Scanning tunneling microscopy and spectroscopy (STM/S) measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well defined features at 0.97 meV and 0.53 meV, located respectively above and below the value for the superconducting gap expected from single band s-wave BCS model (∆=1.76kBTc=0.9 meV). Both features have a continuous temperature evolution and disappear at Tc = 5.7 K. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with the charge density wave order (CDW), absent in 2H-NbS2.PACS numbers: 71.45. Lr, 74.25.Jb,74.50.+r,74.70.Ad The study of the coexistence of superconductivity with competing physical phenomena such as magnetic or charge order has historically produced great interest on the scientific community. Anisotropies or modulations of the superconducting properties (in real and/or reciprocal space) often appear as a consequence of competing orders within the same system [1,2]. In the compound 2H-NbSe 2 , superconductivity appears within a CDW state (T CDW =33K and T c =7.2 K) [3]. Low lying excitations measured deep in the superconducting state long time ago by specific heat [4,5] have been explained by recent experiments and theoretical calculations with a multiband superconductivity and a peculiar anisotropy of the superconducting gap [6,7]. Recent angular resolved photoemission spectroscopy measurements demonstrate that the superconducting gap has, close to T c (at 5.7 K), largest values at k-space positions connected with CDW wavevectors [8,9]. Hess et al. [10,11,12] found that the local superconducting density of states (LDOS) at the center of the vortex core shows a high peak close to the Fermi level highlighting the lowest quasiparticle state bound within the vortex core well [13]. Around the vortex core, the LDOS is far from respecting in-plane symmetry and intriguing vortex lattice images with patterns showing strong in-plane LDOS modulations are obtained [10,11,12]. 2H-NbSe 2 belongs to the transition-metal dichalcogenides (2H-MX 2 with M = Ta, Nb and X = Se, S), a family of systems which is unique to study the interplay between CDW order and superconductivity. The 2H-MX 2 compounds share a double layered structure made of two hexagonal X sheets with an intercalated M sheet (X-M-X), connected through very weak van der Waals bonds [14]. This produces highly anisotropic, quasi two dimensional electronic properties. The features of the Fermi Surface (FS) expected to be common in all systems of the series are two concentric cylindrical FS sheets centered on both Γ and K points, derived from the transition-metal d bands [15,16,17,18]. When going over the series from 2H-TaSe 2 and 2H-TaS 2 to 2H-NbSe 2 and 2H-NbS 2 , the ratio of the intralayer lattice constant with the interlayer distance a/c increases, as well as T c , whereas T CDW decreas...

We report experimental evidences for the observation of the superconducting amplitude mode, socalled 'Higgs' mode in the charge density wave superconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its iso-structural partner 2H-NbS2 which shows superconductivity but lacks the charge density wave order, we demonstrate that the superconducting mode in 2H-NbSe2 owes its spectral weight to the presence of the coexisting charge density wave order. In addition temperature dependent measurements in 2H-NbSe2 show a full spectral weight transfer from the charge density wave mode to the superconducting mode upon entering the superconducting phase. Both observations are fully consistent with a superconducting amplitude mode or Higgs mode.PACS numbers: 74.70. Ad,71.45.Lr ,74.25.nd While the quest for the Higgs boson in particle physics is reaching its goal and its prediction has been rewarded by the Nobel prize, there is growing interest in the search for an analogous excitation in quantum many body systems where the Higgs boson manifests itself as a fundamental collective mode [1][2][3]. When a spontaneous breaking of a continuous symmetry takes place collective excitations of the order parameter emerge: they are the massless Nambu-Goldstone phase modes [4] and the massive amplitude Higgs mode [5]. In quantum many body systems, the Higgs mode was recently identified in ultra-cold 2D bosonic 87 Rb atoms in optical lattice [6] and reported in a the dimer antiferromagnet TlCuCl 3 [7]. Very recently, it has been unveiled in the superconducting Nb 1−x Ti x N films by using terahertz pump probe spectroscopy [8]. The existence of a Higgs mode was proposed more than thirty years ago in the bulk charge density wave (CDW) superconductor (SC) 2H-NbSe 2 [9,10], where a superconducting amplitude mode, or Higgs mode, can be unraveled via its coupling to the coexisting charge density wave mode. Despite this prediction after the first experimental observation by Raman scattering [11,12], unambiguous proofs of its Higgs type nature have remained elusive up to now [13][14][15].As it does not carry any spin or charge, in principle, the amplitude mode of the superconducting order parameter, or the Higgs mode, does not couple directly to any external probe. However, when superconductivity coexists with a charge density wave order, the amplitude mode of the CDW order couples to the Higgs mode by modulating the density of states at the Fermi level, thus "shaking" the SC condensate by modulating the amplitude of the superconducting order parameter. This allows the indirect detection of the 'Higgs' mode by spectroscopic probes [9,10]. Experimentally, the Higgs mode becomes active by removing spectral weight from the CDW amplitude mode upon entering the SC state. The requisite of a coexisting CDW mode and the observation of a transfer of spectral weight from the CDW amplitude mode to the Higgs mode in the SC state can thus be considered as key predictions of the Higgs mode scenario.Raman inelastic light scattering experiments al...

