The Josephson current-phase characteristics, that arise when a supercurrent flows across two fermionic superfluids separated by a potential barrier, can be controlled by varying either the interparticle coupling or the temperature. While the coupling dependence has been addressed in detail both theoretically and experimentally for an attractive Fermi gas undergoing the BCS-BEC crossover, a corresponding study of the temperature dependence of the Josephson characteristics is still lacking in this context. Here, we investigate the combined coupling and temperature dependence of the Josephson characteristics in a systematic way for a wide set of barriers, within ranges of height and width that can be experimentally explored. Our study smoothly connects the two limiting cases, of nonoverlapping composite bosons at low temperature described by the Gross-Piatevskii equation, and of strongly overlapping Cooper pairs near the critical temperature described by the Ginzburg-Landau equation. In this way, we are able to explore several interesting effects related to how the current-phase characteristics evolve along the BCS-BEC crossover as a function of temperature and of barrier shape. These effects include the coherence length outside the barrier and the pair penetration length inside the barrier (which is related to the proximity effect), as well as the temperature evolution of the Landau criterion in the limit of a vanishingly small barrier. A comparison is also presented between the available experimental data for the critical current and our theoretical results over a wide range of couplings along the BCS-BEC crossover.

We propose a general theoretical framework, using two layers of ancilla qubits, for deconfined criticality between a Fermi liquid with a large Fermi surface and a pseudogap metal with a small Fermi surface of electronlike quasiparticles. The pseudogap metal can be a magnetically ordered metal, or a fractionalized Fermi liquid (FL*) without magnetic order. A critical ‘ghost’ Fermi surface emerges (alongside the large electron Fermi surface) at the transition, with the ghost fermions carrying neither spin nor charge but minimally coupled to $(U\left(1\right)\times U\left(1\right))/Z_2$ or $(SU\left(2\right)\times U\left(1\right))/Z_2$ gauge fields. The $(U\left(1\right)\times U\left(1\right))/Z_2$ case describes simultaneous Kondo breakdown and onset of magnetic order: The two gauge fields induce nearly equal attractive and repulsive interactions between ghost Fermi surface excitations, and this competition controls the quantum criticality. Away from the transition on the pseudogap side, the ghost Fermi surface absorbs part of the large electron Fermi surface and leads to a jump in the Hall coefficient. We also find an example of an “unnecessary quantum critical point” between a metal with spin density order and a metal with local moment magnetic order. The ghost fermions contribute an enhanced specific heat near the transition and could also be detected in other thermal probes. We relate our results to the phases of correlated electron compounds.

Using the exact diagonalization technique, we determine the energy spectrum and wave functions for finite chains described by the two-spin (Kugel-Khomskii) model with different types of intersubsystem exchange terms. The obtained solutions provide the possibility to address the problem of quantum entanglement inherent in this class of models. We put the main emphasis on the calculations of the concurrence treated as an adequate numerical measure of the entanglement. We also analyze the behavior of two-site correlation functions considered a local indicator of entanglement. We construct the phase diagrams of the models involving the regions of nonzero entanglement. The pronounced effect of external fields, conjugated to both spin variables in the regions with entanglement, could both enhance and weaken the entanglement depending on the parameters of the models.

Stimulated by anomalous behaviors found in non-Kramers $f$-electron systems in an applied magnetic field, we study a two-channel Kondo lattice model by using a cluster extension of the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. We include the effect of the external magnetic field in two ways: the Zeeman coupling to conduction electron spins and an effective coupling to the quadrupole degree of freedom through the crystalline electric field splitting. We show that the magnetic field suppresses the antiferroic-spin order (physically, corresponding to the antiferroic-quadrupole order) and yields a channel-selective non-Fermi liquid state where one of the two channels (physically, spin-up or spin-down) exhibits non-Fermi liquid behavior while the other shows Fermi liquid behavior, before entering the Fermi liquid regime in higher fields. This anomalous state appears in a dome-shaped region which extends from the antiferroic-spin ordered phase to the paramagnetic phase. We find that the composite correlation, which is a measure of differentiation in the Kondo coupling between the two channels, is enhanced in this dome-shaped region. We also find that the specific heat coefficient is enhanced in this region on the paramagnetic side, indicating heavy fermion behavior not only in the vicinity of the critical field where the antiferroic-spin order vanishes but also in a certain region of the field and temperature. We discuss the results in comparison with the ordinary Kondo lattice model. We also discuss the implication of our findings to the peculiar behaviors observed in the 1-2-20 compounds such as ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20},$ ${\mathrm{PrRh}}_2{\mathrm{Zn}}_{20},$ and ${\mathrm{PrV}}_2{\mathrm{Al}}_{20}$ under a magnetic field.

