A thorough analysis of the emission via the magnetic dipole (MD) transition, called magnetic light below, of trivalent rare-earth ions in or near dielectric homogeneous spheres has been performed. In the search for enhancement of fluorescence from magnetic light, one faces the difficult task of identifying the regions where the combined fluorescence due to multiple electric dipole (ED) transitions becomes negligible compared to the fluorescence of the MD transition. We have succeeded in identifying a number of configurations with dielectric sphere parameters and a radial position of a trivalent rare-earth emitter wherein the branching ratio of the MD transition approaches its limit of one, implying that transitions from a given initial level (e.g., ${}^5\mathrm{D}_0$ level of ${\mathrm{Eu}}^{3+}$) are completely dominated by the MD transition. The dimensionless directivity of the MD emission, the radiative decay rates, and the fluorescence of the magnetic light can be increased by a factor of more than 25, ${10}^3$, and ${10}^4$, respectively.

The Reynolds similitude, a key concept in hydrodynamics, states that two phenomena of different length scales with a similar geometry are physically identical. Flow properties are universally determined in a unified way in terms of the Reynolds number $\mathcal{R}$ (dimensionless, ratio of inertial to viscous forces in incompressible fluids). For example, the drag coefficient $c_D$ of objects with similar shapes moving in fluids is expressed by a universal function of $\mathcal{R}$. Certain studies introduced similar dimensionless numbers, that is, the superfluid Reynolds number ${\mathcal{R}}_s$, to characterize turbulent flows in superfluids. However, the applicability of the similitude to inviscid quantum fluids is nontrivial as the original theory is applicable to viscous fluids. This Letter proposes a method to verify the similitude using current experimental techniques in quantum liquid He II. A highly precise relation between $c_D$ and ${\mathcal{R}}_s$ was obtained in terms of the terminal speed of a macroscopic body falling in He II at finite temperatures across the Knudsen (ballistic) and hydrodynamic regimes of thermal excitations. The Reynolds similitude in superfluids proves the quantum viscosity of a pure superfluid and can facilitate a unified mutual development of classical and quantum hydrodynamics; the concept of quantum viscosity provides a practical correspondence between classical and quantum turbulence as a dissipative phenomenon.

Motivated by the recent discovery of an intermediate quantum critical phase between the antiferromagnetic order and the Fermi liquid in the frustrated Kondo lattice CePdAl, we study here a Kondo-Heisenberg chain with frustrated $J_1\text{−}{J}_2$ XXZ interactions among local spins using the density matrix renormalization group method. Our simulations reveal a global phase diagram with rich ground states including the antiferromagnetic order, the valence-bond-solid and bond-order-wave orders, the pair density wave state, the uniform superconducting state, and the Luttinger liquid state. We show that both the pair density wave and uniform superconductivity belong to the family of Luther-Emery liquids and may arise from pair instability of an intermediate quantum critical phase with medium Fermi volume in the presence of strong quantum fluctuations, while the Luttinger liquid has a large Fermi volume. Our work provides a comprehensive picture of the frustrated Kondo lattice physics at one dimension, and suggests a deep connection between the pair density wave, the unconventional superconductivity, and the non-Fermi liquid quantum critical phase.

Quantum signal processing (QSP) has emerged as a promising framework to manipulate and determine properties of quantum systems. QSP not only unifies most existing quantum algorithms but also provides tools to discover new ones. Quantum signal processing is applicable to single-qubit or multiqubit systems that can be “qubitized” so one can exploit the SU(2) structure of system evolution within special invariant two-dimensional subspaces. In the context of quantum algorithms, this SU(2) structure is artificially imposed on the system through highly nonlocal evolution operators that are difficult to implement on near-term quantum devices. In this work, we propose QSP protocols for the infinite-dimensional Onsager Lie algebra, which is relevant to the physical dynamics of quantum devices that can simulate the transverse-field Ising model. To this end, we consider QSP sequences in the Heisenberg picture, allowing us to exploit the emergent SU(2) structure in momentum space and “synthesize” QSP sequences for the Onsager algebra. Our results demonstrate a concrete connection between QSP techniques and noisy intermediate scale quantum protocols. We provide examples and applications of our approach in diverse fields ranging from space-time dual quantum circuits and quantum simulation to quantum control.

