Search

The Fermi surface of Bi2Sr2CaCu2O8+x(001) and Bi2Sr2CuO6(001) has been mapped at nearly 6000 points in k space using angle-resolved photoemission at 300K. Bi2Sr2CaCu2O8+x(001) exhibits a shadow Fermi surface strongly suggesting short range antiferromagnetic correlations (P. Aebi et al., Phys. Rev. Lett. 72, 2757 (1994)). For its Ca free counter part the photoemission intensity at the Fermi level is weaker, reflecting the presence of only one Ca-O layer and the difference in doping. An estimation of the doping is given directly from the topology of the Fermi surface.

Angle-resolved, synchrotron-radiation photoemission and low-energy electron-diffraction studies of single-crystal Bi2Sr2CaCu2Ox high-temperature superconductors establish that the [100] and [010] directions exhibit quantitatively different structural and electronic properties. Specifically, there is a superlattice modulation in the [010] direction, with superlattice period between 4.7 and 5.2 times the period in the [100] direction. Accompanying this structural difference there is a marked difference in the intensity and amount of dispersion in the two directions.

The comment by Starnberg, Brauer, and Nilsson qualitatively identifies the final-state lifetime as the cause of the ''anomalous'' photoemission broadening, but it neglects the linear energy dependence of the final-state broadening whose effect is of the same magnitude as the purely geometrical contributions.

The capability to perform absorption spectroscopy with hard x-rays was added to the synchrotron imaging Spectromicroscopy technique. We present several successful test cases. First, the EXAFS (extended x-ray absorption fine structure) analysis was implemented in areas as small as few µm2 for transition-metal K edge absorption spectra. The corresponding structural information was complemented by chemical information from the near-edge region. Specifically, in the case of a cross-sectional cut steel different Fe oxidation states were found at different depths. Tests on a PVD grown TiN film revealed areas with oxidized Ti. Overall, these tests demonstrate the feasibility of combining structural and chemicals analysis with hard-x-ray absorption and high lateral resolution.

Note: ISI Document Delivery No.: NN993 Reference LSE-ARTICLE-1994-017doi:10.1103/PhysRevB.49.14085.2 Record created on 2006-10-03, modified on 2017-05-12

Single crystals of the system Ti 1−x Ru x O 2 have been prepared by chemical vapour transport in closed ampoules using halide transport agents. The crystals are single phased for x ≦ 0.02 and 0.98 ≦ x. In the intermediate range a highly ordered two phased system is found. The pure RuO 2 crystals are substantially larger than any grown by open sysem chemical vapour transport. Single crystals of TiO 2 doped with other impurities have also been prepared.

Electrical properties of semiconducting MoSe2 single crystals doped with Re and with Nb are described. Impurity conduction can be characterized by three activation energies ∊1 > ∊2 > ∊3. At low temperature and for high doping levels a transition to a metallic state is observed. A band model is suggested which accounts for the measured activation energies and is in agreement with the photoemission results and optical data.

The temperature dependence of the d.c. resistivity was investigated in bulk ceramics of nominal composition Bi1.6Pb0.4Sr2Ca2Cu3Oy and Bi1.6Sr2Ca2Cu3Oy. The dependence of Tc on the current density was also studied. Our best samples contain a mixture of the 80 K-phase (40%) and 110 K-phase (60%). The corresponding X-ray diffraction lines can be indexed in the pseudo-tetragonal unit cells with parameters 5.40 × 5.41 × 31 A 3 and 5.40 × 5.41 × 37.3 A 3 , respectively. The effects of annealing, quenching and oxygen excess during sintering on the resistivity and granularity of the samples are discussed.

Large single crystals of ReS2 and ReSe2 crystallizing in a layer-type structure (space group P 1 ) have been grown by chemical vapour transport in a temperature gradient of 1 to 5K cm-1 at about 1080°C. The triclinic symmetry and lattice parameters have been checked by X-ray diffraction measurements. The electrical properties were investigated. For the first time, the temperature dependence of the resistivity and Hall voltage is reported for n- and p-type ReS2 and ReSe2. N-type samples had a room- temperature resistivity of about 2 and 5 ohm · cm. The Hall mobility of 22 and 16 cm 2 V · s at 300K varied with temperature as T-1.1 and T-2.4, respectively. The activation energy for the carriers of n-type ReS2 and ReSe2 as a function of the temperature was 40 and 240 meV, respectively. In the case of the p-type crystals, the doping with phosphorus was strongly compensated.

