We report on fabrication of devices integrating ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ with other van der Waals materials, measuring transport properties as well as tunneling spectra at variable magnetic fields and temperatures down to 35 mK. Transport measurements are reliable and repeatable, revealing temperature and magnetic field dependence in agreement with prior results, confirming that fabrication processing does not alter bulk properties. However, cross-sectional scanning transmission microscopy reveals oxidation of the surface, which may explain a lower yield of tunneling device fabrication. We nonetheless observe hard-gap planar tunneling into ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ through a ${\mathrm{MoS}}_2$ barrier. Notably, a minimal hard gap of 0.5 meV persists up to a magnetic field of 9 T in the $ab$ plane and 3 T out of plane. This may be the result of very small junction dimensions or a quantum-limit minimal energy spacing between vortex bound states. We also observe defect-assisted tunneling, exhibiting bias-symmetric resonant states, which may arise due to resonant Andreev processes.

We present a first-principles study of the ${\mathrm{GdRh}}_2{\mathrm{Si}}_2$(001) surface electronic structure. Two surfaces, Si- and Gd-terminated, are considered. The origin of the two-dimensional (2D) electronic states at both terminations is investigated by tracing the band structure evolution by going from individual Si, Rh, and Gd atomic layers to (non)stoichiometric ultrathin films and, finally, to thicker ${\mathrm{GdRh}}_2{\mathrm{Si}}_2$ slabs. We find the conic-like (Dirac-like) resonance state located in the vicinity of the $\overline{\mathrm{\Gamma }}$ point at the Si termination to form via the Tamm mechanism and explain the reasons for the differences in dispersion and energy position of the resonance states at the Si and Gd terminations. Then, we show how the butterfly-like dispersion of the Shockley state, residing in the bulk projected band gap near the $\overline{\mathrm{M}}$ point, appears due to the interaction of the bands localized in the surface and subsurface Gd-Si-Rh-Si blocks of the Si termination. Also, a giant sign-alternating atomic relaxation near both the Si- and Gd-terminated surfaces is revealed and its effect on the dispersion and energy position of the 2D states is discussed. In this way we shed light on the origin of the 2D states and explain their dispersion seen in angle-resolved photoemission spectroscopy experiments.

Optical materials exhibit significant losses over the resonance frequency of their constituent atoms and so they are practically implemented at frequencies far from resonances. Electromagnetically induced transparency (EIT) provides a method for effective suppression of optical loss in a narrow window over the resonance, where the medium exhibits significant dispersion but at the expense of zero susceptibility. The classical or plasmonic analogs of the EIT effect are introduced and widely used in the context of electromagnetic or optical metamaterials (MMs). In another interesting phenomenon in quantum optics known as enhancement of index of refraction (EIR), the optical loss of the medium can be zero or even negative at the region of maximal susceptibility and negligible dispersion. This condition is interesting for applications where a strong electromagnetic response of the medium with negligible loss is required, such as zero- or negative-index metamaterials (MMs). Here we introduce a plasmonic analog of the EIR which allows for coherent control over the polarizability and absorption of plasmonic nanoantennas. It can open up the way for loss-compensated propagation of optical waves in zero-index to high-refractive-index plasmonic MMs. The scheme also offers an approach to all-optical switching and coherent control of transmission, diffraction, and polarization conversion properties of plasmonic nanostructures, as well as propagation properties of surface plasmon polaritons on metasurfaces.

Recent discovery of superconductivity in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ has raised particular interest in its pairing mechanism and gap symmetry. Here we propose a phenomenological theory of its superconductivity and investigate possible gap structures by solving the multiband Eliashberg equations combining realistic Fermi surfaces from first-principles calculations and a quantum critical form of magnetic pairing interactions. The resulting gap symmetry shows sensitive dependence on the in-plane propagation wave vector of the quantum critical fluctuations, suggesting that superconductivity in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ is located on the border of $(p_x+i{p}_y)$ and $d_{x^2-y^2}$-wave solutions. This leads to two candidate phase diagrams: one has only a spin-triplet $(p_x+i{p}_y)$-wave superconducting phase; the other contains multiple phases with a spin-singlet $d_{x^2-y^2}$-wave state at zero field and a field-induced spin-triplet $(p_x+i{p}_y)$-wave state. In addition, the electron pairing is found to be dominated by the “jungle-gym” Fermi surface rather than the “doughnut”-like one, in contrast to previous thought. This requests a more elaborate and renewed understanding of the electronic properties of ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$.

