The $\mathrm{Cu}\mathrm{\Sigma }5$ grain boundary (GB) fracture process has been simulated by the first-principles computational tensile test. An additional metastable state has been discovered during tensile tests. The energy of ideal mirror-symmetric GBs continuously changes with tensile strains. However, at high strains, more stable structures with lower energy are found when some atoms on GBs are artificially relocated. Thus the structures obtained on ideal GBs are not stable and will not occur under actual tensile experiments, which is the exact reason for the occurrence of these unstable structures, which may be considered as additional metastable states. Finally, a large amount of calculations have also been performed to search underlying, more stable GB structures and arrived at almost identical previous results. These results indicate that structures of symmetric GBs under tensile tests should be very carefully optimized by introducing small perturbations even if the energy of the system increases smoothly with increasing tensile strain. In addition, the exclusion of metastable states usually plays a major role in investigating the mechanical properties under tensile test.

We examine the impact of interfacial phonons on the superconducting state of $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ developing a material's specific multiband, full bandwidth, and anisotropic Eliashberg theory for this system. Our self-consistent calculations highlight the importance of the interfacial electron-phonon interaction, which is hidden behind the seemingly weak-coupling constant ${\lambda }_m=0.4$, in mediating the high $T_c$, and explain other puzzling experimental observations, such as the $s$-wave symmetry and replica bands. We discover that the formation of replica bands has a $T_c$ decreasing effect that is nevertheless compensated by deep Fermi-sea Cooper pairing which has a $T_c$ enhancing effect. We predict a strong-coupling dip-hump signature in the tunneling spectra due to the interfacial coupling.

Motivated by the recently renewed interest in the superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds $A{\mathrm{BiO}}_3$ ($A=$ Sr, Ba) using ab initio methods and tight-binding (TB) modeling. We use the density functional theory (DFT) in the local density approximation (LDA) to understand the role of various interactions in shaping the $A{\mathrm{BiO}}_3$ band structure near the Fermi level. It is established that interatomic hybridization involving Bi-$6s$ and O-$2p$ orbitals plays the most important role. Based on our DFT calculations, we derive a minimal TB model and demonstrate that it can describe the properties of the band structure as a function of lattice distortions, such as the opening of a charge gap with the onset of the breathing distortion and the associated condensation of holes onto $a_{1g}$-symmetric molecular orbitals formed by the O-$2p_{\sigma }$ orbitals on collapsed octahedra. We also derive a single band model involving the hopping of an extended molecular orbital involving both Bi-$6s$ and a linear combination of six O-$2p$ orbitals which provides a very good description of the dispersion and band gaps of the low energy scale bands straddling the chemical potential.

We investigated ${\mathrm{SrFeO}}_{3-x}$ thin films on a ${\mathrm{SrTiO}}_3$ (001) substrate prepared via pulsed laser epitaxy using an optical spectroscopy technique. The oxygen vacancy level ($x$) was controlled by post-annealing processes at different oxygen partial pressures. We achieved a brownmillerite (BM) structure at $x=0.5$ and observed the evolution of the crystal structure from BM into perovskite (PV) as the oxygen concentration increased. We observed the evolution of infrared-active phonons with respect to the oxygen concentration, which was closely related to the structural evolution observed via x-ray diffraction. We identified the phonons using the shell-model calculation. Furthermore, we studied temperature-dependent behaviors of the phonon modes of three representative samples: PV and two BMs (${\mathrm{BM}}_{\mathrm{oop}}$ and ${\mathrm{BM}}_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel. In the ${\mathrm{BM}}_{\mathrm{oop}}$ sample, we observed a phonon mode, which exhibited an unusual redshift with decreasing temperature; this behavior may have been due to the apical oxygen instability in the ${\mathrm{FeO}}_6$ octahedron. Our results provide important information regarding the ionic conduction mechanism in ${\mathrm{SrFeO}}_{3-x}$ material systems.