Nb 2 Se 9 single crystals were grown from the liquid solution of niobium in selenium. The crystals have been characterized by X-ray diffraction. Resistivity measurements in the temperature range 200 5 Щcm.

The p-type as grown single crystals of the layered compounds FePS3 and FePSe3 have been studied by the photoelectrochemical method. The current-voltage and capacitance-voltage characteristics indicate the presence of surface states. The analysis of the photocurrent quantum efficiencies give two indirect optical transitions at 1.62 and 2.3 eV for FePS3 and at 1.3 and 1.83 eV for FePSe3. Hole mobilities are in the range 0.5–1.0 cm2 V−1 sec−1 as found from the Hall effect measurements. These results are interpreted in terms of the existing electronic band scheme for the MPX3 (M = transition metal; X = S, Se) layer compounds.

n‐ and p‐type doped crystals grown by chemical vapor transport have been investigated for their electrical and photoelectrochemical properties. The photocurrent‐voltage characteristics, differential capacitance, and spectral response were measured in . The current‐voltage curves of both types of electrodes indicate significant recombination. p‐type is stable during the photocathodic hydrogen evolution while n‐type electrodes undergo photoanodic decomposition. The measured quantum efficiencies are very low due to the localized d‐electronic states involved in the photoelectronic processes. Indirect bandgap analysis of the quantum efficiency data reveals transitions at 1.64 and 1.9 eV. The transition at 1.64 eV is in good agreement with the reported optical absorption edge. These results are related with the electronic energy band scheme of .

Electrical properties of (NbSe4)3I are studied on single crystals prepared under various growth conditions. It is found that, with decreasing temperature, a semiconducting behaviour above about 330 K is followed by a semiconducting state with a very small activation energy or semimetallic state depending on sample. A very sharp resistivity anomaly is observed around 270 K in all the examined crystals and is related with the reported structural phase transition. The origin of the various resistivity behaviours at low temperatures is discussed on the basis of the absence of nonlinear transport phenomena.

We report low field studies of the static magnetic susceptibility and torque measurements in the temperature range 2-330 K for the powder and crystalline CuTe2O5 compound. We show that the observed extraordinarily large magnetic axis rotation below 20 K and small (nearly negligible), but highly anisotropic, low temperature susceptibility upturn (which is thus of intrinsic nature) is a consequence of the changes within the ground state d(9) orbital doublet which take place below 20 K, smoothly.

We have studied by high resolution ARPES the spectroscopic properties of the prototype charge density wave system (TaSe4)(2)I and of the closely related insulating compound (NbSe4)(3)I. We observe features dispersing with the lattice periodicity, including commensurate (in (NbSe4)(3)I) and incommensurate (in (TaSe4)(2)I) superlattice shadow bands. The spectral lineshape is anomalous in the normal metallic state, and the intensity is strongly suppressed near the Fermi surface. In the insulating CDW state the linear extrapolation of the leading edge rather than the peak position identifies the gap energy scale. (C) 2001 Elsevier Science B.V; All rights reserved.

We investigate transport through ionic liquid gated field effect transistors (FETs) based on exfoliated crystals of semiconducting WS$_2$. Upon electron accumulation, at surface densities close to -or just larger than- 10$^{14}$ cm$^{-2}$, transport exhibits metallic behavior, with the surface resistivity decreasing pronouncedly upon cooling. A detailed characterization as a function of temperature and magnetic field clearly shows the occurrence of a gate-induced superconducting transition below a critical temperature $T_c \approx 4$ K, a finding that represents the first demonstration of superconductivity in tungsten-based semiconducting transition metal dichalcogenides. We investigate the nature of superconductivity and find significant inhomogeneity, originating from the local detaching of the frozen ionic liquid from the WS$_2$ surface. Despite the inhomogeneity, we find that in all cases where a fully developed zero resistance state is observed, different properties of the devices exhibit a behavior characteristic of a Berezinskii-Kosterlitz-Thouless transition, as it could be expected in view of the two-dimensional nature of the electrostatically accumulated electron system., 18 pages, 4 figures; Nano Letters (2015)

