed at $\hslash \omega =1.7$ and 12.3 meV. The transition energy of 1.7 meV corresponds to the energy barrier of magnetic relaxation estimated from an ac magnetic susceptibility measurement. This indicates that the magnetization reversal occurs through quantum tunneling between pairs of degenerated excited states at 1.7 meV, which is called a thermally activated tunneling process. Interestingly, these excitations exhibit further peak splitting in energy and their energies are independent of momentum transfer $Q$. Our calculation, which is based on a spin Hamiltonian, suggests that (i) the excitation at 1.7 meV originates from the exchange coupling between Tb${}^{3+}$ and Cu${}^{2+}$, (ii) the excitation at 12.3 meV corresponds to the transition between the multiplets $\mathit{J}$ of the Tb${}^{3+}$ moment, and (iii) the fine peak splitting in energy is due to the hyperfine interaction between the nuclear spin and electron spin and orbital magnetic moments.

The magnetic properties of ThCr${}_2$Si${}_2$-type single crystals $A$Co${}_2$Se${}_2$ ($A=\text{K}$, Rb, and Cs) were investigated by means of magnetic susceptibilities and isothermal magnetizations measurements. The ferromagnetic phase transition temperatures are $\approx$74 and 76 K for $A=\text{K}$ and Rb, respectively. A possible antiferromagnetic phase transition occurs around 80 K in susceptibility and a metamagnetismlike behavior is first discovered at a field of $\approx$3.5 T for $H\parallel ab$ plane in CsCo${}_2$Se${}_2$ below 62 K. The susceptibilities in the paramagnetic state obey the modified Curie-Weiss law quite well in all samples. Moreover, the derived effective magnetic moments of the Co atom are about 2.21, 2.04, and 2.04 ${\mu }_B$/Co for $A=\text{K}$, Rb, and Cs, respectively. The generalized Rhodes-Wohlfarth ratio indicates an itinerant magnetism in this series. Finally, we discussed the magnetic properties in this series within the frameworks of the self-consistent renormalization and Takahashi's theory of spin fluctuations.

Electrical resistivity measurements as a function of temperature between 1 and 300 K were performed at various pressures up to 3 GPa on the superconducting layered compounds $\mathit{Ln}$O${}_{0.5}$F${}_{0.5}$BiS${}_2$ ($\mathit{Ln}$ = La, Ce). At atmospheric pressure, LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ and CeO${}_{0.5}$F${}_{0.5}$BiS${}_2$ have superconducting critical temperatures $T_c$ of 3.3 and 2.3 K, respectively. For both compounds, the superconducting critical temperature $T_c$ initially increases, reaches a maximum value of 10.1 K for LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ and 6.7 K for CeO${}_{0.5}$F${}_{0.5}$BiS${}_2$, and then gradually decreases with increasing pressure. Both samples also exhibit transient behavior in the region between the lower $T_c$ phase near atmospheric pressure and the higher $T_c$ phase at higher pressures. This region is characterized by a broadening of the superconducting transition, in which $T_c$ and the transition width $\Delta$$T_c$ are reversible with increasing and decreasing pressure. There is also an appreciable pressure-induced and hysteretic suppression of semiconducting behavior up to the pressure at which the maximum value of $T_c$ is found. At pressures above the value at which the maximum in $T_c$ occurs, there is a gradual decrease of $T_c$ and further suppression of the semiconducting behavior with pressure, both of which are reversible.

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the $s^{\pm }$-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies ($E\le 2\Delta$) below $T_c$. On the other hand, the $s^{++}$-pairing symmetry expects a broad spin excitation enhancement at an energy above $2\Delta$ below $T_c$. Although the resonance has been observed in iron pnictide superconductors at an energy below $2\Delta$ consistent with the $s^{\pm }$-pairing symmetry, the mode has also been interpreted as arising from the $s^{++}$-pairing symmetry with $E\ge 2\Delta$ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at $E=7$ meV in SC NaFe${}_{0.935}$Co${}_{0.045}$As ($T_c=18$ K). On warming towards $T_c$, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the $s^{\pm }$ and $s^{++}$-pairing symmetries, we conclude that the ground-state resonance in NaFe${}_{0.935}$Co${}_{0.045}$As is only consistent with the $s^{\pm }$ pairing, and is inconsistent with the $s^{++}$-pairing symmetry.

