We revisit the delocalized character of the $4f$ states of ${\mathrm{CeFe}}_2$ in the ferromagnetically ordered phase by x-ray magnetic circular dichroism (XMCD) in x-ray absorption spectroscopy (XAS) with improved data quality using single crystals. Surprisingly, the Ce $M_{4,5}$ XMCD spectral shape changes significantly as a function of temperature and applied magnetic field, with no concomitant changes in the spectral shape of the Ce $M_{4,5}$ XAS as well as the Fe $L_{2,3}$ XAS and XMCD. This unusual behavior is characterized by the $J=7/2$ states in a $4f^1$ configuration mixed into the $J=5/2$ ground state. Such extreme sensitivity of the Ce $4f$ states to the external perturbations can be related to the magnetic instability toward an antiferromagnetic phase in ${\mathrm{CeFe}}_2$. Our experimental data presented here provide valuable insights into the underlying physics in strongly hybridized ferromagnetic Ce compounds.

Based on the tight-binding model we construct a nonlinear Hamiltonian to describe the effective electric system around the $M$ point for the single layer graphene. The local energy spectrum at the $M$ point is approximated by the perfect hyperbolic geometry, and the modification by the screening effect from the substrate is taken into account. With the method based on the concept of the Floquet states and quasienergies (FSQ) we investigate the third order nonlinear conductivity ${\sigma }_3\left(\omega \right),{\sigma }_3\left(3\omega \right)$ for the different frequency ranges, respectively, in which only the $\pi -{\pi }^{*}$ bands are involved. A positive cusplike peak arises at $\hslash \omega ={\varepsilon }_{\mathrm{gap}}\left(\text{M}\right)/3$ for ${\sigma }_3\left(3\omega \right)$ which originates from the three-photon processes. Also, there is a peak at ${\varepsilon }_{\mathrm{gap}}\left(\text{M}\right)/2$ for ${\sigma }_3\left(\omega \right)$ resulting from two-photon-resonant processes. The analysis of the pole processes indicates that a self-energy-like term transition process plays a role in the nonlinear optical response, and the different transition processes interact with each other during the response to the external field. These interactions can be influenced by the polarization of the external field.

We propose a theoretical framework that captures the geometric vector potential emerging from the nonadiabatic spin dynamics of itinerant carriers subject to arbitrary magnetic textures. Our approach results in a series of constraints on the geometric potential and the nonadiabatic geometric phase associated with it. These constraints play a decisive role when studying, e.g., the geometric spin phase gathered by conducting electrons in ring interferometers under the action of in-plane magnetic textures, allowing a simple characterization of the topological transition recently reported by Saarikoski et al. [H. Saarikoski, J. E. Vázquez-Lozano, J. P. Baltanás, F. Nagasawa, J. Nitta, and D. Frustaglia, Phys. Rev. B 91, 241406(R) (2015)].

Aluminum nanostructures have recently been at the focus of numerous studies due to their properties including oxidation stability and surface plasmon resonances covering the ultraviolet and visible spectral windows. In this article, we reveal a facet of this metal relevant for both plasmonic purposes and photothermal conversion. The field distribution of high-order plasmonic resonances existing in two-dimensional Al structures is studied by nonlinear photoluminescence microscopy in a spectral region where electronic interband transitions occur. The polarization sensitivity of the field intensity maps shows that the electric field concentration can be addressed and controlled on demand. We use a numerical tool based on the Green dyadic method to analyze our results and to simulate the absorbed energy that is locally converted into heat. The polarization-dependent temperature increase of the Al structures is experimentally quantitatively measured, and is in an excellent agreement with theoretical predictions. Our work highlights Al as a promising candidate for designing thermal nanosources integrated in coplanar geometries for thermally assisted nanomanipulation or biophysical applications.

Superconducting transmon qubits comprise one of the most promising platforms for quantum information processing due to their long coherence times and to their scalability into larger qubit networks. However, their weakly anharmonic spectrum leads to spectral crowding in multiqubit systems, making it challenging to implement fast, high-fidelity gates while avoiding leakage errors. To address this challenge, we use a protocol for speeding up waveforms by inducing phases to harmful transitions (SWIPHT) [S. E. Economou and E. Barnes, Phys. Rev. B 91, 161405(R) (2015)], which yields smooth, simple microwave pulses designed to suppress leakage without sacrificing gate speed through spectral selectivity. Here, we determine the parameter regimes in which SWIPHT is effective and demonstrate that in these regimes, it systematically produces two-qubit gate fidelities for cavity-coupled transmons in the range 99.6%–99.9% with gate times as fast as 23 ns. Our results are obtained from full numerical simulations that include current experimental levels of relaxation and dephasing. These high fidelities persist over a wide range of system parameters that encompass many current experimental setups and are insensitive to small parameter variations and pulse imperfections.

