Romain Grasset

B22. 00004: Defect-induced low-energy Majorana excitations in the Kitaev magnet α-RuCl 3

We study the effects of flux creep on the linear AC response of the vortex lattice in a low-pinning CaIrSn superconductor by measuring the Campbell penetration depth, $\lambda_\rm \scriptscriptstyle C(T,H,t)$. Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter, which is proportional to the pinning well depth. This effect cannot be detected in conventional magnetic relaxation measurements but is revealed by our observation of a nonmonotonic time evolution of $\lambda_\rm \scriptscriptstyle C(T,H,t)$, which directly probes the average curvature of the occupied pinning centers. The time evolution of $\lambda_\rm \scriptscriptstyle C(T,H,t)$ was measured at different temperatures in samples with different densities of pinning centers produced by electron irradiation. The $\lambda_\rm \scriptscriptstyle C(T,H,t)$ is hysteretic with a zero-field-cool warmed (ZFCW) state that exhibits a noticeable nonmonotonic relaxation but shows a monotonic change in the field-cooled (FC) state. The curves, measured at different temperatures, can be collapsed together when plotted on a logarithmic time scale , quantitatively corroborating the proposed picture of vortex creep based on the strong pinning theory.

In bulk SrRuO, the strong sensitivity of the superconducting transition temperature to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film SrRuO, the relationship between and the residual resistivity , which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film SrRuO, we show that is suppressed in both systems at nearly identical rates. This suggests that part of in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh17S15 (Tc = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature Tc and the upper critical field Hc2(T) in single crystalline Rh17S15. We found a T − linear temperature variation of Δλ(T) below 0.3Tc, which is …

The intrinsically superconducting Dirac semimetal 2 M− WS 2 is a promising candidate for realizing proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential to understand the nature of surface superconductivity in the system. Here, we report a detailed experimental study of the temperature-dependent London penetration depth, λ (T), the upper critical field, H c 2 (T), and the effects of nonmagnetic disorder on these quantities, as well as on the superconducting transition temperature T c in single crystals of 2 M− WS 2. We observe a power-law variation of λ (T)∝ T 3 at temperatures below 0.35 T c. Nonmagnetic pointlike disorder induced by 2.5 MeV electron irradiation at various doses results in a significant suppression of T c. These observations are markedly different from expectations for a fully gapped isotropic s− wave …

The effects of 2.5-MeV electron irradiation were studied in the superconducting phase of single crystals of LaNiGa 2, using measurements of electrical transport and radio-frequency magnetic susceptibility. The London penetration depth is found to vary exponentially with temperature, suggesting a fully gapped Fermi surface. The inferred superfluid density is close to that of a single-gap weak-coupling isotropic s− wave superconductor. Superconductivity is extremely robust against nonmagnetic point-like disorder induced by electron irradiation. Our results place strong constraints on the previously proposed triplet pairing state by requiring fine-tuned impurity scattering amplitudes and are most naturally explained by a sign-preserving, weak-coupling, and approximately momentum-independent singlet superconducting state in LaNiGa 2, which does not break time-reversal symmetry. We discuss how our findings …

Chiral engineering of TeraHertz (THz) light fields and the use of the handedness of light in THz light-matter interactions promise many novel opportunities for advanced sensing and control of matter in this frequency range. Unlike previously explored methods, this is achieved here by leveraging the chiral properties of highly confined THz surface plasmon modes. More specifically, we design ultrasmall surface plasmonic-based THz cavities and THz metasurfaces that display significant and adjustable chiral behavior under modest magnetic fields. For such a prototypical example of non-hermitian and dispersive photonic system, we demonstrate the capacity to magnetic field-tune both the poles and zeros of cavity resonances, the two fundamental parameters governing their resonance properties. Alongside the observed handedness-dependent cavity frequencies, this highlights the remarkable ability to engineer chiral and tunable radiative couplings for THz resonators and metasurfaces. The extensive tunability offered by the surface plasmonic approach paves the way for the development of agile and multifunctional THz metasurfaces as well as the realization of ultrastrong chiral light-matter interactions at low energy in matter with potential far-reaching applications for the design of material properties.