Majorana zero modes (MZMs) emerging at the edges of topological superconducting wires have been proposed as the building blocks of novel, fault-tolerant quantum computation protocols. Coherent detection and manipulation of such states in scalable devices are, therefore, essential in these applications. Recent detection proposals include semiconductor quantum dots (QDs) coupled to the end of these wires, as changes in the QD electronic spectral density due to the MZM coupling could be detected in transport experiments. Here, we propose that multi-QD systems can also be used to manipulate MZMs through precise control over the QDs' parameters. The simplest case where Majorana manipulation is possible is in a double quantum dot (DQD) geometry. By using exact analytical methods and numerical renormalization-group calculations, we show that the QDs' spectral functions can be used to characterize the presence or not of MZMs “leaking” into the DQD. More importantly, we find that these signatures respond to changes in the DQD parameters such as gate voltages and couplings in a consistent fashion. Additionally, we show that different MZM-DQD coupling geometries (“symmetric,” “in-series,” and “T-shaped” junctions) offer distinct ways in which MZMs can be switched from dot to dot. These results highlight the interesting possibilities that DQDs offer for all-electrical MZM control in scalable devices.

We investigated the structural stability of SnSe from 0 to 800 K by the phonon quasiparticle approach combining first-principles molecular-dynamics (MD) simulations and lattice dynamics. At high temperature, we witness the dynamic stability of the $Cmcm$ phase and reveal the coupling of the polarization of phonon modes and the phase transition between the $Cmcm$ and $Pnma$ phases. Specifically, in real space, the probability distribution of atomic displacements from first-principles MD simulations successfully captures the structural instability at low temperature and the structural stability at high temperature for $Cmcm$ SnSe. An analysis of phonon power spectra of several modes also delivers the dynamic stabilization of $Cmcm$ at high temperature. Particularly, the soft modes of the $Y_1$ mode at the $\mathbf{q}=Y(\frac{1}{2},\frac{1}{2},0$) point and the ${\mathrm{\Gamma }}_1$ mode at the $\mathbf{q}=\mathrm{\Gamma }(0,0,0)$ point of the $Cmcm$ phase in the harmonic approximation become relatively rigid at elevated temperature, in agreement with experimental and previous theoretical results. The calculated anharmonic phonon dispersions and density of states are strongly temperature-dependent, and some phonon modes adopt giant frequency shifts. These aspects demonstrate the heavy anharmonicity in SnSe. At low temperature, the transition from the $Cmcm$ to the $Pnma$ phase induces a symmetry breaking of structure. Consequently, the degeneracy of associated electronic states (mainly $p$ states) is lifted, thus lowering the energy of the $Pnma$ phase.

Topological magnetic semimetals promise large Berry curvature through the distribution of the topological Weyl nodes or nodal lines and further novel physics with exotic transport phenomena. We present a systematic study of the structural and magnetotransport properties of ${\mathrm{Co}}_2\mathrm{MnGa}$ films from thin (20 nm) to bulklike behavior (80 nm) in order to understand the underlying mechanisms and the role of topology. The magnetron sputtered ${\mathrm{Co}}_2\mathrm{MnGa}$ films are $L{2}_1$-ordered showing very good heteroepitaxy and a strain-induced tetragonal distortion. The anomalous Hall conductivity was found to be maximum at a value of 1138 S/cm, with a corresponding anomalous Hall angle of 13%, which is comparatively larger than topologically trivial metals. There is good agreement between the theoretical calculations and the Hall conductivity observed for the 80 nm film, which suggests that the effect is intrinsic. Thus, the ${\mathrm{Co}}_2\mathrm{MnGa}$ compound manifests as a promising material for topologically driven spintronic applications.

The electric control of magnetic anisotropy has important applications for nonvolatile memory and information processing. By first-principles calculations, we show a large nonvolatile control of magnetic anisotropy in the ferromagnetic/ferroelectric CoPt/ZnO interface. Using the switched electric polarization of ZnO, the density-of-states and magnetic anisotropy at the CoPt surface show a large change. Due to a strong Co/Pt orbitals hybridization and a large spin-orbit coupling, a large control of magnetic anisotropy was found. We experimentally measured the change of effective anisotropy by tunneling resistance measurements in CoPt/Mg-doped ZnO/Co junctions. Additionally, we corroborate the origin of the control of magnetic anisotropy by observations on tunneling anisotropic magnetoresistance.

We have experimentally studied a magnetopiezoelectric effect (MPE), i.e., dynamic distortion caused by AC electric currents, in the antiferromagnetic conductor ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ at low temperatures down to $77\mathrm{K}$ using laser Doppler vibrometry. When AC electric currents are applied to ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, dynamic displacement signals which increase in proportion to the current amplitude are clearly observed at $77\mathrm{K}$, but their magnitude and temperature dependence strongly depend on Joule heating effects caused by the applied electric currents. Especially, poor thermal contact of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ samples to the sample holder produces second-harmonic displacement signals owing to thermal expansion due to the Joule heating, and tends to affect the temperature dependence of the MPE signals. In a measurement run without any heating effects, a sharp enhancement of the MPE signal is observed at the Néel temperature of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, which suggests that fluctuations of itinerant Mn moments near the Néel temperature play an important role in the MPE. We speculate that electric currents produce nonequilibrium antiferromagnetic moments via the antiferromagnetic Edelstein effect and the induced antiferromagnetic momemts enhance the MPE signal with the help of a large magnetovolume coupling near the Néel temperature. The MPE efficiency as a piezoelectric response reaches 300–$500\mathrm{pC}/\mathrm{N}$ at a maximum, which is comparable to high values of piezoelectric coefficients detected in the piezoelectric (ferroelectric) ceramics.