We study thermoelectric transport at low temperatures in correlated Kondo insulators, motivated by the recent observation of a high thermoelectric figure of merit ($ZT$) in ${\mathrm{FeSb}}_2$ at $T\sim 10\mathrm{K}$ [A. Bentien et al., Eur. Phys. Lett. 80, 17008 (2007)]. Even at room temperature, correlations have the potential to lead to high $ZT$, as in ${\mathrm{YbAl}}_3$, one of the most widely used thermoelectric metals. At low temperature correlation effects are especially worthy of study because fixed band structures are unlikely to give rise to the very small energy gaps $E_g\sim 5\mathrm{kT}$ necessary for a weakly correlated material to function efficiently at low temperature. We explore the possibility of improving the thermoelectric properties of correlated Kondo insulators through tuning of crystal-field and spin-orbit coupling and present a framework to design more efficient low-temperature thermoelectrics based on our results.

Low bandwidth Pr-based cobalt perovskites, such as Pr${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$, have received significant recent attention as they undergo first-order spin-state transitions with a strong influence on magnetic and transport properties. The unique nature of the Pr-O bond has been implicated as the impetus for these transitions, as it is thought that temperature-dependent charge transfer can occur between Pr and Co ions, i.e., a partial Pr${}^{3+}$→Pr${}^{4+}$ and Co${}^{4+}$→Co${}^{3+}$ valence shift. In the present work, we have studied the related compound Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$. The Nd${}^{3+}$ ions have very similar ionic radius to Pr${}^{3+}$ but do not induce a temperature-dependent valence shift (at least in the composition range studied here), enabling deconvolution of the intrinsic low bandwidth physics from the unique effects of Pr-O bonding in Pr${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$. To this end, we have characterized the structural, magnetic, and electronic transport characteristics of Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$ bulk polycrystals, using neutron diffraction, small-angle neutron scattering, dc and ac magnetometry, and magnetotransport, and have established the Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$ magnetic phase diagram. This phase diagram contains regimes of short-range ferromagnetism and long-range ferromagnetism, in addition to ferrimagnetism. We argue that, with the exception of the valence transition that occurs at high $x$ (e.g., $x$ $=$ 0.5) in Pr${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$ and the low-temperature ordering of Nd${}^{3+}$moments that results in the ferrimagnetism in Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$, the two systems are nearly isostructural and have similar magnetic and transport properties. The low bandwidth physics intrinsic to both systems is summarized as encompassing long-range ferromagnetism with a relatively low Curie temperature due to Co-O-Co bond buckling (<60 K for Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$), short-range ferromagnetism that emerges at much higher temperatures (∼270 K for Nd${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$), and likely stems from oxygen deficiency, exchange-spring behavior related to magnetoelectronic phase separation, and a doping-driven insulator-metal transition. In addition to elucidating the essential physics of narrow bandwidth perovskite cobaltites, the results thus further confirm the importance of the unique features of the Pr-O bond in driving the abrupt spin-state transition in Pr${}_{1-x}$Ca${}_x$CoO${}_{3-\delta }$.

First-principles calculations were carried out on the basis of the density functional theory to investigate the structural properties and spontaneous polarization in wurtzite Mg${}_x$Zn${}_{1-x}$O. Random atomic configurations were applied to compute electronic ground states in large $4\times 4\times 3$ unit cells containing 192 atoms. Our theoretical calculation predicted a nonlinear characteristic of the lattice parameters $a$, $c$, and $u$ of wurtzite Mg${}_x$Zn${}_{1-x}$O, indicating the violation of Vegard's law. We also clarified a nonlinear characteristic of spontaneous polarization in the alloys and its almost linear dependence on the internal parameter $u$.