We present a tractable and physically transparent semiclassical theory of matrix-valued Hamiltonians, i.e., those that describe quantum systems with internal degrees of freedoms, based on a generalization of the Gutzwiller trace formula for a $n\times n$ dimensional Hamiltonian $H(\widehat{\mathbf{p}},\widehat{\mathbf{q}})$. The classical dynamics is governed by $n$ Hamilton-Jacobi (HJ) equations that act in a phase space endowed with a classical Berry curvature encoding anholonomy in the parallel transport of the eigenvectors of $H(\mathbf{p},\mathbf{q})$; these vectors describe the internal structure of the semiclassical particles. At the $\mathcal{O}\left({\hslash }^1\right)$ level and for nondegenerate HJ systems, this curvature results in an additional semiclassical phase composed of (i) a Berry phase and (ii) a dynamical phase resulting from the classical particles “moving through the Berry curvature”. We show that the dynamical part of this semiclassical phase will, generally, be zero only for the case in which the Berry phase is topological (i.e., depends only on the winding number). We illustrate the method by calculating the Landau spectrum for monolayer graphene, the four-band model of AB bilayer graphene, and for a more complicated matrix Hamiltonian describing the silicene band structure. Finally, we apply our method to an inhomogeneous system consisting of a strain engineered one-dimensional moiré in bilayer graphene, finding localized states near the Dirac point that arise from electron trapping in a semiclassical moiré potential. The semiclassical density of states of these localized states we show to be in perfect agreement with an exact quantum mechanical calculation of the density of states.

We study the role of finite-size effects on critical behavior near a metallic $q=0$ critical point and compare the results with a recent extensive quantum Monte Carlo (QMC) study [Y. Schattner et al., Phys. Rev. X 6, 031028 (2016)]. This study found several features in both bosonic and fermionic responses, in disagreement with the expected critical behavior with a dynamical exponent $z=3$. We show that finite-size effects are particularly strong for $z=3$ criticality and give rise to a behavior different from that of an infinite system, over a wide range of momenta and frequencies. We argue that by taking finite-size effects into account, the QMC results can be explained within $z=3$ theory.

When a Dirac semimetal is subject to a circularly polarized laser, it is predicted that the Dirac cone splits into two Weyl nodes and a nonequilibrium transient state called the Floquet Weyl semimetal is realized. We focus on the previously unexplored low-frequency regime, where the upper and lower Dirac bands resonantly couple with each other through multiphoton processes, which is a realistic situation in solid-state ultrafast pump-probe experiments. We find a series of new Weyl nodes emerging in pairs when the Floquet replica bands hybridize with each other. The nature of the Floquet Weyl semimetal with regard to the number, locations, and monopole charges of these Weyl nodes is highly tunable with the amplitude and frequency of the light. We derive an effective low-energy theory using Brillouin-Wigner expansion and further regularize the theory on a cubic lattice. The monopole charges obtained from the low-energy Hamiltonian can be reconciled with the number of Fermi arcs on the lattice, which we find numerically.

We identified, by means of coincidence site lattice theory, an evaluative stacking phase with a wavelike Moiré pattern, denoted as $2\mathrm{O}\text{−}\mathrm{t}\alpha \mathrm{P}$, from all potentially twisted bilayer orthorhombic black phosphorus. Such a twisted stacking comes with a low formation energy of $-162.8\mathrm{meV}$, very close to existing AB stacking, according to first-principles calculations. Particularly, classic molecular dynamic simulations verified that the stacking can be directly obtained in an in situ cleavage. The stability of $2\mathrm{O}\text{−}\mathrm{t}\alpha \mathrm{P}$ stacking can be directly attributed to the corrugated configuration of black phosphorus leading to the van der Waals constraining forces, where the top layer can get stuck to the bottom when one layer rotates in plane relative to the other by $\sim 70.5^{\circ }$. Tribological analysis further revealed that the interlayer friction of $2\mathrm{O}\text{−}\mathrm{t}\alpha \mathrm{P}$ stacking reaches up to 1.3 nN, playing a key role in the origin of $2\mathrm{O}\text{−}\mathrm{t}\alpha \mathrm{P}$.

The entanglement entropy (EE) of quantum systems is often used as a test of low-energy descriptions by conformal field theory (CFT). Here we point out that this is not a reliable indicator, as the EE often shows the same behavior even when a CFT description is not correct (as long as the system is asymptotically scale-invariant). We use constraints on the scaling dimension given by the CFT with SU(2) symmetry to provide alternative tests with two- and four-point correlation functions, showing examples for quantum spin models in 1+1 dimensions. In the case of a critical amplitude-product state expressed in the valence-bond basis (where the amplitudes decay as a power law of the bond length and the wave function is the product of all bond amplitudes), we show that even though the EE exhibits the expected CFT behavior, there is no CFT description of this state. We provide numerical tests of the behavior predicted by CFT for the correlation functions in the critical transverse-field Ising chain and the $J\text{−}Q$ spin chain, where the conformal structure is well understood. That behavior is not reproduced in the amplitude-product state.