In this work, a pump-probe experiment is used to study the ultrafast dynamics of heat transfer in thin gold films and gold nanostructures on glass substrates, following local heating by ultrashort laser pulses. Full spectrotemporal differential reflectivity and transmission maps were obtained for different film thicknesses (30, 50, 80, 150, and 200 nm) and different laser fluences $(0.38 \mathrm{to} 9.5{\mathrm{Jm}}^{-2})$. For arrays of gold nanorods, the two orthogonal probe polarizations were also acquired. We propose an improved model for these phenomena based on a modified two-temperature model that integrates thermal conduction and the three-dimensional finite element method model to link the spatiotemporal temperature maps to the spectrotemporal optical response maps. The impact of an underlying titanium adhesion layer is reported. Excellent agreement between numerical and experimental data for both the gold films and the nanostructures is shown.

Polariton lasers are mostly based on planar cavities. Here we focus on an alternative configuration with slab waveguide modes strongly coupled to excitons confined in GaN/AlGaN quantum wells. We study experimentally and theoretically polariton relaxation at temperatures ranging from 4 to 200 K. We observe a good robustness of the lower polariton population peak energy position against temperature changes due to a balance between the shift of the exciton energy and the change in the normal mode splitting, a promising feature for future applications such as lasers and amplifiers where a small temperature drift in the emission wavelength is a desired asset. Finally, at $T=4\mathrm{K}$ we observe the signature of polariton nonlinearities occurring in the continuous wave regime that are assigned to an optical parametric oscillation process.

We investigated third-order correlations between photons from a single quantum dot's resonance fluorescence in the strong excitation regime originating from opposite sidebands of the power spectrum. The three-time correlation measurements for photons filtered using three independent Fabry-Perot etalons resulted in correlation maps with “antibunching” features as a consequence of correlations originating from the same sideband, and “bunching ridges” due to opposite sideband correlated photons.

We study Dyakonov surface waveguide modes in a waveguide represented by an interface of two anisotropic media confined between two air half-spaces. We analyze such modes in terms of perturbation theory in the approximation of weak anisotropy. We show that in contrast to conventional Dyakonov surface waves that decay monotonically with distance from the interface, Dyakonov waveguide modes can have local maxima of the field intensity away from the interface. We confirm our analytical results by comparing them with full-wave electromagnetic simulations. We believe that this work can bring new ideas in the research of Dyakonov surface waves.

We investigate effects of electron-electron interactions on the shape of the Fermi surface in an anisotropic two-dimensional electron gas using the “RPA-GW” self-energy approximation. We find that the interacting Fermi surface deviates from an ellipse but not in an arbitrary way. The interacting Fermi surface has only two qualitatively distinct shapes for most values of $r_s$. The Fermi surface undergoes two distinct transitions between these two shapes as $r_s$ increases. For larger $r_s$, the degree of the deviation from an ellipse rapidly increases, but, in general, our theory provides a justification for the widely used elliptical Fermi-surface approximation, even for the interacting system, since the nonelliptic corrections are quantitatively rather small except for very large $r_s$.