Electrides constitute a unique class of materials that can be developed as conventional superconductors with diverse dimensional superconductivity. However, the transition temperatures $\left(T_c\right)$ of electride superconductors are generally low and promoting their $T_c$ usually requires extremely high external pressures that are formidable for practical applications. Here, based on the first-principles calculations, we proposed that the recently reported electride, ${\mathrm{Be}}_2\mathrm{N}$, can exhibit a two-dimensional (2D) superconductivity, which has a $T_c$ of 10.3 K that is the highest $T_c$ ever found for bulk electride superconductors at ambient pressure. More interestingly, we found that the high $T_c$ of ${\mathrm{Be}}_2\mathrm{N}$ is mainly attributed to the large average phonon frequency, rather than the strong electron-phonon coupling, which can be further understood by the small atomic weight of Be atoms and the strong Be-N bonds. Moreover, compared to most conventional superconductors, we identified an unusual dependence of the superconductivity of ${\mathrm{Be}}_2\mathrm{N}$ on external pressures, originating from a unique charge transfer from its cationic framework to its anionic electron cloud. Our studies provide a deeper understanding of the superconductivity of 2D electrides and suggest a feasible way for the development of high-temperature electride superconductors at ambient pressure.

Spontaneous generation of time-reversal symmetry breaking in unconventional superconductors is currently a topic of considerable interest. While chiral superconducting order is often assumed to be the source of such signatures, they can sometimes also arise from nonmagnetic disorder. Here we perform a theoretical study of the impact of dislocations on the superconducting order parameter within a microscopic one-band model which, in the homogeneous case, features either extended $s$-wave, $d$-wave, or $s+id$-wave superconductivity depending on the electron concentration. We find that the dislocations minimize their impact on the superconducting condensate by inducing localized supercurrents pinned by the dislocations, even well outside the $s+id$ regime. We map out the parameter and density dependence of the induced currents. From these results we conclude that quite generically unconventional superconductors hosting dislocations tend to break time-reversal symmetry locally.

Specific heat can explore low-energy quasiparticle excitations of superconductors, so it is a powerful tool for bulk measurement on the superconducting gap structure and pairing symmetry. Here, we report an in-depth investigation on the specific heat of the multiband superconductors ${\mathrm{KCa}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4{\mathrm{F}}_2$ $(x=0, 0.05, 0.13)$ single crystals and the overdoped nonsuperconducting one with $x=0.17$. Clear specific heat anomalies can be observed at the superconducting transition temperature of 33.6 K and 28.8 K for the samples with $x=0$ and $x=0.05$, respectively. For the two samples, the magnetic-field-induced specific heat coefficient $\mathrm{\Delta }\gamma \left(H\right)$ in the low-temperature limit increases rapidly below 2 T, then it rises slowly above 2 T. Using the nonsuperconducting sample with $x=0.17$ as a reference, the specific heat of phonon background for various superconducting samples can be obtained and subtracted, which allows us to extract the electronic specific heat of the superconducting samples. Through comparative analyses, it is found that the energy gap structure including two $s$-wave gaps and an extended $s$-wave gap with large anisotropy can reasonably describe the electronic specific heat data. According to these results, we suggest that at least one anisotropic superconducting gap with a deep gap minimum should exist in this multiband system. With the doping of Ni, the superconducting transition temperature of the sample decreases along with the decrease of the large $s$-wave gap, but the extended $s$-wave gap increases due to the enlarged electron pockets via adding more electrons. Despite these changes, the general properties of the gap structure remain unchanged when doping Ni. In addition, the calculation of condensation energy of the parent and doped samples shows the rough consistency with the correlation of $U_0\propto {T_c}^n$ with $n=3–4$, which is beyond the understanding of the BCS theory.

The existing literature on the influence of impurities on superconductivity follows two different approaches. In the first one, a simple BCS-like instantaneous electron-electron interaction is considered, and special care is taken to treat the scattering on impurities as accurately as possible. On the other hand, the second approach starts with more realistic phonon-induced retarded electron-electron interactions, but treats the impurities only within the simplest Born approximation. Here we develop a theory combining the strengths of both of these approaches. This allows us to describe the recently introduced Dynes superconductors within an Eliashberg-like theory. One of the results of the developed theory is that the Dynes scattering parameter may depend on temperature, in qualitative agreement with recent experiments.

The unexpected connections between classical wave phenomena and higher-order topological phases have attracted the interest of researchers across various fields. Although attaining lower-dimensional localized states, such as corner states, has been a common achievement in the realm of sonic crystals, a notable limitation arises where wave energy primarily localizes at the outermost site with the degenerate eigenfrequencies, thereby restricting further applications in acoustics. In this paper, we address this limitation by introducing previously overlooked extra next-nearest-neighbor interactions into the system. Remarkably, the inclusion of these extra nonlocal interactions generates an array of unconventional corner-localized states, each with distinct off-site localization positions. Experimental validation of these theoretical predictions is meticulously conducted using fabricated sonic crystals. Notably, in contrast to natural crystals, the advantageous characteristics of artificial crystals provide a promising platform for probing the effect of long-range interactions in topological phases and materials, which is facilitated by distance-independent coupling strengths. Our findings not only expand the boundaries of sound confinement but also provide momentum for exploring long-range interactions in the field of artificial crystals.

