High-$T_{\mathrm{c}}$ cuprate, alkali-doped ${\mathrm{C}}_{60}$, and several other unconventional superconductors have very high transition temperatures $T_{\mathrm{c}}$ with respect to the energy scale of superconducting (SC) charges inferred from the superfluid density (SFD). The observed linear relationship between $T_{\mathrm{c}}$ and the SFD can hardly be expected in BCS superconductors while being reminiscent of Bose-Einstein condensation of preformed bosonic charges. As additional non-BCS-like behaviors, responses similar to those in the bulk SC states have been observed at temperatures well above $T_{\mathrm{c}}$ in the vortexlike Nernst effect, diamagnetic susceptibility, and transient optical conductivity in recent photoexcited pump-probe measurements. In this paper, we propose a coherent picture based on equilibrium and transient SFD to understand these unconventional behaviors in cuprates, ${\mathrm{K}}_3{\mathrm{C}}_{60}$, and organic superconductors. This picture assumes: (1) Dynamic SC responses in the Nernst and photoinduced measurements emerge at the formation of the local phase coherence (LPC) among wave functions of preformed bosonic pairs. (2) Its onset temperature $T_{\mathrm{LPC}}$ is distinct from and lower than the boson formation temperature often denoted as the “pseudogap temperature” $T^{*}$, as $T_{\mathrm{LPC}}$ is determined by the many-body boson density while $T^{*}$ represents attractive interaction between two fermions. (3) The bulk superconducting $T_{\mathrm{c}}$, signaling global phase coherence, is significantly reduced from $T_{\mathrm{LPC}}$, due to the competition between the SC and antiferromagnetic (AF) order. (4) The inelastic magnetic resonance mode (MRM) controls $T_{\mathrm{c}}$ in the SC-AF competition. (5) The transient optical responses can be attributed to a change of the balance between the competing SC and AF orders caused by photoexcitation. The assumptions (1) and (2) explain the relationship between $T_{\mathrm{c}}$ and the transient SFD in photoexcited studies and equilibrium SFD in Nernst effect. (3) and (4) are inferred from the linear dependence of $T_{\mathrm{c}}$ on the MRM energy. (4) and (5) are consistent with the behaviors of the $400-\mathrm{c}{\mathrm{m}}^{-1}$ optical responses in equilibrium and photoexcited studies and temperature dependence of the intensity of this optical mode and the MRM. Unlike previous phase-fluctuation pictures which expect dynamic responses between $T^{*}$ and $T_{\mathrm{c}}$, the present picture involving competing order indicates that dynamic SC responses are seen between $T_{\mathrm{LPC}}$ and $T_{\mathrm{c}}$.

High-critical-temperature $\left(T_{\mathrm{c}}\right)$ superconductivity at 48 K is reported for hydrogen-doped SmFeAsO epitaxial films on MgO single-crystal substrates. The key processes are pulsed laser deposition to grow undoped SmFeAsO epitaxial films and subsequent topotactic chemical reaction using $\mathrm{Ca}{\mathrm{H}}_2$ powders under evacuated silica-glass ampule atmosphere. Based on this post-deposition thermal annealing treatment that we have developed, a maximum hydrogen concentration $x=\sim 0.35$ was realized in $\mathrm{SmFeAs}\left({\mathrm{O}}_{1–x}{\mathrm{H}}_x\right)$. Disordered hydrogen substitution at O sites is experimentally confirmed directly by atomic-scale microstructural observations. Magnetization measurement validates the bulk nature of the high-$T_{\mathrm{c}}$ superconductivity in the films. This method will become an effective and general method to fabricate various high-quality oxyhydride epitaxial films.

The strength of defect-free crystalline materials is known to be close to the theoretical strength of the material and is typically governed by the nucleation of dislocations. Dislocation nucleation is controlled by the energy barrier associated with the nucleation process, which is a function of both the temperature and stress. Previous work has suggested that the energy barrier decreases with stress towards zero as a power-law relationship and that there may be an exponent with universal value for nucleation. In this paper, we analyze atomistic simulations and continuum models to determine if such a universal exponent exists. Fitting of all available atomistic data to the power-law expression does not yield evidence of a universal power-law behavior. The examination of continuum models, however, provides particular interesting insight. Sufficiently far away from the athermal limit, the energy barrier decays with an effective exponent greater than one such that both the energy barrier and activation volume decrease. However, very close to the athermal limit, the activation volume diverges, due to a divergent activation area of the dislocation loop, resulting in a limiting exponent around 0.5. The divergent activation volume is a result of including the generalized stacking fault energy and allowing the Burgers vector to go to zero during nucleation. These results further provide insight into strategies for empirically modeling the activation energy and activation volume, because they contradict the idea that the activation volume goes to zero, which is currently assumed in most empirical forms of the activation energy.