Nontrivial band topology combined with magnetic order can lead to rich emergent phenomena, including quantized anomalous Hall effect and axion insulator state. Here we use scanning tunneling microscopy to image the surface Dirac fermions of the newly discovered magnetic topological insulators $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ and $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$. We have determined the energy dispersion and helical spin texture of the surface states through quasiparticle interference patterns far above Dirac energy. Approaching the Dirac point, the native defects in the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ septuple layer give rise to resonance states which extend spatially and potentially hinder the detection of a mass gap in the spectra. Our results impose tight constraints on the magnitude of the possible mass gap at the nanoscale and provide key ingredients for a comprehensive understanding of the electronic structure in this class of fascinating materials.

We find that thin peptide films and coatings doped with metallic nanoparticles are more stable due to the role of electromagnetic fluctuations. It is shown that for the doped freestanding in vacuum peptide film the Casimir attraction becomes larger in magnitude. For dielectric substrates coated with peptide films, the nanoparticle doping leads to a wider range of film thicknesses where the Casimir pressure is attractive and to larger pressure magnitudes at the points of extremum. The doping of peptide coatings with magnetic nanoparticles preserves all the advantages of nonmagnetic ones and simultaneously imparts superparamagnetic properties to the coating, which could extend significantly the application areas of bioelectronics.

Boltzmann theory deals with the quasiparticle distributions $F_K$ for electrons and $N_Q$ for phonons. It is difficult to reconcile the common situation of “steady state” (where distributions are out of equilibrium but carry steady charge current $\vec{j}$) and Joule heating $\vec{j}·\vec{E}$. This paper offers elementary clarification. The resolution is simplest if a true steady state is abandoned.

Based on symmetry analysis, we propose that filling-enforced Dirac nodal loops (FEDLs) in nonmagnetic systems exist and only exist in five space groups (SGs), namely, SG.57, SG.60, SG.61, SG.62, and SG.205. We explore all possible configurations of the FEDLs in these SGs and classify them accordingly. Furthermore, we study the evolutions of the FEDLs under various types of symmetry-breaking perturbations, such as an applied strain or an external field. The results show that FEDL materials can serve as parent materials of both topological semimetals hosting nodal points/loops and topological insulators/topological crystalline insulators. By means of first-principles calculations, almost all materials possessing FEDLs are predicted.

We report a combined inelastic neutron- and x-ray-scattering study of the phonon density of states of the nano- and microcrystalline lanthanide-based materials ${\mathrm{NaY}}_{0.8}{\mathrm{Yb}}_{0.18}{\mathrm{Er}}_{0.02}{\mathrm{F}}_4$ and $\mathrm{Na}{\mathrm{Gd}}_{0.8}{\mathrm{Yb}}_{0.18}{\mathrm{Er}}_{0.02}{\mathrm{F}}_4$. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at low energies, $E\le 15\mathrm{meV}$, in ultrasmall (5 nm) $\mathrm{Na}{\mathrm{Gd}}_{0.8}{\mathrm{Yb}}_{0.18}{\mathrm{Er}}_{0.02}{\mathrm{F}}_4$ nanocrystals which we assign to an increased relative spectral weight of surface phonon modes. Based on our observations for ultrasmall nanocrystals, we rationalize that an increase of the phonon density of states in large nanocrystals due to surface phonons is too small to be observed in the current measurements. The experimental approach described in this paper constitutes a step toward the rationalization of size effects on the modification of the absolute upconversion quantum yield of upconverting nanocrystals.

The charge-density wave (CDW) transition has been extensively studied in transition metal dichalcogenides (TMDs), the underlying mechanism and the related metal-insulator transition are not as simple as Peierls instability in one dimension and still under hot debate. Here, through electrochemical intercalation of organic ions, we have observed a dimensional crossover induced metal-insulator transition in an organic ion intercalated TMDs: ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. In pristine ${\mathrm{VSe}}_2$, previous studies have revealed a three-dimensional CDW transition at $T_{\mathrm{CDW}}\sim 110\mathrm{K}$ with a metallic ground state. After intercalation of organic ions, the remarkable anisotropy of resistivity indicates a highly two-dimensional electronic state in ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$, which is consistent with our density functional theory (DFT) calculation. Interestingly, the dimensional crossover enhances the CDW transition with $T_{\mathrm{CDW}}$ of 165 K and leads to an insulating ground state in ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. Moreover, a commensurate superstructure with $3a\times 3a$ periodicity is also confirmed in this insulating CDW state. Although the DFT calculation suggests that the commensurate superstructure and the enhanced CDW temperature could be ascribed to the improved Fermi surface nesting, whether the metal-insulator transition is driven by a perfect Fermi surface nesting is still elusive at the present stage. The possible role of electronic correlation and electron-phonon coupling has also been discussed. Our work provides a different material platform to study the metal-insulator transition in TMDs.

