Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals

International audienceWe report on the Nernst effect, specific heat and transportmeasurements performed in high quality (K,Ba)BiO3 single crystals close to optimal doping (Tc ∼ 31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than kBT . Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates

First-Order Transition in the Magnetic Vortex Matter in Superconducting MgB2 Tuned by Disorder

International audienceThe field-driven transition from an ordered Bragg glass to a disordered vortex phase in singlecrystalline MgB2 is tuned by an increasing density of point defects, introduced by electron irradiation. The discontinuity observed in magnetization attests to the first-order nature of the transition. The temperature and defect density dependences of the transition field point to vortex pinning mediated by fluctuations in the quasiparticle mean free path, and reveal the mechanism of the transition in the absence of complicating factors such as layeredness or thermal fluctuations

Probing the exciton condensate phase in 1T-TiSe2 with photoemission

International audienceWe present recent results obtained using angle-resolved photoemission spectroscopy performed on 1T-TiSe2. Emphasis is put on the peculiarity of the bandstructure of TiSe2 compared to other transition metal dichalcogenides, which suggests that this system is an excellent candidate for the realization of the excitonic insulator phase. This exotic phase is discussed in relation to the BCS theory, and its spectroscopic signature is computed via a model adapted to the particular bandstructure of 1T-TiSe2. A comparison between photoemission intensity maps calculated with the spectral function derived for this model and experimental results is shown, giving strong support for the exciton condensate phase as the origin of the charge density wave transition observed in 1T-TiSe2. The temperature-dependent order parameter characterizing the exciton condensate phase is discussed, both on a theoretical and an experimental basis, as well as the chemical potential shift occurring in this system. Finally, the transport properties of 1T-TiSe2 are analyzed in the light of the photoemission results

Fermi surface induced lattice distortion in NbTe2_2

The origin of the monoclinic distortion and domain formation in the quasi two-dimensional layer compound NbTe$_2$ is investigated. Angle-resolved photoemission shows that the Fermi surface is pseudogapped over large portions of the Brillouin zone. Ab initio calculation of the electron and phonon bandstructure as well as the static RPA susceptibility lead us to conclude that Fermi surface nesting and electron-phonon coupling play a key role in the lowering of the crystal symmetry and in the formation of the charge density wave phase

Etude par photoémission et microscopie à effet tunnel<br />des relations entre propriétés structurales et<br />électroniques des interfaces Ce/Sc(0001) et Ag/Au(111)

In this thesis we have studied the structural and electronic properties of intermetallic interfaces by STM and angle-resolved photoemission (ARPES). In the first part we have studied the spin-orbit splitting (SOS) of the Shockley state in the Ag/Au(111) interface. Ag growth at 300 K follows a layer-by-layer mode, whereas annealing at higher temperatures leads to interdiffusion. ARPES measurements, supported by a modelisation of the interface, show that the SOS is proportional to the relative amounts of Au and Ag probed by the surface state wave function. This behaviour confirms the mainly atomic nature of the spin-orbit coupling in Shockley states. In the second part we have tried unsuccessfully to elaborate a strongly hybridized monocristalline a-Ce phase. RHEED measurements lead to the conclusion that the epitaxial film is in the weakly hybridized g phase, but ARPES measurements show a strong dispersive band near the Fermi level, arising from a surface state of d-symmetry.Dans cette thèse nous avons étudié les propriétés structurales et électroniques d'interfaces intermétalliques par STM et photoémission résolue en angle. La première partie est dédiée à l'étude du splitting de spin-orbite (SSO) de l'état de Shockley dans l'interface Ag/Au(111). A 300 K la croissance de l'Ag s'effectue couche-par-couche, et un recuit à plus haute température active l'interdiffusion. Les mesures de photoémission, étayées par une modélisation de l'interface, montrent que le SSO est proportionnel aux quantités relatives d'Au et d'Ag sondées par la fonction d'onde de l'état de surface, confirmant le caractère majoritairement atomique du couplage spin-orbite dans ces états. La deuxième partie est dédiée à l'étude du Ce, épitaxié sur Sc(0001) pour tenter d'obtenir la phase a. Les mesures RHEED concluent à la phase g faiblement hybridée, qui présente toutefois en photoémission une bande dispersive intense au niveau de Fermi, issue d'un état de surface de symétrie d