The magnetic and magnetoresonance properties of a new single-crystal compound Ni-5(TeO3)(4)Cl-2 are studied. Measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5-300 K) make it possible to conclude that Ni-5(TeO3)(4)Cl-2 is a quasi-two-dimensional antiferromagnet with the easy magnetization axis a* directed perpendicular to the crystallographic plane bc and a magnetic ordering temperature T-N approximate to 21 K. Resonance measurements at 4.2 K in wide range of frequencies (25-105 GHz) and magnetic field (up to 200 kOe) permit us to obtain the frequency-field dependence of the AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that a magnetic field directed along the anti ferromagnetic axis (H parallel to a*) induces a magnetic phase transition of the spin-flop type, which is found to occur at H-sf approximate to 120 kOe. The magnetic resonance experimental data are described qualitatively in the model of a biaxial anti ferro magnet. (c) 2008 American Institute of Physics.

Time reversal and spatial inversion are two key symmetries for conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity. Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties. Two-dimensional (2D) NbSe$_2$ has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin-orbit coupling (SOC). Here, we report the observation of an unusual continuous paramagnetic-limited superconductor-normal metal transition in 2D NbSe$_2$. Using tunneling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin film superconductors. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe$_2$. Our findings establish 2D NbSe$_2$ as a promising platform for exploring novel spin-dependent superconducting phenomena and device concepts, such as equal-spin Andreev reflection and topological superconductivity.

Electrodeposited bismuth telluride (Bi2Te3) nanowires using anodic alumina (AAO) templates show anisotropic thermal properties in a direction parallel and perpendicular to the nanowire/nanochannel axis. The present study reports thermal diffusivity measurement of templated Bi2Te3 nanowires in a direction perpendicular to the nanowire axis, using an active infrared thermography based noncontact technique. Measurements were performed on empty AAO templates of varying pore dimensions, AAO/Bi2Te3 nanocomposites and a single crystal Bi2Te3 sample. A first order lower bound model estimation showed nearly five-fold reduction of thermal conductivity in 200 nm Bi2Te3 nanowires as compared to the bulk values. (C) 2013 Elsevier B.V. All rights reserved.

We performed a spectromicroscopy experiment to find evidence of lateral inhomogeneities in the band bending for the Al/GaS system. Microimages of the interface tuned on the Al 2p con levels revealed localized chemical inhomogeneities in proximity of the edge of the Al overlayer. A careful analysis of the photoemission spectra supports these conclusions and relates the inhomogeneities to band bending variations. The micron-size lateral scale for these features is similar to that of other interfaces. (C) 1999 Elsevier Science B.V. All rights reserved.

We present Fermi surface (FS) mapping measurements obtained by photoemission for the quasi-commensurate charge density wave (CDW) phase of layered 1T-TaS2. The topology of the FS is rather dominated by the undistorted (1×1) symmetry than by the CDW-induced ( 13 × 13 ) phase. CDW effects can be seen in terms of nesting properties of the FS. Compared to one-electron band structure calculations we observe changed FS contours of Ta 5d-derived electron pockets around the M,M′ points of the (1×1) Brillouin zone. Low energy electron diffraction measurements support the view of a quasi-commensurate CDW.

The possibility to inject spin currents in topological insulators (TIs) by ultrashort optical pulses has stimulated intense studies of their out-of-equilibrium electronic properties. However, a comprehensive description of the electronic relaxation dynamics has been elusive, so far. In order to reveal the role of the bulk and surface states in the microscopic scattering mechanisms, we have investigated, by means of time- and angle-resolved photoemission spectroscopy, a wide set of TIs. These have been selected in order to display different positions of the Fermi energy (${E}_{\text{F}}$) within the bulk band structure. When three-dimensional bulk states lie at ${E}_{\text{F}}$, we observe a fast relaxation dynamics with a characteristic time scale of a few picoseconds (ps). On the contrary, a long lasting excited state is recorded when only the two-dimensional surface state crosses ${E}_{\text{F}}$. These findings suggest the important role played by spatial diffusion in the direction orthogonal to the surface in governing the relaxation mechanisms. We propose that this electron diffusive mechanism is driven by the optically induced temperature gradient that is at play only for electrons residing in bulk states.

