Experimental results of electron emission from surfaces of aluminum fluoride impacted by keV noble gas ions (${\mathrm{He}}^{+}, {\mathrm{Ne}}^{+}, {\mathrm{Ar}}^{+}$) show a high-energy structure that increases in intensity with ion energy in the case of ${\mathrm{He}}^{+}$ and ${\mathrm{Ar}}^{+}$ and is nearly independent on the impact energy for ${\mathrm{Ne}}^{+}$ ions. It is also observed that this high-energy secondary electron peak is less pronounced in the case of impact by ${\mathrm{Ar}}^{+}$. While ion-induced electron emission in metal surfaces is relatively well understood, in the case of wide band-gap insulators, such as metal fluorides, the nature of high-energy excitations is still an open question. In this work, we propose a mechanism of high-energy electron emission strongly linked to the ion neutralization process and the collision induced excitation of the electrons in the solid.

We demonstrate that the defect mode in the bulk plasmon-polariton gap of one-dimensional defective metamaterial photonic crystals can be used to achieve a giant enhancement of more than four orders of magnitude in the second harmonic (SH) conversion efficiency only by changing the incidence angle. Furthermore, the one-dimensional photonic crystal may be designed in order for the SH wave to coincide with the edge of the Bragg gap or with the defect mode inside this gap, in which case the enhancement is even higher. Because of the robustness of the bulk plasmon-polariton gap to scaling effects, the present proposal may inspire different routes for frequency upconversion, signal filtering, and switching photonic devices.

We investigate the effect of blinking on the two-photon interference measurement from two independent quantum emitters. We find that blinking significantly alters the statistics in the Hong-Ou-Mandel second-order intensity correlation function ${\mathrm{g}}^{\left(2\right)}\left(\tau \right)$ and the outcome of two-photon interference measurements performed with independent quantum emitters. We theoretically demonstrate that the presence of blinking can be experimentally recognized by a deviation from the ${\mathrm{g}}_D^{\left(2\right)}\left(0\right)=0.5$ value when distinguishable photons from two emitters impinge on a beam splitter. Our findings explain the significant differences between linear losses and blinking for correlation measurements between independent sources and are experimentally verified using a parametric down-conversion photon-pair source. We show that blinking imposes a mandatory cross-check measurement to correctly estimate the degree of indistinguishability of photons emitted by independent quantum emitters.

By using first-principles electronic structure calculations, we find that material LaSb with extreme magnetoresistance (XMR) undergoes a topological phase transition without breaking any symmetry under a hydrostatic pressure applied between 3 and 4 GPa; meanwhile, the electron-hole compensation remains in its electronic band structure. This makes LaSb an ideal platform for studying what role the topological property plays in the XMR phenomenon, in addition to the electron-hole compensation.

We investigate a class of multilayered metamaterials characterized by moderate-permittivity inclusions and low average permittivity. Via first-principles calculations, we show that in such a scenario, first- and second-order spatial dispersions may exhibit a dramatic and nonresonant enhancement, and may become comparable with the local response. Their interplay gives access to a wealth of dispersion regimes encompassing additional extraordinary waves and topological phase transitions. In particular, we identify a configuration featuring bound and disconnected isofrequency contours. Since they do not rely on high-permittivity inclusions, our proposed metamaterials may constitute an attractive and technologically viable platform for engineering nonlocal effects in the optical range.

Strain tuning is increasingly being recognized as a clean tuning parameter to induce novel behavior in quantum matter. Motivated by the possibility of straining graphene up to 20 percent, we investigate novel quantum criticality due to interplay between strain-induced anisotropic band structure and critical antiferromagnetic (AF) spin fluctuations in this setting. We detail how this interplay drives (i) a quantum phase transition (QPT) between the Dirac-semimetal-incoherent pseudogapped metal-correlated insulator as a function of strain ($\epsilon$), and (ii) critical AF spin fluctuations-driven divergent nematic susceptibility near critical strain (${\epsilon }_c$) manifesting as critical singularities in magnetothermal expansion and Grüneisen coefficients. The correlated band insulator at large strain affords realization of a two-dimensional dimerized spin-singlet state due to this interplay, and doping such an insulator can lead to a spin-charge separated metal, leading to anomalous metallicity and possible unconventional superconductivity. On a wider front, our work serves to illustrate the range of novel states realizable by strain-tuning quantum materials.