The delafossite metal PtCoO 2 is among the highest-purity materials known, with low-temperature mean free path up to 5 μ m in the best as-grown single crystals. It exhibits a strongly faceted, nearly hexagonal Fermi surface. This property has profound consequences for nonlocal transport within this material, such as in the classic ballistic-regime measurement of bend resistance in mesoscopic squares. Here, we report the results of experiments in which high-energy electron irradiation was used to introduce pointlike disorder into such squares, reducing the mean free path and therefore the strength of the ballistic-regime transport phenomena. We demonstrate that high-energy electron irradiation is a well-controlled technique to cross from nonlocal to local transport behavior and therefore determine the nature and extent of unconventional transport regimes. Using this technique, we confirm the origins of the …

The ability to confine THz photons inside deep-subwavelength cavities promises a transformative impact for THz light engineering with metamaterials and for realizing ultrastrong light-matter coupling at the single emitter level. To that end, the most successful approach taken so far has relied on cavity architectures based on metals, for their ability to constrain the spread of electromagnetic fields and tailor geometrically their resonant behavior. Here, we experimentally demonstrate a comparatively high level of confinement by exploiting a plasmonic mechanism based on localized THz surface plasmon modes in bulk semiconductors. We achieve plasmonic confinement at around 1 THz into record breaking small footprint THz cavities exhibiting mode volumes as low as , excellent coupling efficiencies and a large frequency tunability with temperature. Notably, we find that plasmonic-based THz cavities …

The recently discovered kagome superconductors AV3Sb5 (A = K, Rb, Cs) exhibit unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries with sign-changing and chiral order parameters. Experimentally, however, superconducting phase information in AV3Sb5 is still lacking. Here we report the impurity effects in CsV3Sb5 using electron irradiation as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, an anisotropic fully-gapped state changes to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under pressure show that the double …

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1 T− TaS 2. When heating the sample, the surface displays an intermediate insulating phase that persists for∼ 10 K on top of a metallic bulk. The weaker screening of Coulomb repulsion and a stiffer charge density wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time-resolved angle-resolved photoelectron spectroscopy and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low-temperature phase and weaker in the intermediate one.

In this article, we report some examples of how high-energy electron irradiation can be used as a tool for shaping material properties turning the generation of point-defects into an advantage beyond the presumed degradation of the properties. Such an approach is radically different from what often occurs when irradiation is used as a test for radiation hard materials or devices degradation in harsh environments. We illustrate the potential of this emerging technique by results obtained on two families of materials, namely semiconductors and superconductors.

The layered delafossite metal PdCrO is a natural heterostructure of highly conductive Pd layers Kondo coupled to localized spins in the adjacent Mott insulating CrO layers. At high temperatures, T, it has a T-linear resistivity which is not seen in the isostructural but nonmagnetic PdCoO. The strength of the Kondo coupling is known, as-grown crystals are extremely high purity and the Fermi surface is both very simple and experimentally known. It is therefore an ideal material platform in which to investigate “Planckian metal” physics. We do this by means of controlled introduction of point disorder, measurement of the thermal conductivity and Lorenz ratio, and studying the sources of its high-temperature entropy. The T-linear resistivity is seen to be due mainly to elastic scattering and to arise from a sum of several scattering mechanisms. Remarkably, this sum leads to a scattering rate within 10 of the Planckian value of k …

Here we perform angle and time-resolved photoelectron spectroscopy on the commensurate charge density wave (CDW) phase of 1T-TaS 2. Data with different probe pulse polarization are employed to map the dispersion of electronic states below and above the chemical potential. Upon photoexcitation, the fluctuations of CDW order erase the band dispersion and squeeze the electronic states near to the chemical potential. This transient phase sets within half a period of the coherent lattice motion and is favored by strong electronic correlations. The experimental results are compared to density-functional theory calculations with a self-consistent evaluation of the Coulomb repulsion. Our simulations indicate that the screening of Coulomb repulsion depends on the stacking order of the TaS 2 layers. The entanglement of such degrees of freedom suggest that both the structural order and electronic repulsion are …