When electrons are subject to a potential with two incommensurate periods, translational invariance is lost, and no periodic band structure is expected. However, model calculations based on nearly free one-dimensional electrons and experimental results from high-resolution photoemission spectroscopy on a quasi–one-dimensional material do show dispersing band states with signatures of both periodicities. Apparent band structures are generated by the nonuniform distribution of electronic spectral weight over the complex eigenvalue spectrum.

Thermal conductivity (κ) plays an essential role in functional devices. It is advantageous to design materials where one can tune κ in a wide range according to its function: single-crystals and nanowires of anatase polymorph of titanium dioxide, broadly used in applications ranging from photovoltaics, reflective coatings to memristors, have been synthesized in large quantities. Here we identify a new, strong diffusion mechanism of heat by polaronic structures due to oxygen vacancies, which considerably influences both the absolute value and the temperature dependence of κ. The additional decrease of κ is achieved in anatase nanowires organized into foam, where porosity and the quasi-one-dimensional size-effect dramatically hinder the propagation of heat, resulting in an extremely low κ = 0.014 W/Km at room-temperature. Doping this anatase foam could herald promising applications, in particular in thermoelectricity. The thermal properties of a material often determine its suitability for application and use in devices. Here, the thermal conductivity of anatase TiO2 is tuned over three orders of magnitude from bulk crystals to foam samples, by controlling polaronic effects and texturing.

Precision measurements of the magnetization and ac susceptibility of Cu2OSeO3 are reported for magnetic fields along different crystallographic directions, focusing on the border between the conical and the field-polarized state for a magnetic field along the axis, complemented by selected specific heat data. Clear signatures of the emergence of a second skyrmion phase and a tilted conical phase are observed, as recently identified by means of small angle neutron scattering. The low-temperature skyrmion phase displays strongly hysteretic phase boundaries, but no dissipative effects. In contrast, the tilted conical phase is accompanied by strong dissipation and higher-harmonic contributions, while the transition fields are essentially nonhysteretic. The formation of the second skyrmion phase and tilted conical phase are found to be insensitive to a vanishing demagnetization factor. A quantitative estimate of the temperature dependence of the magnetocrystalline anisotropy may be consistently inferred from the magnetization and the upper critical field and agrees well with a stabilization of the low-temperature skyrmion phase and tilted conical state by conventional cubic magnetic anisotropies.

Single crystal growth by chemical vapour transport has resulted samples with Fe1.35Ge stoichiometry. Structural study has shown that the large number of vacancies introduces an incommensuately modulated structure. The electrical resistivity is in the 200 mu Omega cm range at room temperature, and although the ferromagnetic transition temperature at 425 K is clearly visible, it hardly varies down to 4.2 K. It is suggested that the large number of vacancies (and the incommensurate modulation) introduce a strong backscattering, and the system is at the brink of a Mooij correlation. The thermal conductivity and Seebeck coefficient carry also the consequences of the high concentration of vacancies. (c) 2019 Elsevier B.V. All rights reserved.

The search for the coexistence between superconductivity and other collective electronic states in many instances promoted the discovery of novel states of matter. The manner in which the different types of electronic order combine remains an ongoing puzzle. 1T-TaS(2) is a layered material, and the only transition-metal dichalcogenide (TMD) known to develop the Mott phase. Here, we show the appearance of a series of low-temperature electronic states in 1T-TaS(2) with pressure: the Mott phase melts into a textured charge-density wave (CDW); superconductivity develops within the CDW state, and survives to very high pressures, insensitive to subsequent disappearance of the CDW state and, surprisingly, also the strong changes in the normal state. This is also the first reported case of superconductivity in a pristine 1T-TMD compound. We demonstrate that superconductivity first develops within the state marked by a commensurability-driven, Coulombically frustrated, electronic phase separation.

We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO2 planes as a function of the carrier density, in optimally doped and underdoped Bi2Sr2CaCuO8+x and in Sr2CuO2Cl2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.