To solve the puzzle of metamagnetic phenomena in heavy-fermion systems, we have compared paramagnetic isostructural Ce and Yb systems, ${\mathrm{CeNi}}_2{\mathrm{Ge}}_2$ and ${\mathrm{YbNi}}_2{\mathrm{Ge}}_2$, both of which are located near a magnetic instability. The most intriguing result is the discovery of a metamagneticlike anomaly for isomorphic Ce and Yb paramagnetic systems from magnetization measurements in a pulsed magnetic field. Similar to other metamagnets, the metamagnetic transition fields for both compounds are well scaled by the temperature $T_{\chi }^{\max }$, at which the magnetic susceptibility shows a maximum. In addition, for ${\mathrm{CeNi}}_2{\mathrm{Ge}}_2$, a peak of nonlinear susceptibility ${\chi }_3$ appears at approximately $T_{\chi }^{\max }/2$, as for other heavy-fermion metamagnets. In contrast, ${\mathrm{YbNi}}_2{\mathrm{Ge}}_2$ shows only a sign change for ${\chi }_3$ at $T_{\chi }^{\max }$, as observed in itinerant metamagnets located near the ferromagnetic critical point. The metamagnetism of ${\mathrm{CeNi}}_2{\mathrm{Ge}}_2$ corresponds to a typical Kondo lattice system, whereas that of ${\mathrm{YbNi}}_2{\mathrm{Ge}}_2$ is similar to the nearly ferromagnetic itinerant systems. Other possibilities for the metamagnetic behavior of ${\mathrm{YbNi}}_2{\mathrm{Ge}}_2$ are also discussed.

Split-ring resonators (SRRs) are a fundamental building block of many electromagnetic metamaterials. Typically the response of a metamaterial is assumed to be independent of interelement interactions in the material. We show that SRRs in close proximity to each other exhibit a rich coupling that involves both electric and magnetic interactions. We study experimentally and computationally the strength and nature of the coupling between two identical SRRs as a function of their separation and relative orientation. We characterize the electric and magnetic couplings and find that, when SRRs are close enough to be in each other's near field, the electric and magnetic couplings may either reinforce each other or act in opposition. At larger separations retardation effects become important.

We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated $\mathrm{Fe}\left(001\right)\text{−}p(1\times 1)\mathrm{O}$ surface and characterized by the formation of an ordered array of Cr vacancies producing a $\left(\sqrt{5}\times \sqrt{5}\right)R{27}^{\circ }$ superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of $\mathrm{O}2p$ states and (ii) the orbital and spin character of $\mathrm{Cr}3d$ states. We show that the experimental $\mathrm{O}2p$ dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate.

Upon adsorption onto epitaxial graphene on Ir(111) at room temperature, Cs adatoms donate part of their charge to the substrate, which effectively creates mutually repelling entities that can diffuse across the graphene surface. By using scanning tunneling microscopy at 6 K, an ordered hexagonal structure of Cs adatoms with several rotated domains has been identified. The amount of charge transferred to graphene has been determined from the presented valence-band photoemission data. Density-functional calculations show that the repulsive interaction is additionally modulated by preferential adsorption of Cs to the atop region within the moiré unit cell. Our results provide direct atomistic insight into the self-assembly of individual atoms on epitaxial graphene, and they demonstrate how the moiré structure acts as a perturbation to that process.

We consider the full nonequilibrium response of a mesoscopic capacitor in the large transparency limit, exactly solving a model with electron-electron interactions appropriate for a cavity in the quantum Hall regime. For a cavity coupled to the electron reservoir via an ideal point contact, we show that the response to any time-dependent gate voltage $V_{\mathrm{g}}\left(t\right)$ is strictly linear in $V_{\mathrm{g}}$. We analyze the charge and current response to a sudden gate voltage shift and find that this response is not captured by a simple circuit analogy. In particular, in the limit of strong interactions a sudden change in the gate voltage leads to the emission of a sequence of multiple charge pulses, the width and separation of which are controlled by the charge-relaxation time ${\tau }_{\mathrm{c}}=h{C}_{\mathrm{g}}/e^2$ and the time of flight ${\tau }_{\mathrm{f}}$. We also consider the effect of a finite reflection amplitude in the point contact, which leads to nonlinear-in-gate-voltage corrections to the charge and current response.