Low-loss crystals with defects due to paramagnetic or rare earth impurity ions are a major area of investigation for quantum hybrid systems at both optical and microwave frequencies. In this work we examine the single-crystal yttrium aluminium perovskite ${\mathrm{YAlO}}_3$ using the whispering gallery mode technique. Multiple resonant microwave modes were measured from room temperature to 20 mK allowing precise characterization of the permittivity tensor at microwave frequencies. We show that it is biaxial and characterize the tensor as a function of temperature with estimated uncertainties below 0.26%. Electron spin resonance spectroscopy was also performed at 20 mK, with transitions identified with zero-field splittings of 16.72 and 9.92 GHz. Spin-photon couplings of order 4.2 and 8.4 MHz were observed for residual levels of concentration, which are stronger than the photon cavity linewidths of 116 kHz but of the same order as the linewidths of the discovered spin transitions.

We adapt the DFT+$U$ method in the gauge-including projector augmented-wave NMR chemical shift calculations within the plane wave pseudopotential implementation. The nonlocal Hubbard correction potential has been reexamined in order to comply with the gauge-including projector augmented-wave transformation under an external uniform magnetic field. The resulting expression is suitable for chemical shift calculations using both norm-conserving and ultrasoft pseudopotentials in the proector augmented-wave scheme. The implementation is applied to the ${}^{17}\mathrm{O}$ solid-state NMR chemical shift calculations for transition-metal and rare-earth oxides, including ${\mathrm{TiO}}_2$, ZnO, ${\mathrm{Ti}}_2{\mathrm{O}}_3, {\mathrm{La}}_2{\mathrm{O}}_3$, and ${\mathrm{CeO}}_2$. A comparison between the DFT and DFT+$U$ NMR chemical shifts for the selected materials is presented.

We propose a strong-disorder renormalization-group approach to study the Anderson localization transition in disordered tight-binding models in any dimension. Our approach shifts the focus from the lower to the upper critical dimension, thus emphasizing the strong-coupling/strong-disorder nature of the transition. By studying the two-point conductance, we (i) show that our approach is in excellent agreement with exact numerical results, (ii) confirm that the upper critical dimension for the Anderson transition is $d_c^{+}=\infty$, (iii) find that the scaling function shows a previously reported ‘mirror symmetry’ in the critical region, and (iv) demonstrate that the range of conductances for which this symmetry holds increases with the system dimensionality. Our results open an efficient avenue to explore the critical properties of the Anderson transition using the strong-coupling high-dimension limit as a starting point.

The collective decay of excitons from initial Bose-Einstein condensate state is investigated theoretically. As a practically more interesting case we consider excitons of the yellow series in the semiconductor cuprous oxide where we have collective photon and phonon-assisted decay of excitons. It is shown that because of intrinsic instability of recoilless two-boson decay of Bose-Einstein condensate, the spontaneously emitted bosonic pairs are amplified, leading to an exponential buildup of a macroscopic population into the certain modes. The collective decay rate has a nonlinear dependence on the excitonic density being comparable or larger than the Auger recombination loss rate up to the high densities, which makes obtainable its observation. The phenomenon considered can also be applied for the realization of a phonon laser.

Topological insulators are ideally represented as having an insulating bulk with topologically protected, spin-textured surface states. However, it is increasingly becoming clear that these surface transport channels can be accompanied by a finite conducting bulk, as well as additional topologically trivial surface states. To investigate these parallel conduction transport channels, we studied Shubnikov–de Haas oscillations in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films, in high magnetic fields up to 30 T so as to access channels with a lower mobility. We identify a clear Zeeman-split bulk contribution to the oscillations from a comparison between the charge-carrier densities extracted from the magnetoresistance and the oscillations. Furthermore, our analyses indicate the presence of a two-dimensional state and signatures of additional states the origin of which cannot be conclusively determined. Our findings underpin the necessity of theoretical studies on the origin of and the interplay between these parallel conduction channels for a careful analysis of the material's performance.

Utilizing suitable precursor molecules, a thermally activated and surface-assisted synthesis results in the formation of defect-free graphene nanoribbons (GNRs), which exhibit electronic properties that are not present in extended graphene. Most importantly, they have a band gap in the order of a few electron volts, depending on the nanoribbon width. In this study, we investigate the electronic structure changes during the formation of GNRs, nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111). Thus we determine the optical gaps of the precursor molecules, the intermediate nonaromatic polymers, and finally the aromatic GNRs, using high-resolution electron energy loss spectroscopy and density functional theory calculations. As expected, we find no influence of N-doping on the size of the optical gaps. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanced delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons, since they exhibit an extended delocalized $\pi$-electron system.