High-resolution angle-resolved (ARPES) photoemission in the quasi-one dimensional (1D) Peierls systems (TaSe4)2I and K0.3MoO3 reveals spectral features with clear 1D character. Their dispersion and polarization dependence is consistent with open Fermi surfaces and with the symmetry properties of the lattice, However the measured spectral lineshapes are unusual in various respects, as shown also by a comparison with the quasiparticle spectra of a `good' metal. A consistent description of the normal metallic phase and of the low-temperature insulating phases is possible within a polaronic scenario, where the carriers are strongly renormalized by the interaction with the lattice and the spectrum is largely incoherent. The quasiparticle signatures are hidden at the spectral leading edge.

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we examined the charge density wave (CDW) compound $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ as a function of the photon energy (energy range from 11 up to 33 eV) for both quasicommensurate (QC) and commensurate (C) CDW phases. While the QC-phase normal emission ARPES spectra coincide with previous results, normal emission spectra of the C phase reveal evidence for a fine structure in the CDW-induced features between 1 eV binding energy and the Fermi level. This fine structure may be explained by the subband manifold of the Ta $5d$ band, split off due to the CDW-induced $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure.

Local-probe weak-transverse-field and zero-field mu SR measurements have been employed to investigate magnetic ordering in the new magnetoelectric compound FeTe2O5Br. Below the Neel transition temperature T-N=10.6 K a static local magnetic field starts to develop at the mu(+) sites. Fast mu(+) polarization decay below T-N speaks in favor of a broad distribution of internal magnetic fields, in agreement with the incommensurate magnetic structure suggested by neutron diffraction experiments. Above T-N the presence of short-range order is detected as high as at 2T(N), which suggests only weak interlayer magnetic coupling. On the other hand, strong Fe3+ spin fluctuations likely reflect geometrically frustrated structure of [Fe4O16](20-) spin clusters, which are the main building blocks of the layered FeTe2O5Br structure. (C) 2010 American Institute of Physics. [doi:10.1063/1.3362909]

We report a comprehensive study of transport, magnetotransport, and magnetic properties of single crystals of BaVSe3. The paramagnetic metal-ferromagnetic metal transition at 43 K was followed as a function of pressure by measuring the electrical resistivity and the thermoelectric power. The exponent of the low-temperature power-law dependence of the resistivity increases with pressure. The effective magnetic moment obtained from magnetic susceptibility in the paramagnetic regime is µ eff=1.40µ B. The study was completed by band-structure calculations based on density-functional theory both at ambient and high pressures. Transport coefficients of BaVSe3 resemble the high-pressure phase of BaVS3, which suggest that the replacement of sulfur with selenium can be viewed as chemical pressure.

In quasi-two-dimensional electron systems of layered transition metal dichalcogenides (TMDs) there is still controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e., whether they are described by a Peierls-type mechanism or by a lattice-driven model. By performing scanning tunneling microscopy experiments on canonical TMD-CDW systems, we image the electronic modulation and the lattice distortion separately in 2H-TaS2, TaSe2, and NbSe2. Across the three materials, we found dominant lattice contributions instead of the electronic modulation expected from Peierls transitions, in contrast to the CDW states, which show the hallmark of contrast inversion between filled and empty states. Our results imply that periodic lattice distortion plays a vital role in the formation of CDW phases in TMDs and illustrate the importance of taking into account the more complicated lattice degrees of freedom when studying correlated electron systems.

Oxygen vacancies created in anatase TiO2 by UV photons (80 - 130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.

The quasi-one-dimensional metallic system BaVS3 with a metal-insulator transition at T_MI=70 K shows large changes of the optical phonon spectrum, a central peak, and an electronic Raman scattering continuum that evolve in a three-step process. Motivated by the observation of a strongly fluctuating precursor state at high temperatures and orbital ordering and a charge gap at low temperatures we suggest a concerted action of the orbital, electronic, and lattice subsystems dominated by electronic correlations., Comment: 4 pages, 4 figures