Spin-torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.

The Dirac magnon system ${\mathrm{CrI}}_3$ with a honeycomb lattice is a potential host of topological edge magnons. It ideally orders ferromagnetically (FM) $(T_c=61 \mathrm{K})$ on cooling from a monoclinic (M) to a rhombohedral (R) phase, but antiferromagnetic (AFM) order has been detected in nanometer thin flakes, attributed to M-type layer stacking. There remains confusion, however, as to the extent to which such behavior is present in bulk samples. Using a powder sample in which the sliding transition to the R phase was largely inhibited (2:1 M:R ratio), clear evidence for M-type AFM order $(T_N\sim 50 \mathrm{K})$ coexisting with R-type FM order is observed in the bulk. From inelastic neutron scattering, a lower magnon energy is observed compared to the R phase, consistent with smaller interlayer interactions expected in the M phase. While a gap at the Dirac points has been reported in the R phase, the gap is clearly observed even when the majority is M type, as in our sample, suggesting that the same nontrivial magnon topology of the R phase is present in the M phase as well.

We theoretically consider magnetization dynamics in a ferromagnetic slab induced by the magnetic field of a strong femtosecond laser pulse. The longitudinal geometry, in which the initial magnetization lies in both the plane of incidence and the sample plane, is studied. The magnetization oscillations at the optical wave frequency are calculated with the use of the Kapitza pendulum approach taking into account that the optical frequency is much greater than the magnetization oscillation eigenfrequency. We study the reflection of the electromagnetic wave from a ferromagnet and show that this laser-induced low-frequency magnetization dynamics leads to the appearance of the second-order nonlinearity in the Maxwell's equations, which in turn gives rise to both the second harmonic generation (SHG) and rectification effect. Although the amplitude of the magnetization oscillations is small, the considered effect may be responsible for the SHG with the efficiency comparable to that of nonmagnetic SHG from metal surfaces. Our estimations show that the suggested mechanism may explain the recent experiments on magnetization induced modulation of the SHG intensity in a “forbidden” $P_{\text{in}}{P}_{\text{out}}$ combination of incident and reflected waves in cobalt/heavy metal systems, where it can be even more pronounced due to the spin current flow through the ferromagnet/heavy metal interface.

Alterations of magnetic properties associated with a large surface-to-volume ratio are of great importance for all magnetic nanosystems. In particular, amorphous rare-earth–transition-metal thin films with a thickness of 10 nm or less demonstrate modified $M\left(T\right)$ characteristics and magnetization switching behavior in comparison with their bulk counterparts. Understanding and experimentally identifying which particular surface phenomenon is responsible for this behavior are challenges due to the variety of possible overlapping contributions and effects. In this paper, we report on a study of surface magnetism in amorphous ferrimagnetic TbFe thin films and nanoparticles with varying diameters. We demonstrate how the reduction of the average number of surface neighbors, preferential oxidation, and chemical segregation can change compensation and Curie temperatures as well as magnetic reversal characteristics of the alloy. Our results indicate that the preferential oxidation plays a leading role, whereas reduction of the average number of neighbors and chemical segregation are found to be the second most important factors.

We study the ground state of the doped Hubbard model on honeycomb lattice with both nearest $\left(t\right)$ and next-nearest-neighbor hoppings $\left(t^{\prime }\right)$ in the small doping and strongly interacting region. Previous study on the model without $t^{\prime }$ showed the ground state is a half-filled stripe. We employ density matrix renormalization group and extrapolate the results with truncation errors in the converged region. In the $t^{\prime }<0$ side, we find the half-filled stripe phase at $t^{\prime }=0$ is stable against the frustration of $t^{\prime }$ until a critical point $-0.4<t_c^{\prime }<-0.3$, beyond which the ground state switches to antiferromagnetic Néel phase with charge modulation. With further increase of $t^{\prime }$ to $-0.7$, the ground state becomes paramagnetic. In the $t^{\prime }>0$ side, the half-filled stripe stretches to $t^{\prime }\approx 0.7$. We do not find obvious enhancement of pairing for the range of $t^{\prime }$ studied. We study width-4 cylinders in this paper but the results for spin, charge, and pairing correlation agree qualitatively for periodic and antiperiodic boundary conditions in the half-filled stripe and antiferromagnetic Néel phases, suggesting the results are likely to be representative for true two-dimensional systems. The half-filled stripe to antiferromagnetic Néel phase transition can be realized on real materials or ultracold atom platform.