Evaluating many-body correlation effects beyond the commonly applied local or semilocal density functionals has received tremendous attention over the past few years. Using the random phase approximation to describe the correlation energy combined with the exact exchange energy, we have investigated 20 cubic $AB{\mathrm{O}}_3$-type perovskites and three prototypical ferroelectric (tetragonal) perovskites. A quantitative analysis and comparison of the performance of various local and semilocal exchange-correlation functionals shows that the inclusion of dynamical correlation effects allows for an excellent account of the structure and energetics of complex $AB{\mathrm{O}}_3$-type oxides.

First-principles molecular dynamics is employed to investigate thermal transport in glassy ${\mathrm{GeTe}}_4$, a subsystem of several ternary phase-change materials. As a first result, we found modes localized on a few atoms in the vibrational density of states. The thermal transport is further rationalized by calculating the thermal conductivity for a range of system sizes and shapes via the approach-to-equilibrium methodology. By considering the length dependence of the thermal conductivity, we provide evidence of propagative modes with mean free paths as long as 6 nm, i.e., well beyond short-range order distances. Extrapolation of our bulk thermal conductivity to macroscopic sizes is in full agreement with the experimental values. Finally, we assess phenomenological models developed for the thermal conductivity of disordered materials by enriching their intrinsic significance via the insertion in their analytical expression of values obtained via first-principles molecular dynamics.

Ab initio density-functional theory is used to study the structural changes of a crystalline silicon nanowire during lithiation and delithiation processes. Based on the obtained results quantum transport calculations are performed to shed light on the electrical current flowing at varying lithium concentrations. It is found that inserting lithium atoms into the crystalline silicon system leads to a steplike process, where each Si shell is successively lithiated from the outside. While increasing the Li concentration, the volume of the nanowire quadruples and the silicon network becomes fully amorphous. During delithiation the Li ions are initially homogeneously distributed throughout the nanowire cross section, eliminating the steplike behavior observed in the lithiation cycle. Hereby, the volume of the system decreases to 113% of its initial value, with a small amount of Li ions remaining in the Si network and a loss of storage capacity as a consequence. A simultaneous analysis of the current trajectories within the lithiated nanowires reveals that the main contribution shifts from the center to the surface of the nanostructure as the Li concentration increases.

The magnetic and transport properties of Fe-deficient ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$ single crystals (${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ with $x\approx 0.3$) were studied and the impact of thermal processing was explored. Quenching crystals from the growth temperature has been previously shown to produce a metastable state that undergoes a strongly hysteretic first-order transition upon cooling below $\approx 100$ K. The first-order transition impacts the magnetic properties, yielding an enhancement in the Curie temperature $T_{\mathrm{C}}$ from 270 to 310 K. In the present work, $T_{HT}\approx 550$ K has been identified as the temperature above which metastable crystals are obtained via quenching. Diffraction experiments reveal a structural change at this temperature, and significant stacking disorder occurs when samples are slowly cooled through this $T$ range. The transport properties are demonstrated to be similar regardless of the crystal's thermal history. The scattering of charge carriers appears to be dominated by moments fluctuating on the Fe(1) sublattice, which remain dynamic down to $\approx 100–120$ K. Maxima in the magnetoresistance and anomalous Hall resistance are observed near 120 K. The Hall and Seebeck coefficients are also impacted by magnetic ordering on the Fe(1) sublattice. The data suggest that both electrons and holes contribute to conduction above 120 K, but that electrons dominate at lower $T$ when all of the Fe sublattices are magnetically ordered. This study demonstrates a strong coupling of the magnetism and transport properties in ${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ and complements the previous results that demonstrated strong magnetoelastic coupling as the Fe(1) moments order.

In traditional semiconductors with large effective Bohr radius, an electron donor creates a hydrogen-like bound state just below the conduction band edge. The properties of the impurity band arising from such hydrogenic impurities have been studied extensively during the last 70 years. In this paper we consider whether a similar bound state and a similar impurity band can exist in mixed valence insulators, where the gap arises at low temperature due to strong electron-electron interactions. We find that the structure of the hybridized conduction band leads to an unusual bound state that can be described using the physics of the one-dimensional hydrogen atom. The properties of the resulting impurity band are also modified in a number of ways relative to the traditional semiconductor case; most notably, the impurity band can hold a much larger concentration without inducing an insulator-to-metal transition. We estimate the critical doping associated with this transition, and then proceed to calculate the DC and AC conductivities and the specific heat. We discuss our results in light of recent measurements on the mixed valence insulator ${\mathrm{SmB}}_6$ and find them to be consistent with the experiments.