Polarization dependent vanadium L edge X-ray absorption spectra of BaVS$_3$ single crystals are measured in the four phases of the compound. The difference between signals with the polarization \textbf{E}$\perp$\textbf{c} and \textbf{E}$\parallel$\textbf{c} (linear dichroism) changes with temperature. Besides increasing intensity of one of the maxima, a new structure appears in the pre-edge region below the metal-insulator transition. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in $E_{g}$ and $A_{1g}$ states, and is compatible with the formation of four inequivalent V sites along the V-S chain., Comment: 6 pages, 6 figures

Shubnikov-de Haas (SdH) oscillations, Hall effect, and optical reflectance (R(omega)) measurements have been performed on single crystals of BiTeBr. Under magnetic fields up to 32 tesla and at temperatures as low as 0.4K, the SdH data shows a single oscillation frequency F = 102 +/- 5 tesla. The combined transport and optical studies establish that the SdH effect originates from the Rashba spin-split bulk conduction band, with the chemical potential situated about 13 meV below the crossing (Dirac) point. The bulk carrier concentration was n(e) approximate to 5 x 10(18) cm(-3) and the effective mass m(1)* = 0.16m(0). Combining SdH and optical data, we reliably determine the Rashba parameters for the bulk conduction band of BiTeBr: the Rashba energy E-R = 28meV and the momentum spin-split k(R) = 0.033 angstrom(-1). Hence, the bulk Rashba coupling strength alpha(R) = 2E(R)/k(R) is found to be 1.7 eV angstrom. Copyright (C) EPLA, 2016

We show atomically resolved scanning tunneling microscopy (STM) images of charge density waves (CDWs) at room temperature together with angle-resolved photoelectron band-mapping of 1T-TaSe2. By comparing the results of these two techniques, we demonstrate the atomic structure of the CDW-features observed by the STM and atomic origin of the reconstructed band-structure in this material. (c) 2007 Elsevier B.V. All rights reserved.

The semimetal MoTe$_2$ is studied by spin- and angle- resolved photoemission spectroscopy to probe the detailed electronic structure underlying its broad range of response behavior. A novel spin-texture is uncovered in the bulk Fermi surface of the non-centrosymmetric structural phase that is consistent with first-principles calculations. The spin-texture is three-dimensional, both in terms of momentum dependence and spin-orientation, and is not completely suppressed above the centrosymmetry-breaking transition temperature. Two types of surface Fermi arc are found to persist well above the transition temperature. The appearance of a large Fermi arc depends strongly on thermal history, and the electron quasiparticle lifetimes are greatly enhanced in the initial cooling. The results indicate that polar instability with strong electron-lattice interactions exists near the surface when the bulk is largely in a centrosymmetric phase.

Photoemission spectromicroscopy experiments on a series of Y-doped Bi2Sr2Ca1Cu2O8 single crystals show that the doping changes the valence of copper as required for the observed modification of the critical temperature. The doping, however, affects with substitutional reactions not only the CaO planes, but also the SrO planes.

The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nanoelectronics. With the exception of LaAlO3/SrTiO3 interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS2 by performing tunnelling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density n. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap Δ, which is maximum (Δ ~2 meV) for n of ~1 × 1014 cm−2 and decreases monotonously for larger n. A perpendicular magnetic field B generates states at E

We review recent angle-scanned photoemission experiments on Bi 2 Sr 2 CaCu 2 O 8 + x . The Fermi surface has been mapped completely revealing previously unobserved “shadow bands” interpreted as due to antiferromagnetic correlations. Since Bi 2 Sr 2 CaCu 2 O 8 + x exhibits a relatively complicated incommensurate lattice modulation new measurements on a Pb-doped, unmodulated Bi-cuprate are presented for comparison.

We have realized ambipolar ionic liquid gated field-effect transistors based on WS2 mono- and bilayers, and investigated their opto-electronic response. A thorough characterization of the transport properties demonstrates the high quality of these devices for both electron and hole accumulation, which enables the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the operation of the transistors in the ambipolar injection regime with electrons and holes injected simultaneously at the two opposite contacts of the devices in which we observe light emission from the FET channel. A quantitative analysis of the spectral properties of the emitted light, together with a comparison with the band gap values obtained from transport, show the internal consistency of our results and allow a quantitative estimate of the excitonic binding energies to be made. Our results demonstrate the power of ionic liquid gating in combination with nanoelectronic systems, as well as the compatibility of this technique with optical measurements on semiconducting transition metal dichalcogenides. These findings further open the way to the investigation of the optical properties of these systems in a carrier density range much broader than that explored until now., Comment: 22 pages, 6 figures, Nano Letters (2014)