Recent progress in extremely correlated Fermi liquid theory (ECFL) and the dynamical mean field theory (DMFT) enables us to accurately compute in the $d\to \infty$ limit the resistivity of the $t-J$ model after setting $J\to 0$. This is also the $U=\infty$ Hubbard model. Since $J$ is set to zero, our study isolates the dynamical effects of the single occupation constraint enforced by the projection operator originally introduced by Gutzwiller. We study three densities $n=.75,.8,.85$ that correspond to a range between the overdoped and optimally doped Mott insulating state. We delineate four distinct regimes separated by three crossovers, which are characterized by different behaviors of the resistivity $\rho$. We find at the lowest temperature $T$ a Gutzwiller correlated Fermi liquid regime with $\rho \propto T^2$ extending up to an effective Fermi temperature that is dramatically suppressed from the noninteracting value by the proximity to half filling, $n\sim 1$. This is followed by a Gutzwiller correlated strange metal regime with $\rho \propto (T-T_0)$, i.e., a linear resistivity extrapolating back to $\rho =0$ at a positive $T_0$. At a higher temperature scale this crosses over into the bad metal regime with $\rho \propto (T+T_1)$, i.e., a linear resistivity extrapolating back to a finite resistivity at $T=0$ and passing through the Ioffe-Regel-Mott value where the mean free path is a few lattice constants. This regime finally gives way to the high $T$ metal regime, where we find $\rho \propto T$, i.e., a linear resistivity extrapolating back to zero at $T=0$. The present work emphasizes the first two, i.e., the two lowest temperature regimes, where the availability of an analytical ECFL theory is of help in identifying the changes in related variables entering the resistivity formula that accompanies the onset of linear resistivity, and the numerically exact DMFT helps to validate the results. We also examine thermodynamical variables such as the magnetic susceptibility, compressibility, heat capacity, and entropy and correlate changes in these with the change in resistivity. This exercise casts valuable light on the nature of charge and spin correlations in the Gutzwiller correlated strange metal regime, which has features in common with the physically relevant strange metal phase seen in strongly correlated matter.

The tetragonal copper oxide ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ has an unusual crystal structure with a three-dimensional network of well separated ${\mathrm{CuO}}_4$ plaquettes. The spin structure of ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ in the magnetically ordered state below $T_{\text{N}}\sim 43$ K remains controversial. Here we present the results of detailed studies of specific heat, magnetic, and dielectric properties of ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ single crystals grown by the floating zone technique, combined with the polarized neutron scattering and high-resolution x-ray measurements. Down to 3.5 K our polarized neutron scattering measurements reveal ordered magnetic Cu moments which are aligned within the $ab$ plane. Below the onset of the long range antiferromagnetic ordering we observe an electric polarization induced by an applied magnetic field, which indicates inversion symmetry breaking by the ordered state of Cu spins. For the magnetic field applied perpendicular to the tetragonal axis, the spin-induced ferroelectricity is explained in terms of the linear magnetoelectric effect that occurs in a metastable magnetic state. A relatively small electric polarization induced by the field parallel to the tetragonal axis may indicate a more complex magnetic ordering in ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$.

The discovery of an enhanced superconducting transition temperature $T_c$ in monolayers of FeSe grown on several oxide substrates has opened a different route to high-$T_c$ superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon $(e$-ph) interaction across the interface that is peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon line shape, which may result in charge instabilities at long wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the $e$-ph interaction has a strong peak in the forward-scattering (small $\mathbf{q})$ direction.

