T. Klimczuk

Superconductivity in CuxTiSe2

Charge density waves (CDWs) are periodic modulations of the density of conduction electrons in solids. They are collective states that arise from intrinsic instabilities often present in low-dimensional electronic systems. The most well-studied examples are the layered dichalcogenides–an example of which is TiSe2, one of the first CDW-bearing materials to be discovered. At low temperatures, a widely held belief is that the CDW competes with another collective electronic state, superconductivity. But despite much exploration, a detailed study of this competition is lacking. Here we report how, on controlled intercalation of TiSe2 with Cu to yield CuxTiSe2, the CDW transition can be continuously suppressed, and a new superconducting state emerges near x=0.04, with a maximum transition temperature Tc of 4.15u2009K at x=0.08. CuxTiSe2 thus provides the first opportunity to study the CDW to superconductivity transition in detail through an easily controllable chemical parameter, and will provide fundamental insight into the behaviour of correlated electron systems.

Superconductivity in Cu_xTiSe_2

Charge density waves (CDWs) are periodic modulations of the density of conduction electrons in solids. They are collective states that arise from intrinsic instabilities often present in low-dimensional electronic systems. The most well-studied examples are the layered dichalcogenides–an example of which is TiSe2, one of the first CDW-bearing materials to be discovered. At low temperatures, a widely held belief is that the CDW competes with another collective electronic state, superconductivity. But despite much exploration, a detailed study of this competition is lacking. Here we report how, on controlled intercalation of TiSe2 with Cu to yield CuxTiSe2, the CDW transition can be continuously suppressed, and a new superconducting state emerges near x=0.04, with a maximum transition temperature Tc of 4.15u2009K at x=0.08. CuxTiSe2 thus provides the first opportunity to study the CDW to superconductivity transition in detail through an easily controllable chemical parameter, and will provide fundamental insight into the behaviour of correlated electron systems.

Superconductivity phase diagram of Na(x)CoO2*1.3H2O.

The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity near 4u2009K in the layered sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO2·1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

Pressure-induced superconductivity in?CaFe2As2

We report pressure-induced superconductivity in a single crystal of CaFe2As2. At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature Tc exceeding 10 K. The rate of Tc suppression with applied magnetic field is -0.7 K/T, giving an extrapolated zero-temperature upper critical field of 10-14T.

Synthesis and properties of CaFe2As2 single crystals

We report the synthesis and basic physical properties of single crystals of CaFe2As2, a compound isostructural to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystallizes in the ThCr2Si2 structure with lattice parameters a = 3.887(4) A and c = 11.758(23) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T0 = 171 K. The magnetic susceptibility is nearly isotropic from 2 to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 ± 0.3 mJ mol−1 K−2, and does not reveal any evidence for the presence of high frequency (>300 K) optical phonon modes. The Hall coefficient is negative below the transition, indicating dominant n-type carriers.

Magnetism and structure of LixCoO2 and comparison to NaxCoO2

Received 19 October 2007; revised manuscript received 19 January 2008; published 19 February 2008 The magnetic properties and structure of LixCoO2 for 0.5x1.0 are reported. Co 4+ is found to be high spin in LixCoO2 for 0.94x1.00 and low spin for 0.50x0.78. Weak antiferromagnetic coupling is observed, and at x0.65 the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. Comparison of the structures of NaxCoO2 and LixCoO2 reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter, and that the CoO6 octahedra in LixCoO2 are less distorted.

Superconductivity in the Heusler Family of Intermetallics

Przeprowadzono badania szeregu związkow nadprzewodzących z klasy związkow Heuslera, w szczegolności rodziny (Sc, Y, Lu)Pd2Sn i APd2M (A= Hf, Zr, i M = In, Al). Zwrocono uwage na istotny wplyw sprzezenia elektron - fonon na obserwowane nadprzewodnictwo.

The first order phase transition and superconductivity in BaNi2As2 single crystals

We report the synthesis and physical properties of single crystals of stoichiometric BaNi2As2 that crystalizes in the ThCr2Si2 structure with lattice parameters a = 4.112(4) AA and c = 11.54(2) AA. Resistivity and heat capacity show a first order phase transition at T_0 = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4x10^{-10} Omega-cm/Oe. Resistivity, ac-susceptibility, and heat capacity find evidence for bulk superconductivity at T_c = 0.7 K. The Sommerfeld coefficient at T_c is 11.6 pm 0.9 mJ/molK^2. The upper critical field is anisotropic with initial slopes of dH_{c2}^{c}/dT = -0.19 T/K and dH_{c2}^{ab}/dT = -0.40 T/K, as determined by resistivity.

Synthesis and Properties of CaFe

We report the synthesis and basic physical properties of single crystals of CaFe2As2, a compound isostructural to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystallizes in the ThCr2Si2 structure with lattice parameters a = 3.887(4) A and c = 11.758(23) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T0 = 171 K. The magnetic susceptibility is nearly isotropic from 2 to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 ± 0.3 mJ mol−1 K−2, and does not reveal any evidence for the presence of high frequency (>300 K) optical phonon modes. The Hall coefficient is negative below the transition, indicating dominant n-type carriers.

_2

We review the properties of Ni-based superconductors which contain Ni2X2 (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in Tc in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.