Based on operations prescribed under the paradigm of complex transformation optics (CTO) [F. Teixeira and W. Chew, J. Electromagn. Waves Appl. 13, 665 (1999); F. L. Teixeira and W. C. Chew, Int. J. Numer. Model. 13, 441 (2000); H. Odabasi, F. L. Teixeira, and W. C. Chew, J. Opt. Soc. Am. B 28, 1317 (2011); B.-I. Popa and S. A. Cummer, Phys. Rev. A 84, 063837 (2011)], it was recently shown in [G. Castaldi, S. Savoia, V. Galdi, A. Alù, and N. Engheta, Phys. Rev. Lett. 110, 173901 (2013)] that a complex source point (CSP) can be mimicked by parity-time ($\mathcal{PT}$) transformation media. Such coordinate transformation has a mirror symmetry for the imaginary part, and results in a balanced loss/gain metamaterial slab. A CSP produces a Gaussian beam and, consequently, a point source placed at the center of such a metamaterial slab produces a Gaussian beam propagating away from the slab. Here, we extend the CTO analysis to nonsymmetric complex coordinate transformations as put forth in [S. Savoia, G. Castaldi, and V. Galdi, J. Opt. 18, 044027 (2016)] and verify that, by using simply a (homogeneous) doubly anisotropic gain-media metamaterial slab, one can still mimic a CSP and produce Gaussian beam. In addition, we show that a Gaussian-like beams can be produced by point sources placed outside the slab as well. By making use of the extra degrees of freedom (the real and imaginary parts of the coordinate transformation) provided by CTO, the near-zero requirement on the real part of the resulting constitutive parameters can be relaxed to facilitate potential realization of Gaussian-like beams. We illustrate how beam properties such as peak amplitude and waist location can be controlled by a proper choice of (complex-valued) CTO Jacobian elements. In particular, the beam waist location may be moved bidirectionally by allowing for negative entries in the Jacobian (equivalent to inducing negative refraction effects). These results are then interpreted in light of the ensuing CSP location.

Monolayer ${\mathrm{NbSe}}_2$ has recently been shown to be a two-dimensional superconductor, with a competing charge-density wave (CDW) order. This work investigates the electronic structure of monolayer ${\mathrm{NbSe}}_2$ based on first-principles calculations, focusing on charge and magnetic orders. It is found that decreased screening in the monolayer ${\mathrm{NbSe}}_2$ with a perfect lattice exhibits magnetic instability, which is removed by the formation of CDW. Two energetically competitive but distinct $3\times 3$ CDW structures are revealed computationally, which have a significant impact on the Fermi surface. The relations of the potential CDW phases with experimental structure and the coexisting superconductivity are discussed.

We experimentally observe asymmetric electromagnetic field profiles along two-dimensional chiral metamaterials. The asymmetric field profiles depending on the chirality and the operation frequency have been reproduced well by the numerical simulation. Around a chiral meta-atom, distribution of a Poynting vector is found to be shifted asymmetrically. These results are explained in terms of an analogy with the side-jump mechanism in the electronic anomalous Hall systems.

The electronic structure of ${\mathrm{YbCu}}_{4.5}$ under pressure has been studied with x-ray emission spectroscopy. Pressure-induced first-order valence transition to the divalent Yb state is found around 0.6–2.7 GPa, accompanied by the structural transition at the same pressure range of 0–1.0 GPa suggested by the change in the x-ray diffraction pattern. We also measured temperature dependencies of the Yb valence and magnetic susceptibility, which are compared with the single impurity Anderson model.

We present a study of the spin properties of dense layers of near-surface nitrogen-vacancy (NV) centers in diamond created by nitrogen ion implantation. The optically detected magnetic resonance contrast and linewidth, spin coherence time, and spin relaxation time, are measured as a function of implantation energy, dose, annealing temperature, and surface treatment. To track the presence of damage and surface-related spin defects, we perform in situ electron spin resonance spectroscopy through both double electron-electron resonance and cross-relaxation spectroscopy on the NV centers. We find that, for the energy ($4–30$ keV) and dose ($5\times {10}^{11}–{10}^{13}{\mathrm{ions}/\mathrm{cm}}^2$) ranges considered, the NV spin properties are mainly governed by the dose via residual implantation-induced paramagnetic defects, but that the resulting magnetic sensitivity is essentially independent of both dose and energy. We then show that the magnetic sensitivity is significantly improved by high-temperature annealing at $\ge 1100{}^{\circ }\mathrm{C}$. Moreover, the spin properties are not significantly affected by oxygen annealing, apart from the spin relaxation time, which is dramatically decreased. Finally, the average NV depth is determined by nuclear magnetic resonance measurements, giving $\approx 10–17$ nm at 4–6 keV implantation energy. This study sheds light on the optimal conditions to create dense layers of near-surface NV centers for high-sensitivity sensing and imaging applications.

In this paper, we investigate the transient quantum transport in a nanoscale Aharonov-Bohm interferometer consisting of laterally coupled double quantum dots (DQDs) coupled to the source and drain electrodes. The tran- sient linear conductance is derived at arbitrary temperature of the leads. The resulting transient conductance can be divided into three terms contributed from different transport channels. From the transient linear conductance and the reduced density matrix elements of the DQD system, we show the buildup of Fano resonances in the time domain.