Monolayer films of FeSe grown on ${\mathrm{SrTiO}}_3$ substrates exhibit significantly higher superconducting transition temperatures than those of bulk FeSe. Interaction of electrons in the FeSe layer with dipolar ${\mathrm{SrTiO}}_3$ phonons has been suggested as the cause of the enhanced transition temperature. In this paper we systematically study the coupling of ${\mathrm{SrTiO}}_3$ longitudinal-optical phonons to the FeSe electron, including also electron-electron Coulomb interactions at the random phase approximation level. We find that the electron-phonon interaction between FeSe and ${\mathrm{SrTiO}}_3$ substrate is almost entirely screened by the electronic fluctuations in the FeSe monolayer, so that the net electron-phonon interaction is very weak and unlikely to lead to superconductivity.

We present a consistent microscopic study of spin pumping effects for both metallic and insulating ferromagnets. As for the metallic case, we present a simple quantum mechanical picture of the effect as due to the electron spin flip as a result of a nonadiabatic (off-diagonal) spin gauge field. The effect of interface spin-orbit interaction is briefly discussed. We also carry out a field-theoretic calculation to discuss on equal footing the spin current generation and torque effects such as an enhanced Gilbert damping constant and a shift of precession frequency both in metallic and insulating cases. For thick ferromagnetic metals, our study reproduces the results of previous theories such as the correspondence between the dc component of the spin current and the enhancement of the damping. For thin metals and insulators, the relation turns out to be modified. For the insulating case, driven locally by interface $sd$ exchange interaction due to magnetic proximity effect, the physical mechanism is distinct from the metallic case. Further study of the proximity effect and interface spin-orbit interaction would be crucial to interpret experimental results in particular for insulators.

We report a variation with temperature $T$ of the effective interdimeric interaction $J_{\mathrm{eff}}^{\prime }$ in the antiferromagnetic (AFM) copper dimeric organic compound ${\mathrm{Cu}}_2{\left[\mathrm{TzTs}\right]}_4$ $(N$-thiazol-2-yl-toluenesulfonamidate ${\mathrm{Cu}}^{\mathrm{II}})$. This $T$ dependence was obtained from measurements of the effects in the electron paramagnetic resonance (EPR) spectra of the proposed quantum phase transition associated with the exchange-narrowing processes. ${\mathrm{Cu}}_2{\left[\mathrm{TzTs}\right]}_4$ contains exchange-coupled pairs of ${\mathrm{Cu}}^{\mathrm{II}}$ spins ${\mathit{S}}_{\mathrm{A}}$ and ${\mathit{S}}_{\mathrm{B}}$ $\left(S=1/2\right)$, with intradimeric AFM exchange coupling $J_0=(-115\pm 1)$ ${\mathrm{cm}}^{-1}$ $\left({\mathcal{H}}_{\mathrm{ex}}=-J_0{\mathit{S}}_{\mathrm{A}}·{\mathit{S}}_{\mathrm{B}}\right)$. The variation of the EPR linewidth of single crystals with field orientation around a “magic angle” where the transitions intersect and the integrated signal intensity of the so-called $\mathsf{U}$ peak of the powder spectrum were measured as a function of $T$. Modeling these data using arguments of exchange narrowing in the adiabatic regime considering the angular variation of the single-crystal spectra and a geometric description, we find that the effective interdimeric coupling $|J_{\mathrm{eff}}^{\prime }|$ associated with the exchange frequency ${\omega }_{\mathrm{ex}}$ is negligible for $T\ll |J_0/k_{\mathrm{B}}|$ when the units are uncoupled and $|J_{\mathrm{eff}}^{\prime }|=(0.080\pm 0.005)$ ${\mathrm{cm}}^{-1}$ $\left(|J_{\mathrm{eff}}^{\prime }/J_0|=7.0\times {10}^{-4}\right)$ at 298 K. Within this $T$ interval, two ranges of $|J_{\mathrm{eff}}^{\prime }|$ with linear temperature variation but different slopes, with a kink at $\sim 80$ K, are observed and discussed. This $T$ dependence arises from the growing population of the triplet state, and its relevance to the properties of various arrays of dimeric units is discussed. Our experimental procedures and results are compared with those of previous works in ion radical salts and dimeric metal compounds. The relation between the effective coupling $|J_{\mathrm{eff}}^{\prime }|$ and the real interdimeric exchange coupling $|J^{\prime }|$ related to the chemical paths connecting neighbor units is discussed.