A recent theory of the disorder-dependent slope of the upper critical field at the superconducting transition temperature $T_c$, defined as $S\equiv |d{H}_{c2}{/dT|}_{T\to T_c}$, is extended to multiband superconductors aimed at iron-based superconductors, considering two constant gaps of different magnitudes and, potentially, different signs. We show that the slope $S$ decreases with increasing nonmagnetic scattering rate $P$ in the $s_{\pm }$ pairing state and increases in the $s_{++}$ superconductor for a reasonable range of parameters. The experiment shows that in a typical iron-based superconductor, ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ (BaK122), the nonmagnetic disorder induced by electron irradiation leads to an increasing $S\left(P\right)$ across the superconducting “dome,” at different $x$. This implies that ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ is likely an $s_{++}$ superconductor with two effective gaps of different magnitudes, at least at moderate doping levels, $x<0.6$. This work reopens a decade-long discussion about the nature of the superconducting order parameter in iron pnictides.

Conductivities and Hall coefficients of two-dimensional hard-core bosons (HCB) are calculated using the thermodynamic expansions of Kubo formulas. At temperatures above the superfluid transition, the resistivity rises linearly and is weakly dependent on boson filling. The zeroth-order Hall coefficient diverges toward zero and unit fillings, and reverses its sign at half filling. The correction terms, which are calculated up to fourth (Krylov) orders, do not alter this behavior. The high temperature thermal Hall coefficient is reversed relative to the electric Hall coefficient. We discuss relevance of HCB transport to the metallic state of short coherence length superconductors.

The sine-Gordon model captures the low-energy effective dynamics of a wealth of one-dimensional quantum systems, stimulating the experimental efforts in building a versatile quantum simulator of this field theory and fueling the parallel development of new theoretical toolkits able to capture far-from-equilibrium settings. In this work, we analyze the realization of the sine-Gordon model from the interference pattern of two one-dimensional quasicondensates: we argue that the emergent field theory is well described by its classical limit, and we develop its large-scale description based on generalized hydrodynamics. We show how, despite the sine-Gordon model being an integrable field theory, trap-induced inhomogeneities cause instabilities of excitations and provide exact analytical results to capture this effect.

The scalefree non-Hermitian skin effect (NHSE) refers to the phenomenon that the localization length of skin modes scales proportionally with system size in non-Hermitian systems. Authors of recent studies have demonstrated that the scalefree NHSE can be induced through various mechanisms, including the critical NHSE, local non-Hermiticity, and the boundary impurity effect. Nevertheless, these methods require careful modeling and precise parameter tuning. In contrast, in this paper, we suggest that the scalefree NHSE is a universal phenomenon, observable in extensive systems if these systems can be described by non-Bloch band theory and host Bloch points on the energy spectrum in the thermodynamic limit. Crucially, we discover that the geometry of the generalized Brillouin zone determines the scaling rule of the localization length, which can scale either linearly or quadratically with the system size. In this paper, we enriche the phenomenon of the scalefree NHSE.

Infrared (IR) spectra of thin films of fullerene ${\mathrm{C}}_{60}$ molecules of icosahedral ($I_h$) symmetry are recorded as thermal emission of radiation at elevated temperatures in the range of $27{}^{\circ }\mathrm{C}$–$88{}^{\circ }\mathrm{C}$. Temperature dependence is analyzed for relative intensities of four IR-active emission bands of vibrational $T_{1u}$ modes of ${\mathrm{C}}_{60}$. Theoretical simulations are performed on the basis of the temperature-dependent thermal population of vibrationally excited states in a ${\mathrm{C}}_{60}$ molecule, counting explicitly the number of overtones and combinations for all possible energy levels by the excitation of up to seven vibrational quanta. The observed IR emission intensity increases with increasing temperature for all the bands of $T_{1u}\left(i\right)$, with $i=1–4$, at 526, 575, 1182, and 1427 ${\mathrm{cm}}^{-1}$, whereas the intensity of higher-frequency modes, $T_{1u}\left(3\right)$ and $T_{1u}\left(4\right)$, increases more drastically than that of $T_{1u}\left(1\right)$ and $T_{1u}\left(2\right)$. The analyses show clearly how and why the IR emission spectrum of ${\mathrm{C}}_{60}$ changes with temperature. For the thick sample of ${\mathrm{C}}_{60}$, saturation of the IR emission bands is observed due to self-absorption. The molecular IR emission of ${\mathrm{C}}_{60}$ is shown to be an ideal system for consideration of the vibrational temperature and an appropriate probe for astrophysical conditions of cosmic ${\mathrm{C}}_{60}$ molecules.