Correlated electron systems

Superconductors based on transition metal dichalcogenides are of substantial current relevance towards the material realization of topological superconductivity. Here, we report a detailed study on the synthesis and characterization of single crystals of 2H-TaSeS. A superconducting transition is confirmed at $4.15\,\mathrm{K}$ that coexists with a charge-density wave ordering at $66\,\mathrm{K}$. The temperature dependence of the RF penetration depth indicates s-wave characteristics in the weak-coupling limit. Moderate electronic anisotropy is observed in the upper critical fields. DFT calculations confirm that the most stable structure belongs to the $P6_3\textrm{mc}$ space group. Negative values in the phonon dispersion curves verify the possibility of coexisting superconductivity with a charge-density wave in 2H-TaSeS. We also study vortex dynamics in this novel superconductor. Overall, our analysis suggests that 2H-TaSeS is a conventional Type-II superconductor without any evidence for topological superconductivity.

Time-resolved ultrafast spectroscopy has emerged as a promising tool to dynamically induce and manipulate non-trivial electronic states of matter out-of-equilibrium. Here we theoretically investigate light pulse driven dynamics in a Kondo lattice system close to quantum criticality. Based on a time-dependent auxiliary fermion mean-field calculation we show that light can dehybridize the local Kondo screening and induce oscillating magnetic order out of a previously paramagnetic state. Depending on the laser pulse field amplitude and frequency the Kondo singlet can be completely deconfined, inducing a dynamic Lifshitz transition that changes the Fermi surface topology. These phenomena can be identified in harmonic generation and time-resolved angle-resolved photoemission spectroscopy spectra. Our results shed new light on non-equilibrium states in heavy fermion systems.

A nodal surface semimetal (NSSM) features symmetry enforced band crossings along a surface within the three-dimensional (3D) Brillouin zone (BZ) and a presence of a nonsymmorphic symmetry there pushes such surfaces to stick to the BZ center or boundaries. The topological robustness of the same does not always come with nonzero Berry fluxes. We consider two such NS, one with zero and another with nonzero topological charges and investigate the effect of light irradiation on them. We find that depending on the state of polarization, one can obtain additional Weyl points/NS in the corresponding Floquet Hamiltonians. Particularly, using a simple two band spinless/spin polarized models with no spin orbit coupling, we emphasize the low energy behavior of the continuum Hamiltonians close to the band crossings and its evolution in a Floquet system in the high frequency limit. In the Floquet system, we also find the NS to perish or new multi Weyl points to get popped up for different polarization scenario or different NSSM Hamiltonians. Our findings open up important avenues on what out of equilibrium NSSM systems can offer in many active fields including quantum computations.

Experimental determination of the structure of the superconducting order parameter in the kagome lattice compound CsV₃Sb₅ is an essential step towards understanding the nature of the superconducting pairing in this material. Here we report measurements of the temperature dependence of the in-plane magnetic penetration depth, $\lambda(T)$, in crystals of CsV₃Sb₅ down to ${\sim} 60\,\mathrm{mK}$. We find that $\lambda(T)$ is consistent with a fully-gapped state but with significant gap anisotropy. The magnitude of the gap minima are in the range ${\sim} 0.2$–0.3  $T_\mathrm{c}$ for the measured samples, markedly smaller than previous estimates. We discuss different forms of potential anisotropy and how these can be linked to the V and Sb Fermi surface sheets. We highlight a significant discrepancy between the calculated and measured values of $\lambda(T = 0)$ which we suggest is caused by spatially suppressed superconductivity.

Recently, the use of circularly polarized radiation for on-demand switching between distinct quantum states in a superconducting nanoring exposed to half-quantum magnetic flux has been proposed. However, the effectiveness of this method depends on the system's stability against local variations in the superconducting characteristics of the ring and flux fluctuations. In this study, we utilize numerical simulations based on the time-dependent Ginzburg–Landau equation to evaluate the influence of these inevitable factors on the switching behavior. The results obtained demonstrate that the switching phenomena remain remarkably robust, providing confidence in their experimental observation.

We investigate the effects of off-resonant THz-frequency laser light coupling to bound few-body electron–hole system, i.e. the exciton and negatively charged trion confined in quantum wire. To solve this problem, we first conduct a unitary Hennerberger-Kramers transformation of the Hamiltonian and diagonalize its perturbative approximation to obtain the exciton and trion Floquet states. Within this framework, the light-matter coupling renormalizes an attractive eh interaction, leaving the repulsive ee unchanged, thus modifying corresponding two-particle correlation energies. Generally, the correlation energy of eh would exceed the ee one for a semiconductor material with strongly localized heavy holes. However, as the former is weakened by increasing laser intensity, this relation can be reversed. Consequently, the trion may dissociate unconventionally, the hole gradually decouples from still strongly interacting electrons, and adequate energy and optical spectra changes accompany this process. The energy levels of the exciton and trion Floquet states are raised, while their optical brightness smoothly decreases for stronger laser intensities. We also show this process can be further modified by breaking the mirror symmetry of wire with a static electric field, and then the occurrence of the avoided crossings between the lowest energy levels of the trion depends on the laser intensity. These anticrossings shall be observed experimentally, confirming thus the usefulness of Floquet engineering for fast manipulations of the few-particle states in electron–hole systems on a subpicosecond time scale.