A framework for computing the anharmonic free energy (FE) of metallic crystals using Born-Oppenheimer ab initio molecular dynamics (AIMD) simulation, with unprecedented efficiency, is introduced and demonstrated for the hcp phase of iron at extreme conditions (up to $\approx 290$ GPa and 5000 K). The advances underlying this work are: (1) A recently introduced harmonically-mapped averaging temperature integration (HMA-TI) method that reduces the computational cost by order(s) of magnitude compared to the conventional TI approach. The TI path starts from zero Kelvin, where it assumes the behavior is given exactly by a harmonic treatment; this feature restricts application to systems that have no imaginary phonons in this limit. (2) A Langevin thermostat with the HMA-TI method that allows the use of a relatively large MD time step (4 fs, which is about eight times larger than the size needed for the Andersen thermostat) without loss of accuracy. (3) AIMD sampling is accelerated by using density functional theory (DFT) with a low-level parameter set, then the measured quantities of selected configurations are robustly reweighted to a higher level of DFT. This introduces a speedup of about 20–$30\times$ compared to directly simulating the accurate system. (4a) The temperature ($T$) dependence of the hcp equilibrium shape (i.e., $c/a$ axial ratio) is determined (including anharmonicity), with uncertainty less than $\pm 0.001$. (4b) Electronic excitation is included through Mermin's finite-temperature extension of the $T=0$ K DFT. A simple FE perturbation method is introduced to handle the difficulty associated with applying the TI method with a $T$-dependent geometry and (due to electronic excitation) potential-energy surface. (5) The FE in the thermodynamic limit is attained through extrapolation of only the (computationally inexpensive) quasiharmonic FE, because the anharmonic FE contribution has negligible finite-size effects. All methods introduced here do not affect the AIMD sampling—results are obtained through post-processing—so established AIMD codes can be employed without modification. Analytical formulas fitted to the results for the variation of the equilibrium $c/a$ ratio and FE components with $T$ are provided. Notably, effects of magnetic excitations are not included and may yet prove important to the overall FE; if so, it is plausible that such contributions can be added perturbatively to the FE values reported here. Notwithstanding these considerations, FE values are obtained with an estimated accuracy and precision of 2 meV/atom, suggesting that the capability to compute the phase diagram of iron at Earth's inner core conditions is within reach.

We report on thermodynamic, magnetization, and muon spin relaxation measurements of the strong spin-orbit coupled iridate ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$, which constitutes a distinct frustration motif made up of a mixture of edge- and corner-sharing triangles. In spite of a strong antiferromagnetic exchange interaction of the order of 100 K, we find no hint for long-range magnetic order down to 23 mK. The magnetic specific heat data unveil $T$-linear and $T$-squared dependences at low temperatures below 1 K. At the respective temperatures, the zero-field muon spin relaxation features a persistent spin dynamics, indicative of unconventional low-energy excitations. A comparison to the $4d$ isostructural compound ${\mathrm{Ba}}_3{\mathrm{RuTi}}_2{\mathrm{O}}_9$ suggests that a concerted interplay of compasslike magnetic interactions and frustrated geometry promotes a dynamically fluctuating state in a triangle-based iridate.

We calculate the Josephson current between two one-dimensional (1D) nanowires oriented along $x$ with proximity induced $s$-wave superconducting pairing and separated by a narrow dielectric barrier in the presence of both Rashba spin-orbit interaction (SOI) characterized by strength $\alpha$ and Zeeman fields ($h$ along $\widehat{z}$ and $\mathbf{B}$ in the $x\text{−}y$ plane). We formulate a general method for computing the Andreev bound states energy, which allows us to obtain analytical expressions for the energy of these states in several asymptotic cases. We find that in the absence of the magnetic fields the energy gap between the Andreev bound states decreases with increasing Rashba SOI constant leading eventually to touching of the levels. In the absence of Rashba SOI, the Andreev bound states depend on the magnetic fields and display oscillatory behavior with the orientational angle of B, leading to the magneto-Josephson effect. We also present analytic expressions for the dc Josephson current charting out their dependence on $\mathbf{B}$, $h$, and $\alpha$. We demonstrate the existence of finite spin-Josephson current in these junctions in the presence of external magnetic fields and provide analytic expressions for its dependence on $\alpha$, $\mathbf{B}$, and $h$. Finally, we study the ac Josephson effect in the presence of the SOI (for $\left|\mathbf{B}\right|=h=0$) and an external radiation and show that the width of the resulting Shapiro steps in such a system can be tuned by varying $\alpha$. We discuss experiments that can test our theoretical results.