Etude par photoémission et microscopie à effet tunnel des relations entre propriétés structurales et électroniques des interfaces Ce/Sc(0001) et Ag/Au(111)

Dans cette thèse nous avons étudié les propriétés structurales et électroniques d'interfaces intermétalliques par STM et photoémission résolue en angle. La première partie est dédiée à l'étude du splitting de spin-orbite (SSO) de l'état de Shockley dans l'interface Ag/Au(111). A 300 K la croissance de l'Ag s'effectue couche-par-couche, et un recuit à plus haute température active l'interdiffusion. Les mesures de photoémission, étayées par une modélisation de l'interface, montrent que le SSO est proportionnel aux quantités relatives d'Au et d'Ag sondées par la fonction d'onde de l'état de surface, confirmant le caractère majoritairement atomique du couplage spin-orbite dans ces états. La deuxième partie est dédiée à l'étude du Ce, épitaxié sur Sc(0001) pour tenter d'obtenir la phase a. Les mesures RHEED concluent à la phase g faiblement hybridée, qui présente toutefois en photoémission une bande dispersive intense au niveau de Fermi, issue d'un état de surface de symétrie d.In this thesis we have studied the structural and electronic properties of intermetallic interfaces by STM and angle-resolved photoemission (ARPES). In the first part we have studied the spin-orbit splitting (SOS) of the Shockley state in the Ag/Au(111) interface. Ag growth at 300 K follows a layer-by-layer mode, whereas annealing at higher temperatures leads to interdiffusion. ARPES measurements, supported by a modelisation of the interface, show that the SOS is proportional to the relative amounts of Au and Ag probed by the surface state wave function. This behaviour confirms the mainly atomic nature of the spin-orbit coupling in Shockley states. In the second part we have tried unsuccessfully to elaborate a strongly hybridized monocristalline a-Ce phase. RHEED measurements lead to the conclusion that the epitaxial film is in the weakly hybridized g phase, but ARPES measurements show a strong dispersive band near the Fermi level, arising from a surface state of d-symmetry.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Vortex creep down to 0.3 K in superconducting Fe(Te,Se) single crystals

International audienceWe report on a study of the vortex creep in Fe1+δ(TexSe1−x ) single crystals (x = 0.5 and 0.4) down to 0.28 K (∼Tc/50) and up to μ0Ha = 2T. The relaxation of the current density [J (t )] has been measured during 20 hours and the decay of J (t ) can be well described by a J (t ) ∝ [ln(t/t0)]−1/μ law.We show that the relaxation exponent μ tends towards 0 forT <2K and μ0Ha < 0.1T [i.e. J (t ) → (t0/t)α] and increases for increasing T and/or Ha . Our measurements strongly suggest that the logarithmic creep rate R = −d ln(J )/d ln(t ) remains finite at zero temperature (R|T→0 → 2%) and hence that quantum creep plays a dominant role in the relaxation process at low temperature. A maximum is observed in both the temperature and field dependence of R(t = 100s,T,Ha ), which can be associated to a crossover from a single vortex (one-dimensional) to a bundle (three-dimensional) creep regime

Wide critical fluctuations of the field-induced phase transition in graphite

S.M on requestInternational audienceIn the immediate vicinity of the critical temperature (T$_c$) of a phase transition, there are fluctuations of the order parameter, which reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second-order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20 percent) reduction in the amplitude of the magnetic field (from 33 T to 27 T) leads to a threefold decrease of T$_c$ and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T$_c$. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the $3DXY$ universality class, similar to the case of $^4$He superfluid transition