We study the creep behavior of ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ glass-crystal nanocomposites under elastostatic loading conditions in molecular dynamics simulations. By manipulating the glass-crystal interfaces of a precipitation-annealed glass containing Laves-type crystallites, we show that the creep behavior can be tuned. Specifically, we find that for the same microstructure the creep rate scales exponentially with the excess energy in the interfaces, which we raise artificially by disturbing the local short-range order in the atomistic model. The behavior shows analogies to Coble creep in crystalline metals, which depends on grain boundary diffusivity and implicitly on grain boundary energies.

Bridgman single-crystal growth of ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ resulted in systematic Fe-Al lattice site substitution in the ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ (${\mathrm{Mn}}_2{\mathrm{AlB}}_2$-type structure, aka $1\text{−}2\text{−}2$) phase along the direction of solidification. The Fe:Al ratio varied from 1.94 to 2.06. The $1\text{−}2\text{−}2$ phase lattice parameters, magnetic transition temperatures ($T_t$) $280-315$ K, magnetic entropy changes $\left(\mathrm{\Delta }S\right)2.3-4.0\mathrm{J}/\mathrm{kg}\mathrm{K}$, heat capacity ($C_p$) $117-147\mathrm{J}/\mathrm{mol}\mathrm{K}$, and thermal conductivity $\left(\kappa \right)\sim 2.4-11.5\mathrm{W}/\mathrm{m}\mathrm{K}$ all varied monotonically across the region of solubility. These findings confirm the sensitivity of the magnetic and thermal properties of the ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ compound to antisite defects. The interplay between Al and Fe lattice site occupancy and the resultant magnetic and electronic responses facilitates tailoring the properties of the ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ system for potential functional applications, including magnetic cooling.

Control of nanoscale architecture provides unique opportunities to develop novel devices with better performance. Here, we construct a helical architecture by rolling up a graphene nanoribbon. We find that the helical graphene nanoribbon (h-GNR) exhibits direct band gap, high carrier mobility, and efficient photon absorption under infrared illumination. In addition, the electronic structures and optical properties of h-GNR can be precisely tuned by external strains. The band gaps of h-GNRs are given approximately by $E_g\propto t_0\left|\varepsilon \right|$, where $t_0$ is the hopping constant and $\varepsilon$ is the applied strain. The performance of photoresponse in h-GNR is then evaluated by using density-functional theory combined with Keldysh nonequilibrium Green's functions. Under illumination, h-GNR systems exhibit high photoresponsivity in a broad wavelength range, spanning from 1550 nm to $5.90\mu \mathrm{m}$. The predicated photoresponsibility and external quantum efficiency are up to 98 mA/W and 3.45%, respectively. Such outstanding optoelectronic merits combined with exceptional electronic properties make h-GNRs promising candidates for fabricating advanced appliances of infrared photodetectors. Our work may pave the way for designing next-generation devices for infrared light detecting and harvesting.

The synthesis of stoichiometric ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied polarized x-ray absorption spectroscopy at the oxygen $K$ edge and confocal Raman spectroscopy to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ thin films synthesized under different conditions. The results show that these methods can serve as sensitive probes of the electronic and vibrational properties of Ru vacancies, respectively. The intensities of the vacancy-related spectroscopic features extracted from these measurements are well correlated with the transport properties of the films. The methodology introduced here can thus help one to understand and control the stoichiometry and transport properties in films of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and other ruthenates.

Mixed ionic-electronic conducting cathode material plays an important role in the operation of efficient low-temperature solid oxide fuel cells (LT-SOFCs). In the present study, we address the issue of achieving ionic conduction pathways and high mixed ionic-electronic conductivity below 500 °C in cathode materials for LT-SOFC technology. We report high mixed ionic-electronic conductivity in the low temperature range ($\sim {10}^1-{10}^3\mathrm{S}\mathrm{c}{\mathrm{m}}^{-1}$ at 350–500 °C) in the calcium-substituted yttrium iron garnets $\mathrm{C}{\mathrm{a}}_x{\mathrm{Y}}_{3-x}\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12-\delta }$ ($x=0.15$). Oxide ion conduction is governed by an oxygen vacancy concentration mechanism presented here by soft bond valence sum distribution analysis of experimental neutron diffraction data up to 450 °C. The continuous minimum energy conduction pathways for oxide ions in the (100) plane have been visualized by soft bond valence sum analysis. The electronic conduction in present garnets occurs due to hopping of holes from $\mathrm{F}{\mathrm{e}}^{4+}$ to $\mathrm{F}{\mathrm{e}}^{3+}$ chemical states confirmed by Mössbauer spectroscopy. Importantly, the studied materials are found to be thermodynamically stable at least up to high temperatures of 800 °C, which supports well to frequent thermal cycles in SOFC devices. Another important parameter, viz. the thermal expansion coefficient, is found to be comparable with that for the conventionally used anode, electrolyte, and interconnect materials. On the basis of this study, we conclude that the present cathode materials would be useful for an efficient LT- SOFC technology.