The novel physics, such as the pressure-induced electronic topological transition (ETT), topological superconductivity and Majorana fermions in the isostructural $R\text{−}3m$ phase of three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, holds considerable interest in condensed-matter physics. We carried out a combined investigation of single-crystal x-ray diffraction, high-quality x-ray absorption fine structure, and first-principles theoretical calculations to decipher the puzzling origin of the intriguing electronic states in the isostructural $R\text{−}3m$ phase of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ at high pressure. Three distinct regions with two isostructural phase transitions (IPTs) in the $R\text{−}3m$ phase have been identified. The first IPT, which is known as the ETT, occurs at the boundary of region I (0–2 GPa) and region II (2–5 GPa) with a sharp minimum in the $c/a$ ratio of $R\text{−}3m$ structure, while the second IPT happens as pressure increases from region II (2–5 GPa) to region III (5–7 GPa). The positions of the Bi ($6c$) and Se ($6c$) sites in the unit cell change rapidly in region II (2–5 GPa), but there is little change at these sites in region III (5–7 GPa). The band-gap closure in region I reflects the pressure-induced metallization. At higher pressures, the band gap opens in region II but remains almost constant after the second IPT in region III, which agrees well with the topological superconductivity of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results demonstrate that the combination of local structure, long-range crystal structure, and first-principles calculation is critically important for understanding the isostructural electronic states and the connection between the structure and function as in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ at high pressure.

The notion of fidelity susceptibility, introduced within the context of quantum metric tensor, has been an important quantity to characterize the criticality near quantum phase transitions. We demonstrate that for topological phase transitions in Dirac models, provided the momentum space is treated as the manifold of the quantum metric, the fidelity susceptibility coincides with the curvature function whose integration gives the topological invariant. Thus the quantum criticality of the curvature function near a topological phase transition also describes the criticality of the fidelity susceptibility, and the correlation length extracted from the curvature function also gives a momentum scale over which the fidelity susceptibility decays. To map out the profile and criticality of the fidelity susceptibility, we turn to quantum walks that simulate one-dimensional class BDI and two- dimensional class D Dirac models and demonstrate their accuracy in capturing the critical exponents and scaling laws near topological phase transitions.

The bulk-boundary correspondence is a defining feature of topological states of matter. However, for quantum magnets in two dimensions such as spin liquids or topological magnon insulators, a direct observation of topological surface states has proven challenging because of the charge-neutral character of the excitations. Here we propose spin-polarized scanning tunneling microscopy as a spin-sensitive local probe to provide direct information about charge-neutral topological edge states. We show how their signatures, imprinted in the local structure factor, can be extracted by specifically employing the strengths of existing technologies. As our main example, we determine the dynamical spin correlations of the Kitaev honeycomb model with open boundaries. We show that by contrasting conductance measurements of bulk and edge locations, one can extract direct signatures of the existence of fractionalized excitations and nontrivial topology. The broad applicability of this approach is corroborated by a second example of a kagome topological magnon insulator.

An antiferromagnetic domain wall in a thermal gradient is found to experience a force towards colder regions upon the application of a uniform magnetic field along the easy axis. This force increases with the strength of the applied field and, for sufficiently high values, it overcomes the entropic force that pushes the wall towards the hotter regions. The force is proportional to the thermal gradient and it shows a linear dependence with the net magnetic moment of the domain wall induced by the field. The origin of this force lies in the increase of the domain wall reflectivity due the field-induced sizable break of the antiferromagnetic order inside it. Thus, reflected magnons effectively transfer linear momentum to the domain wall and push it to the cold regions.