We report on ultrafast electron diffraction on high quality single crystal silicon. The ultrafast dynamics of the Bragg peaks exhibits a giant photoinduced response which can only be explained in the framework of dynamical diffraction theory, taking into account multiple scattering of the probing electrons in the sample. In particular, we show that lattice heating following photoexcitation can cause an unexpected increase of the Bragg peak intensities, in contradiction with the well-known Debye-Waller effect. We anticipate that multiple scattering should be systematically considered in ultrafast electron diffraction on high quality crystals as it dominates the Bragg peak dynamics. In addition, taking into account multiple scattering effects opens the way to quantitative studies of nonequilibrium dynamics of defects in quasiperfect crystals.

Carbide-derived carbon (CDC) is prepared by etching high purity 4H-SiC single crystals in a mixed atmosphere of 5% ${\mathrm{Cl}}_2$ and 95% Ar for 120 min and 240 min. The secondary ion mass spectroscopy (SIMS) bulk analysis technique excludes the possibility of ferromagnetic transition metal (TM) contamination arising during the experimental process. The paramagnetic and ferromagnetic components are separated from the measured magnetization-magnetic field curves of the samples. Through the use of the Brillouin function, paramagnetic centers carrying a magnetic moment of $\sim 1.3{\mu }_B$ are fitted. A resolvable hysteresis loop in the low magnetic field area is preserved at room temperature. The temperature dependence of the relative intensity of the Lorentzian-like electron spin resonance (ESR) line observed by electron spin spectroscopy reveals the existence of exchange interaction between the localized paramagnetic centers. First-principles calculations show the dominant configuration of defects in the graphitic CDC films. By calculating the energy difference between the antiferromagnetic and ferromagnetic phases, we deduce that the ferromagnetic coupling is sensitive to the concentration of defects.

We experimentally study magnetization aging in a thin-film NiO/permalloy bilayer. Aging characteristics are nearly independent of temperature below the exchange bias blocking temperature $T_B$, but rapidly vary above it. The dependence on the magnetic history qualitatively changes across $T_B$. The observed behaviors are consistent with the spin glass transition at $T_B$, with significant implications for magnetism and magnetoelectronic phenomena in antiferromagnet/ferromagnet bilayers.

A fundamental motif in frustrated magnetism is the fully mutually coupled cluster of $N$ spins, with each spin coupled to every other spin. Clusters with $N=2$ and 3 have been extensively studied as building blocks of square and triangular lattice antiferromagnets. In both cases, large-$S$ semiclassical descriptions have been fruitfully constructed, providing insights into the physics of macroscopic magnetic systems. Here, we develop a semiclassical theory for the $N=4$ cluster. This problem has rich mathematical structure with a ground-state space that has nontrivial topology. We show that ground states are appropriately parametrized by a unit vector order parameter and a rotation matrix. Remarkably, in the low-energy description, the physics of the cluster reduces to that of an emergent free spin-$S$ spin and a rigid rotor. This successfully explains the spectrum of the quadrumer and its associated degeneracies. However, this mapping does not hold in the vicinity of collinear ground states due to a subtle effect that arises from the nonmanifold nature of the ground-state space. We demonstrate this by an analysis of soft fluctuations, showing that collinear states have a larger number of soft modes. Nevertheless, as these singularities only occur on a subset of measure zero, the mapping to a spin and a rotor provides a good description of the quadrumer. We interpret thermodynamic properties of the quadrumer that are accessible in molecular magnets, in terms of the rotor and spin degrees of freedom. Our study paves the way for field theoretic descriptions of systems such as pyrochlore magnets.

We examine the effect of boron doping in superconducting forms of amorphous carbon. By judiciously optimizing boron substitutional sites in simulated amorphous carbon, we predict a superconducting transition temperature near 37 K at 14% boron concentration. Our findings have direct implications for understanding the recently discovered high-$T_c$ superconductivity in Q-carbon.

We study the interplay between superconductivity and non-Fermi liquid behavior of a Fermi surface coupled to a massless $\text{SU}\left(N\right)$ matrix boson near the quantum critical point. The presence of thermal infrared singularities in both the fermionic self-energy and the gap equation invalidates the Eliashberg approximation, and makes the quantum-critical pairing problem qualitatively different from that at zero temperature. Taking the large $N$ limit, we solve the gap equation beyond the Eliashberg approximation, and obtain the superconducting temperature $T_c$ as a function of $N$. Our results show an anomalous scaling between the zero-temperature gap and $T_c$. For $N$ greater than a critical value, we find that $T_c$ vanishes with a Berezinskii-Kosterlitz-Thouless scaling behavior, and the system retains non-Fermi liquid behavior down to zero temperature. This confirms and extends previous renormalization-group analyses done at $T=0$, and provides a controlled example of a naked quantum critical point. We discuss the crucial role of thermal fluctuations in relating our results with earlier work where superconductivity always develops due to the special role of the first Matsubara frequency.