We study the evolution of temperature-dependent resistivity with added pointlike disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the “3-4-13” stannides,(Ca, Sr) 3 (Ir, Rh) 4 Sn 13. Three of these cubic compounds exhibit a proposed microscopic coexistence of charge density wave (CDW) order and superconductivity (SC), while Ca 3 Rh 4 Sn 13 does not develop CDW order. As expected, the CDW transition temperature T CDW is universally suppressed by irradiation in all three compositions. The superconducting transition temperature, T c, behaves in a more complex manner. In Sr 3 Rh 4 Sn 13, it increases initially in a way consistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In the other three compounds, T c is monotonically suppressed by irradiation. The strongest suppression is found in Ca 3 Rh 4 Sn 13, which does not have CDW …

Irradiation with high-energy electrons (HEE) at cryogenic temperatures is a subtle tool for shaping matter. Unlike irradiation with heavy particles, e.g. protons, neutrons, or ions, HEE irradiation produces very low local damage generating exclusively point lattice defects. In the interaction process, the primary high-energy electron transfers a minute quantity of energy to a lattice ion, just enough for displacing it from its lattice site. The concentration of induced vacancies depends on the irradiation dose and in this way can be carefully adjusted. Since the lattice defects can act as donor or acceptor states in semiconductors, electron irradiation enables accurately-controlled compensation of electrically-active impurities introduced in a semiconductor crystal during growth. In this article, we present a study of the evolution of electronic properties of β-gallium oxide with step-by-step compensation of initial n-type doping through …

We investigate the collective mode response of the iron-based superconductor Ba1−xKxFe2As2 using intense terahertz (THz) light. In the superconducting state a THz Kerr signal is observed and assigned to nonlinear THz coupling to superconducting degrees of freedom. The polarization dependence of the THz Kerr signal is remarkably sensitive to the coexistence of a nematic order. In the absence of nematic order the C4 symmetric polarization dependence of the THz Kerr signal is consistent with a coupling to the Higgs amplitude mode of the superconducting condensate. In the coexisting nematic and superconducting state the signal becomes purely nematic with a vanishing C4 symmetric component, signaling the emergence of a superconducting collective mode activated by nematicity.

Terahertz (THz) spin‐to‐charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here, nanometric trilayers of Co/X/Pt (X = Ti, Au or an Au:W alloy) are investigated as a function of the “X” layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the “X” layer it is shown that the ultrafast spin current injected in the non‐magnetic layer defines a direct‐spin‐conductance, whereas the Gilbert damping leads to an effective spin‐mixing‐conductance of the trilayer. Importantly, it is shown that these two parameters are connected to each other and that spin‐memory‐losses can be modeled via an …

THz emission spectroscopy reveals to be a very powerful experimental method to investigate the properties of Rashba or topological insulator surface states. The THz emission can be also used in heavy metallic or in more general Rashba systems. We prove here the ability of the present method. In 3d/5d transient metal bilayers and beyond heavy metal structures, Rashba states and Topological insulators are expected candidates for spintronic-terahertz domains due to their high spin to charge conversion properties. In this scheme, we are interested in the samples based on 2D electron gas, topological insulators and Heusler alloys with strong spin-orbit coupling.

The superconducting phase of the HgBa 2 CuO 4+ δ (Hg-1201) and HgBa 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of T c with pressure is slower in the Hg-1223 cuprate compared to Hg-1201 due to a charge carrier concentration imbalance accentuated by pressure in Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the CuO 2 layer, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the interlayer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the d-wave superconducting gap at the antinodes does not follow T c with pressure. It decreases while T c increases. In the particular case of Hg-1201, the binding energy collapses from …