We theoretically study the $(1,1)$ triplet to $(0,2)$ singlet relaxation rate in a lateral gate-defined double quantum dot tuned to the regime of Pauli spin blockade. We present a detailed derivation of the effective phonon density of states for this specific charge transition, keeping track of the contribution from piezoelectric as well as deformation potential electron-phonon coupling. We further investigate two different spin-mixing mechanisms which can couple the triplet and singlet states: a magnetic field gradient over the double dot (relevant at low external magnetic field) and spin-orbit interaction (relevant at high field), and we also indicate how the two processes could interfere at intermediate magnetic field. Finally, we show how to combine all results and evaluate the relaxation rate for realistic system parameters.

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of the chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magnetotransport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov–de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov–de Haas oscillation were observed, giving extremal Fermi surface areas of $A_S=9.1\times {10}^{12}$ cm${}^{-2}$ and $A_L=3.47\times {10}^{14}$ cm${}^{-2}$ with corresponding cyclotron masses $m_S^{*}=0.0353m_e$ and $m_L^{*}=0.178m_e$, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin-splitting effect in BiTeI.

Based on first-principles calculations, the Ga vacancy and antisite defect Se${}_{\mathrm{Ga}}$ are the only intrinsic shallow acceptors in Ga${}_2$Se${}_3$. The Ga interstitial is always a donor, $+$3 charge state under $p$-type conditions or $+$1 charge state under $n$-type conditions. Both the Se vacancy and Se interstitial are neutral defects except under an extremely $p$-type condition. Both the Ga interstitial and Se interstitial are predicted to be a very fast diffuser under $n$-type conditions, with a migration barrier value of $<$0.3 eV. Under $p$-type conditions, the calculated migration barrier of the Ga interstitial has a quite large value of $>$1.0 eV. The $+$2 charge state is stable for the Se interstitial under the extremely $p$-type condition, and the corresponding migration barrier is 0.68 eV. The migration barriers of the Ga vacancy and Se vacancy are $>$0.8 eV and $>$1.3 eV, respectively.

Bulk and multilayered thin film crystals of II-VI semiconductor compounds are the leading materials for infrared sensing, γ-ray detection, photovoltaics, and quantum dot lighting applications. The key to achieving high performance for these applications is reducing crystallographic defects. Unfortunately, past efforts to improve these materials have been prolonged due to a lack of understanding with regards to defect formation and evolution mechanisms. To enable high-fidelity and high-efficiency atomistic simulations of defect mechanisms, this paper develops a Stillinger-Weber interatomic potential database for semiconductor compounds composed of the major II-VI elements Zn, Cd, Hg, S, Se, and Te. The potential's fidelity is achieved by optimizing all the pertinent model parameters, by imposing reasonable energy trends to correctly capture the transformation between elemental, solid solution, and compound phases, and by capturing exactly the experimental cohesive energies, lattice constants, and bulk moduli of all binary compounds. Verification tests indicate that our model correctly predicts crystalline growth of all binary compounds during molecular dynamics simulations of vapor deposition. Two stringent cases convincingly show that our potential is applicable for a variety of compound configurations involving all the six elements considered here. In the first case, we demonstrate a successful molecular dynamics simulation of crystalline growth of an alloyed (Cd${}_{0.28}$Zn${}_{0.68}$Hg${}_{0.04}$) (Te${}_{0.20}$Se${}_{0.18}$S${}_{0.62}$) compound on a ZnS substrate. In the second case, we demonstrate the predictive power of our model on defects, such as misfit dislocations, stacking faults, and subgrain nucleation, using a complex growth simulation of ZnS/CdSe/HgTe multilayers that also contain all the six elements considered here. Using CdTe as a case study, a comprehensive comparison of our potential with literature potentials is also made. Finally, we also propose unique insights for improving the Stillinger-Weber potential in future developments.