The wide ranging magnetic behavior in intermetallic compounds continues to attract broad interest. Effective control of their magnetic properties is of great importance for fundamental research and potential applications. In this work the structural and magnetic properties of $\mathrm{TbC}{\mathrm{o}}_2\mathrm{M}{\mathrm{n}}_x$ compounds are studied by a combination of temperature dependent synchrotron x-ray diffraction, neutron powder diffraction, specific heat, and magnetic measurements. Magnetization measurements show that the addition of Mn can modify the magnetic behavior significantly: first, the magnetic transition temperatures increase from ∼227 K to 332 K with $x=0.0\mathrm{to}0.3$; secondly, the nature of the magnetic transitions change from the first order to second order, as identified by three methods (Banerjee criterion, master curves of magnetic entropy changes, and detailed crystal structure analysis through neutron diffraction). Both synchrotron x-ray diffraction and neutron diffraction confirm that a structural transition, from cubic $Fd\overline{3}m$ to rhombohedral $R\overline{3}m$ on cooling, occurred accompanying the magnetic transition. To further clarify the nature of the second order magnetic phase transitions, we have carried out a detailed critical exponent analysis. The derived critical exponents are close to the theoretical prediction from the mean-field model, indicating the magnetic interactions are long range. This work benefits our general understanding of magnetic interactions in intermetallic compounds and provides guidance to design a functional magnetic material for room temperature magnetic devices.

We report measurements of the de Haas–van Alphen effect in the layered heavy-fermion compound ${\mathrm{CePt}}_2{\mathrm{In}}_7$ in high magnetic fields up to 35 T. Above an angle-dependent threshold field, we observed several de Haas–van Alphen frequencies originating from almost ideally two-dimensional Fermi surfaces. The frequencies are similar to those previously observed to develop only above a much higher field of 45 T, where a clear anomaly was detected and proposed to originate from a change in the electronic structure [M. M. Altarawneh et al., Phys. Rev. B 83, 081103 (2011)]. Our experimental results are compared with band structure calculations performed for both ${\mathrm{CePt}}_2{\mathrm{In}}_7$ and ${\mathrm{LaPt}}_2{\mathrm{In}}_7$, and the comparison suggests localized $f$ electrons in ${\mathrm{CePt}}_2{\mathrm{In}}_7$. This conclusion is further supported by comparing experimentally observed Fermi surfaces in ${\mathrm{CePt}}_2{\mathrm{In}}_7$ and ${\mathrm{PrPt}}_2{\mathrm{In}}_7$, which are found to be almost identical. The measured effective masses in ${\mathrm{CePt}}_2{\mathrm{In}}_7$ are only moderately enhanced above the bare electron mass $m_0$, from $2m_0$ to $6m_0$.

Motivated by recent experiments, we present a comprehensive theoretical study of the geometrically frustrated strongly correlated magnetic insulator ${\mathrm{Mn}}_3{\mathrm{O}}_4$ spinel oxide based on a microscopic Hamiltonian involving lattice, spin, and orbital degrees of freedom. Possessing the physics of degenerate $e_g$ orbitals, this system shows a strong Jahn-Teller effect at high temperatures. Further, careful attention is paid to the special nature of the superexchange physics arising from the ${90}^{\circ }$ Mn-O-Mn bonding angle. The Jahn-Teller and superexchange-based orbital-spin Hamiltonians are then analyzed in order to track the dynamics of orbital and spin ordering. We find that a high-temperature structural transition results in orbital ordering the nature of which is mixed with respect to the two originally degenerate $e_g$ orbitals. This ordering of orbitals is shown to relieve the intrinsic geometric frustration of the spins on the spinel lattice, leading to ferrimagnetic Yafet-Kittel ordering at low temperatures. Finally, we develop a model for a magnetoelastic coupling in ${\mathrm{Mn}}_3{\mathrm{O}}_4$, enabling a systematic understanding of the experimentally observed complexity in the low-temperature structural and magnetic phenomenology of this spinel. Our analysis predicts that a quantum fluctuation-driven orbital-spin liquid phase may be stabilized at low temperatures upon the application of pressure.