We report on specific heat ͑C p ͒, transport, Hall probe, and penetration depth measurements performed on Fe͑Se 0.5 Te 0.5 ͒ single crystals ͑T c ϳ 14 K͒. The thermodynamic upper critical field H c2 lines has been deduced from C p measurements up to 28 T for both H ʈ c and H ʈ ab, and compared to the lines deduced from transport measurements ͑up to 55 T in pulsed magnetic fields͒. We show that this thermodynamic H c2 line presents a very strong downward curvature for T → T c which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that H c2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T / T c ϳ 0.99 ͑for H ʈ ab͒ which is the consequence of a very small value of the coherence length c ͑0͒ϳ4 Å ͓and ab ͑0͒ϳ15 Å͔, confirming the strong renormalization of the effective mass ͑as compared to DMFT calculations͒ previously observed in ARPES measurements ͓A. Phys. Rev. Lett. 104, 097002 ͑2010͔͒. H c1 measurements lead to ab ͑0͒ = 430Ϯ 50 nm and c ͑0͒ = 1600Ϯ 200 nm and the corresponding anisotropy is approximatively temperature independent ͑ϳ4͒, being close to the anisotropy of H c2 for T → T c . The temperature dependence of both ͑ϰT 2 ͒ and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.

The pressure and temperature dependence of the phonon dispersion of 2H-NbSe2 is measured by inelastic X-ray scattering. A strong temperature dependent soft phonon mode, reminiscent of the Charge Density Wave (CDW), is found to persist up to a pressure as high as 16 GPa, far above the critical pressure at which the CDW disappears at 0 K. By using ab initio calculations beyond the harmonic approximation, we obtain an accurate, quantitative, description of the (P,T) dependence of the phonon spectrum. Our results show that the rapid destruction of the CDW under pressure is related to the zero mode vibrations -or quantum fluctuations -of the lattice renormalized by the anharmonic part of the lattice potential. The calculations also show that the low-energy longitudinal acoustic mode that drives the CDW transition barely contributes to superconductivity, explaining the insensitivity of the superconducting critical temperature to the CDW transition.The interplay between charge density wave order, i.e. a static modulation of the electronic density close to the Fermi level, and superconductivity has attracted much attention 1,2 . This is the central issue of a long standing debate in simple transition metal dichalcogenides without strong electronic correlations, such as 2H-NbSe 2 3,4 . At T CDW =33.5 K, 2H-NbSe 2 undergoes a second order phase transition towards an incommensurate CDW phase which coexists with superconductivity below T c = 7.2 K. The anisotropy and temperature dependence of the electronic band structure has been widely studied by ARPES and STM measurements 5-11 . The CDW is coupled to a periodic lattice distortion through a strong electronphonon coupling. The transition is associated with a softening of a longitudinal acoustic phonon mode as temperature is lowered to T CDW 12 . It was already noticed that the high order phonon fluctuations and strong electronphonon interactions explain some of the key features of the formation of the CDW in this system 13,14 . In the specific case of NbSe 2 at ambient pressure, the reduction of the phonon lifetime through the electron-phonon coupling is a key ingredient on the temperature dependence of the phonon dispersion 15 .At ambient pressure, published ab initio calculations reproduce successfully the lattice instability on a wide part of the Brillouin zone around the experimentally observed q CDW 12,16,17 . However, these calculations are carried out at the harmonic level, and therefore at T=0K. The temperature dependence can be qualitatively reproduced using a Gaussian smearing of the Fermi surface which reduces the contribution of the electron-phonon interaction. Quantitatively however, the temperatures that would effectively yield this smearing are unphysical, as they exceed the experimentally observed one by several orders of magnitude.The quantitative failure of the harmonic ab-initio calculations is even worse under pressure. Experimentally, by applying an hydrostatic pressure, T CDW is driven to 0 K and the CDW instability disappears above the critical press...

We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths (lambda(a) and lambda(c), respectively) in 2H-NbSe2. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast with some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.

The temperature dependence of the phonon spectrum in the superconducting transition-metal dichalcogenide 2H-NbS 2 is measured by diffuse and inelastic x-ray scattering. A deep, wide, and strongly temperature-dependent softening of the two lowest-energy longitudinal phonon bands appears along the M symmetry line in reciprocal space. In sharp contrast to the isoelectronic compound 2H-NbSe 2 , the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab initio calculations. We show that 2H-NbS 2 is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.

In 1957, Abrikosov described how quanta of magnetic flux enter the interior of a bulk type II superconductor. It was subsequently predicted that, in an isotropic superconductor, the repulsive forces between the flux lines would cause them to order in two dimensions, forming a hexagonal lattice. Flux-line lattices with different geometry can also be found in conventional (type II) superconductors; however, the ideal hexagonal lattice structure should always occur when the magnetic field is applied along a hexagonal crystal direction. Here we report measurements of the orientation of the flux-line lattice in the heavy-fermion superconductor UPt3, for this special case. As the temperature is increased, the hexagonal lattice, which is initially aligned along the crystal symmetry directions, realigns itself with the anisotropic superconducting gap. The superconductivity in UPt3 is unusual (even compared to unconventional oxide superconductors) because the superconducting gap has a lower rotational symmetry than the crystal structure. This special feature enables our data to demonstrate clearly the link between the microscopic symmetry of the superconductivity and the mesoscopic physics of the flux-line lattice. Moreover, our observations provide a stringent test of the theoretical description of the unconventional superconductivity in UPt3.

We present measurements of the superconducting and charge-density-wave (CDW) critical temperatures (T c and T CDW ) as a function of pressure in the transition metal dichalchogenides 2H -TaSe 2 and 2H -TaS 2 . Resistance and susceptibility measurements show that T c increases from temperatures below 1 K up to 8.5 K at 9.5 GPa in 2H -TaS 2 and 8.2 K at 23 GPa in 2H -TaSe 2 . We observe a kink in the pressure dependence of T CDW at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate T CDW slowly decreases, coexisting with superconductivity within our full pressure range.