Using a microscopic model of lattice vibrations in the $\mathrm{STO}\left(001\right)$ substrate, an additional ${\mathrm{\Omega }}_s=50$ meV longitudinal optical (LO) interface mode is identified. The soft mode propagating mainly in the first $\mathrm{TiO}{}_2$ layer (“$O$ chains”) has stronger electron-phonon coupling to electron gas in FeSe than a well-known ${\mathrm{\Omega }}_h=100$ meV hard mode. The coupling constant, critical temperature, and replica band are calculated. Although there exists a forward in the electron-phonon scattering peak, it is clearly not as sharp as assumed in recent theories (delta-function-like). The critical temperature is obtained by solution of the gap equation and agrees with the observed one. The corresponding electron phonon coupling constant $\lambda =0.23$. The quasiparticle normal state “satellite” in spectral weight is broad and its peak appears at frequency much higher than ${\mathrm{\Omega }}_s$ consistent with observations usually associated with ${\mathrm{\Omega }}_h$. Possible relation of the transversal counterpart of the surface LO soft mode with known phonons is discussed.

Anomalous hysteresis loops of ferrimagnetic amorphous alloys in high magnetic field in the vicinity of the compensation temperature have so far been explained by sample inhomogeneities. We obtain the $H\text{−}T$ magnetic phase diagram for the ferrimagnetic GdFeCo alloy using a two-sublattice model in the paramagnetic rare-earth ion approximation and taking into account rare-earth (Gd) magnetic anisotropy. It is shown that if the magnetic anisotropy of the $f$ sublattice is larger than that of the $d$ sublattice, the tricritical point can be at higher temperature than the compensation point. The obtained phase diagram explains the observed anomalous hysteresis loops as a result of high-field magnetic phase transition, the order of which changes with temperature. It also implies that in the vicinity of the magnetic compensation point the shape of magnetic hysteresis loop strongly depends on temperature.

Understanding the statistics of quasiparticle excitations in magnetic systems is essential for exploring new magnetic phases and collective quantum phenomena. While the chemical potential of a ferromagnetic gas has been extensively investigated both theoretically and experimentally, its antiferromagnetic counterpart remains uncharted. Here, we derive the statistics of a two-component $U\left(1\right)$-symmetric Bose gas and apply our results to an axially symmetric antiferromagnetic insulator. We find that the two magnon eigenmodes of the system are described by an equal and opposite chemical potential, in analogy with a particle-antiparticle pair. Furthermore, we derive the thermomagnonic torques describing the interaction between the coherent and incoherent antiferromagnetic spin dynamics. Our results show that the magnitude and sign of the chemical potential can be tuned via an AC magnetic field driving resonantly one of the magnon modes. Finally, we propose NV-center relaxometry as a method to experimentally test our predictions.

The effect of systematic Au doping on the onset of enhanced $1/f$ noise in the electrical resistance fluctuations of CuMn spin-glass alloys is reported. The purpose of the Au doping is to add a unidirectional anisotropy to that already present from the Mn in Cu. We find that the ratio of the noise onset temperature to the spin-glass temperature is not affected by the increase in anisotropy.

The nontrivial magnon band topology and its consequent responses have been extensively studied in two-dimensional magnetisms. However, the triangular lattice antiferromagnet (TLAF), the best-known frustrated two-dimensional magnet, has received less attention than the closely related Kagome system, because of the spin-chirality cancellation in the umbrella ground state of the undistorted TLAF. In this paper, we study the band topology and the thermal Hall effect (THE) of the TLAF with (anti-)trimerization distortion under the external perpendicular magnetic field using the linearized spin wave theory. We show that the spin-chirality cancellation is removed in such case, giving rise to the nontrivial magnon band topology and the finite THE. Moreover, the magnon bands exhibit band topology transitions tuned by the magnetic field. We demonstrate that such transitions are accompanied by the logarithmic divergence of the first derivative of the thermal Hall conductivity. Finally, we examine the above consequences by calculating the THE in the hexagonal manganite ${\mathrm{YMnO}}_3$, well known to have antitrimerization.