Key properties of nine possible defect sites in hexagonal boron nitride (h-BN), $V_{\mathrm{N}},{V_{\mathrm{N}}}^{-1},{\mathrm{C}}_{\mathrm{N}},V_{\mathrm{N}}{\mathrm{O}}_{2\mathrm{B}},V_{\mathrm{N}}{\mathrm{N}}_{\mathrm{B}},V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}},V_{\mathrm{B}}{C}_{\mathrm{N}},V_{\mathrm{B}}{\mathrm{C}}_{\mathrm{N}}\mathrm{S}{\mathrm{i}}_{\mathrm{N}}$, and $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}\mathrm{S}{\mathrm{i}}_{\mathrm{B}}$, are predicted using density-functional theory and are corrected by applying results from high-level ab initio calculations. Observed h-BN electron-paramagnetic resonance signals at 22.4, 20.83, and 352.70 MHz are assigned to $V_{\mathrm{N}},{\mathrm{C}}_{\mathrm{N}}$, and $V_{\mathrm{N}}{\mathrm{O}}_{2\mathrm{B}}$, respectively, while the observed photoemission at 1.95 eV is assigned to $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$. Detailed consideration of the available excited states, allowed spin-orbit couplings, zero-field splitting, and optical transitions is made for the two related defects $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ and $V_{\mathrm{B}}{C}_{\mathrm{N}}$. $V_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ is proposed for realizing long-lived quantum memory in h-BN. $V_{\mathrm{B}}{C}_{\mathrm{N}}$ is predicted to have a triplet ground state, implying that spin initialization by optical means is feasible and suitable optical excitations are identified, making this defect of interest for possible quantum-qubit operations.

We study the localization and oscillation properties of the Majorana fermions that arise in a two-dimensional electron gas (2DEG) with spin-orbit coupling (SOC) and a Zeeman field coupled with a $d$-wave superconductor. Despite the angular dependence of the $d$-wave pairing, localization and oscillation properties are found to be similar to the ones seen in conventional $s$-wave superconductors. In addition, we study a microscopic lattice version of the previous system that can be characterized by a topological invariant. We derive its real space representation that involves nearest and next-to-nearest-neighbors pairing. Finally, we show that the emerging chiral Majorana fermions are indeed robust against static disorder. This analysis has potential applications to quantum simulations and experiments in high-$T_c$ superconductors.

We use the Nernst effect to delineate the boundary of the pseudogap phase in the temperature-doping phase diagram of hole-doped cuprate superconductors. New data for the Nernst coefficient $\nu \left(T\right)$ of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO), ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Eu-LSCO), and ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Nd-LSCO) are presented and compared with previously published data on YBCO, Eu-LSCO, Nd-LSCO, and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO). The temperature $T_{\nu }$ at which $\nu /T$ deviates from its high-temperature linear behavior is found to coincide with the temperature at which the resistivity $\rho \left(T\right)$ deviates from its linear-$T$ dependence, which we take as the definition of the pseudogap temperature $T^{★}$—in agreement with the temperature at which the antinodal spectral gap detected in angle-resolved photoemission spectroscopy (ARPES) opens. We track $T^{★}$ as a function of doping and find that it decreases linearly vs $p$ in all four materials, having the same value in the three LSCO-based cuprates, irrespective of their different crystal structures. At low $p,T^{★}$ is higher than the onset temperature of the various orders observed in underdoped cuprates, suggesting that these orders are secondary instabilities of the pseudogap phase. A linear extrapolation of $T^{★}\left(p\right)$ to $p=0$ yields $T^{★}(p\to 0)\simeq T_{\mathrm{N}}$(0), the Néel temperature for the onset of antiferromagnetic order at $p=0$, suggesting that there is a link between pseudogap and antiferromagnetism. With increasing $p,T^{★}\left(p\right)$ extrapolates linearly to zero at $p\simeq p_{\mathrm{c}2}$, the critical doping below which superconductivity emerges at high doping, suggesting that the conditions which favor pseudogap formation also favor pairing. We also use the Nernst effect to investigate how far superconducting fluctuations extend above the critical temperature $T_{\mathrm{c}}$, as a function of doping, and find that a narrow fluctuation regime tracks $T_{\mathrm{c}}$, and not $T^{★}$. This confirms that the pseudogap phase is not a form of precursor superconductivity, and fluctuations in the phase of the superconducting order parameter are not what causes $T_{\mathrm{c}}$ to fall on the underdoped side of the $T_{\mathrm{c}}$ dome.