In this paper we present a process of forming monolithic GaN surface from an ordered nanowire array by means of material redistribution. This process, referred to as reformation, is performed in a conventional MOVPE crystal growth system with the gallium supply turned off and allows a crystal nanostructure to change shape according to differences in surface energies between its facets. Using reformation, coalescence may proceed closer to thermodynamic equilibrium, which is required for fabrication of high-quality substrate material. Scanning probe techniques are utilized, complemented by cathodoluminescence and electron microscopy, to investigate structural and electrical properties of the surface after reformation, as well as to assess densities, location, and formation of different types of defects in the GaN film. Spatial variations in material properties such as intrinsic majority-carrier types can be attributed to the radical changes in growth conditions required for sequential transition between nanowire growth, selective shell growth, and reformation. These properties enable us to assess the impact of the process on densities, locations, and formation of different types of dislocations in the GaN film. We find a fraction of the nanowires to comprise of a single electrically neutral edge dislocation, propagating from the GaN buffer, while electrically active dislocations are found at coalesced interfaces between nanowires. By decreasing the mask aperture size and changing the nucleation conditions the prevalence of nanowires comprising edge dislocation was significantly reduced from $6\%$ to $3\%$, while the density of interface dislocations was reduced from $6\times {10}^8$ to $4\times {10}^7{\mathrm{cm}}^{-2}$. Using a sequential reformation process was found to create inversion domains with low surface potential N-polar regions in an otherwise Ga-polar GaN film. The inversion domains were associated with pinned dislocation pairs, and were further confirmed by selective wet etching in NaOH. This lateral polarity inversion was thoroughly eliminated in samples formed by a continuous reformation process. These results reveal a path and challenges for growing GaN substrates of superior crystal quality through nanowire reformation.

The description of the chemical bond between a solid surface and an atom or a molecule is the fundamental basis for understanding a broad range of scientific problems in heterogeneous catalysis, semiconductor device fabrication, and fuel cells. Widespread understandings are based on the molecular orbital theory and focused on the degree of filling of antibonding surface-adsorbate states that weaken bonding on surfaces. The unoccupied antibonding surface-adsorbate states are often tacitly assumed to be irrelevant. Here, we show that the unoccupancy of those antibonding surface-adsorbate states is related to antibonding electron transfer: Electrons that would occupy these antibonding states are transferred to the lower-energy Fermi level. This antibonding electron transfer leads to an energy gain, which plays a critically important role in controlling the trends of bond formation on surfaces that are often inhomogeneous and defective. This finding is illustrated from the first-principles study of hydrogen adsorption on ${\mathrm{MoS}}_2$ surfaces. A clear linear relationship between the energies of antibonding electron transfer and hydrogen adsorption is identified. Active sites of hydrogen evolution reaction on ${\mathrm{MoS}}_2$ are found to originate from the in-gap states induced by sulfur vacancies or edges. The emerging picture is general and suited for both metal and semiconductor surfaces. It also offers a physically different explanation for the well-known $d$-band model for hydrogen adsorption on transition-metal surfaces.

Lattice anharmonicity is thought to strongly affect vacancy concentrations in metals at high temperatures. It is however nontrivial to account for this effect directly using density functional theory (DFT). Here we develop a deep neural network potential for aluminum that overcomes the limitations inherent to DFT, and we use it to obtain accurate anharmonic vacancy formation free energies as a function of temperature. While confirming the important role of anharmonicity at high temperatures, the calculation unveils a markedly nonlinear behavior of the vacancy formation entropy and shows that the vacancy formation free energy only violates Arrhenius law at temperatures above 600 K, in contrast with previous DFT calculations.

Recently, the oxygen-reconstructed tantalum surface $\mathrm{Ta}\left(001\right)\text{−}\mathrm{p}(3\times 3)\text{−}\mathrm{O}$ has experienced considerable attention due its use as a potential platform for studying spin chains on superconductors. Experimental studies using scanning tunneling microscopy and spectroscopy found rich atomic and electronic structures already for the clean Ta(001)-O surface, which we combine here with ab initio methods. We found a significant reconstruction of the surface into a 2D polymorph oxide with two distinct patterns of 1-nm scale. One of the patterns represents an unusual defect structural state. This state appears only in the simulations with the effective presence of oxygen vacancies, which we also discuss in the context of the oxide formation. A specific combination of structural and electronic properties was established behind the diverse shapes detected in topographic maps. We also observed the energy-dependent partial-charge localization effect under applied bias voltages, which includes a contrast reversal. These effects originate solely in miscellaneous ionic and metallic properties of the electronic system. The charge distribution and polarization properties of this atomically thin oxide layer is shown to vary significantly between normal and defective superstructures, possibly contributing to the rich phenomena related to topological superconductivity recently discussed for Fe adsorbates.