In order to investigate the low-energy antiferromagnetic Cu-spin correlations and their relation to the superconductivity, we have performed muon-spin-relaxation ($\mu \mathrm{SR}$) measurements using single crystals of the electron-doped high-$T_{\mathrm{c}}$ cuprate ${\mathrm{Pr}}_{1-x}{\mathrm{LaCe}}_x{\mathrm{CuO}}_4$ in the overdoped regime. The $\mu \mathrm{SR}$ spectra have revealed that the Cu-spin correlations are developed in the overdoped samples where the superconductivity appears. The development of the Cu-spin correlations weakens with increasing $x$ and is negligibly small in the heavily overdoped sample where the superconductivity almost disappears. Considering that the Cu-spin correlations also exist in the superconducting electron-doped cuprates in the undoped and underdoped regimes [T. Adachi et al., J. Phys. Soc. Jpn. 85, 114716 (2016)], our findings suggest that the mechanism of the superconductivity is related to the low-energy Cu-spin correlations in the entire doping regime of the electron-doped cuprates.

Bayesian parametric analytic continuation (BPAC) is proposed for the analytic continuation of noisy imaginary-time Green's function data as, e.g., obtained by continuous-time quantum Monte Carlo simulations (CTQMC). Within BPAC, the spectral function is inferred from a suitable set of parametrized basis functions. Bayesian model comparison then allows to assess the reliability of different parametrizations. The required evidence integrals of such a model comparison are determined by nested sampling. Compared to the maximum entropy method (MEM), routinely used for the analytic continuation of CTQMC data, the presented approach allows to infer whether the data support specific structures of the spectral function. We demonstrate the capability of BPAC in terms of CTQMC data for an Anderson impurity model (AIM) that shows a generalized Kondo scenario and compare the BPAC reconstruction to the MEM as well as to the spectral function obtained from the real-time fork tensor product state impurity solver where no analytic continuation is required. Furthermore, we present a combination of MEM and BPAC and its application to an AIM arising from the ab initio treatment of ${\mathrm{SrVO}}_3$.

The so-called minimal models of unconventional superconductivity are lattice models of interacting electrons derived from materials in which electron pairing arises from purely repulsive interactions. Showing unambiguously that a minimal model actually can have a superconducting ground state remains a challenge at nonperturbative interactions. We make a significant step in this direction by computing ground states of the 2D U-V Hubbard model—the minimal model of the quasi-1D superconductors—by parallelized DMRG, which allows for systematic control of any bias and that is sign-problem-free. Using distributed-memory supercomputers and leveraging the advantages of the U-V model, we can treat unprecedented sizes of 2D strips and extrapolate their spin gap both to zero approximation error and the thermodynamic limit. Our results for the spin gap are shown to be compatible with a spin excitation spectrum that is either fully gapped or has zeros only in discrete points, and conversely that a Fermi liquid or magnetically ordered ground state is incompatible with them. Coupled with the enhancement to short-range correlations that we find exclusively in the $d_{xy}$ pairing channel, this allows us to build an indirect case for the ground state of this model having superconducting order in the full 2D limit, and ruling out the other main possible phases, magnetic orders, and Fermi liquids.

We have studied the electron pair emission process from surfaces due to the impact of a primary low-energy positron. The existence of this process implies a two-step scattering event. We find that the electron pair intensity is on the same scale as the positron-electron pair intensity. This suggests that a significant contribution of the pair intensity is actually due to the emission of three particles, of which two are detected. We also observe a strong material dependence of the coincidence intensity. Similarly to our previous studies with primary electron excitation or photon absorption, NiO is the material that displays the highest coincidence rates. We also note that the electron pair intensity scales with the single electron rate. This was also observed in electron pair emission with primary photons or electrons. This suggests that the coincidence intensity may provide a measure of the electron correlation strength.

In this work, we study the near-field heat transfer between composite nanostructures. It is demonstrated that thermally excited surface plasmon polaritons, surface phonon polaritons, and hyperbolic phonon polaritons in such composite nanostructures significantly enhance the near-field heat transfer. To further analyze the underlying mechanisms, we calculate energy transmission coefficients and obtain the near-field dispersion relations. The dispersion relations of composite nanostructures are substantially different from those of isolated graphene, silicon carbide (SiC) films, and SiC nanowire arrays due to the strong coupling effects among surface polaritonic modes. We identify four pairs of strongly coupled polaritonic modes with considerable Rabi frequencies in graphene/SiC film composite structures that greatly broaden the spectral peak. We confirm that near-field strong coupling effects between surface plasmon polaritons and hyperbolic phonon polaritons in the in-plane Reststrahlen band are different from those in the out-of-plane Reststrahlen band due to the different types of hyperbolicity. In addition, we analyze the effective tunability of the near-field heat transfer of graphene/SiC nanowire arrays composite structures by adjusting the chemical potential of graphene, the height and volume filling factor of the SiC nanowire arrays. This work provides a method to manipulate the near-field heat transfer with the use of strongly coupled surface polaritonic modes.