Carrier and collective mode dynamics in the quasi one-dimensional charge density wave (CDW) system (TaSe4)2I have been investigated by means of time-resolved optical pump-probe spectroscopy. In the low excitation, linear, regime we focus on the temperature dependence of the collective amplitude modes, originating from linear coupling of the electronic modulation to phonons at q CDW. Numerous amplitude modes are observed, ranging from 100 GHz to several THz. The modes’ softening near T c is rather weak, which could be related to strong decoupling of electronic and lattice subsystems. Alternatively, the data could be reconciled also in case the CDW phase transition is of the first-order type where a nearly constant order parameter below T c would prevent softening. In the high excitation regime we investigated the energetics of the photoinduced CDW-normal phase transition. Similarly to the elaborately investigated one-dimensional CDW system K0.3MoO3 we observe two characteristic energy scales, related to melting the electronic modulation alone (100 meV per unit cell) and to the overall (electronic modulation and the periodic lattice distortion) collapse of the CDW (> 400 meV per unit cell). While the latter coincides with the thermal energy needed to heat the sample from 5 K above T c the former is consistent with the mean field estimate for the electronic condensation energy, suggesting that the weak coupling description of the CDW in (TaSe4)2I is appropriate.

As carriers are introduced into the cuprates (by doping the insulating “parent” compounds) spectral weight appears in the optical spectrum at photon energies below the charge-transfer gap. This spectral weight increases as the doping level increases. Magnetic penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-doped part of the phase diagram. Optical measurements allow independent determination of the total doping-induced spectral weight and the superfluid density. These measurements, made on cuprates with transition temperatures from 40 to 110 K, find that in optimally doped materials only about 20% of the doping-induced spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials, the superfluid fraction is even smaller. This result implies extremely strong coupling for these superconductors.

Motifs of periodic modulations are encountered in a variety of natural systems, where at least two rival states are present. In strongly correlated electron systems such behaviour has typically been associated with competition between short- and long-range interactions, e.g., between exchange and dipole-dipole interactions in the case of ferromagnetic thin films. Here we show that spin-stripe textures may develop also in antiferromagnets, where long-range dipole-dipole magnetic interactions are absent. A comprehensive analysis of magnetic susceptibility, high-field magnetization, specific heat, and neutron diffraction measurements unveils $\beta$-TeVO$_4$ as a nearly perfect realization of a frustrated (zigzag) ferromagnetic spin-1/2 chain. Strikingly, a narrow spin stripe phase develops at elevated magnetic fields due to weak frustrated short-range interchain exchange interactions possibly assisted by the symmetry allowed electric polarization. This concept provides an alternative route for the stripe formation in strongly correlated electron systems and may help understanding other widespread, yet still elusive, stripe-related phenomena., Comment: accapted in Nature Communications

Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in ordered spin structures, and spinons, their fractional counterparts that emerge in disordered, yet correlated spin states. Here we report on the discovery of elementary excitation inherent to spin-stripe order that represents a bound state of two phason quasiparticles, resulting in a wiggling-like motion of the magnetic moments. We observe these excitations, which we dub "wigglons", in the frustrated zigzag spin-1/2 chain compound $\beta$-TeVO$_4$, where they give rise to unusual low-frequency spin dynamics in the spin-stripe phase. This result provides insights into the stripe physics of strongly-correlated electron systems., Comment: published in an open access journal

International audience; We 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.

We have measured the pressure and temperature dependences of the resistivity and the thermoelectric power of anatase, TiO2. The resistivity varies with T-3 at high temperatures, and its absolute value is in the 1 Omega cm range. Below 60 K, the resistivity is activated. Most surprisingly, the activation energy shows non-monotonic pressure dependence. The thermoelectric power has a very high value and its temperature dependence resembles that of polaronic materials. We suggest a large polaronic model to describe the temperature and pressure dependence of the two transport coefficients. Copyright (c) EPLA, 2012

We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as well as the related system 2H-NbS2, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations., Comment: 4 pages, 3 figures