Quasi Two-dimensional Metal Research Articles

Unconventional magneto-transport in ultrapure PdCoO2 and PtCoO2

The single-band, quasi-two dimensional metals PdCoO2 and PtCoO2 have recently come to prominence because of their extremely long mean free paths, which establish them as some of the most electronically pure materials known, and as potential hosts of previously unobservable regimes of electronic transport. To fully establish their magnetotransport properties, we have studied the magnetoresistance and Hall effect in bulk single crystals to which electrical contacts have been made with high precision using focused ion beam machining. We observe a strong temperature dependence of the Hall resistivity in small applied fields, linked to a large violation of Kohler’s rule in the magnetoresistance. We discuss the extent to which these observations can be accounted for by standard transport theory.

Metal-insulator transition in the dimerized organic conductor κ−(BEDT-TTF)2Hg(SCN)2Br

The organic charge-transfer salt $\kappa$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br is a quasi two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80$ K it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $\kappa$-(BEDT-TTF) salts, in particular the Cl-analogue, where a charge-order transition takes place at $T_{\rm CO}=30$ K.

Non-Lifshitz–Kosevich field- and temperature-dependent amplitude of quantum oscillations in the quasi-two dimensional metal θ-(ET)4ZnBr4(C6H4Cl2)

According to band structure calculations, the Fermi surface of the quasi-two dimensional metal -(ET)4ZnBr4(C6H4Cl2) illustrates the linear chain of coupled orbits model. Accordingly, de Haas–van Alphen oscillations spectra recorded in pulsed magnetic field of up to 55 T evidence many Fourier components, the frequency of which are linear combinations of the frequencies relevant to the closed and the magnetic breakdown orbits. The field and temperature dependence of their amplitude are quantitatively accounted for by analytic calculations including, beyond the Lifshitz–Kosevich formula, second-order terms in damping factors due to the oscillation of the chemical potential as the magnetic field varies. Whereas these second-order terms are negligible for the orbits , and , they are solely responsible for the ‘forbidden orbit’ and its harmonic and have a significant influence on Fourier components such as and , yielding strongly non-Lifshitz–Kosevich behaviour in the latter case.

Model of anisotropic scattering in a quasi-two-dimensional metal

A simple model of anisotropic scattering in a quasi two-dimensional metal is studied. Its simplicity allows an analytic calculation of transport properties using the Boltzmann equation and relaxation time approximation. We argue that the c-axis magnetoresistance provides the key test of this model of transport. We compare this model with experiments on overdoped Tl-2201 and find reasonable agreement using only weak scattering anisotropy. We argue that optimally doped Tl-2201 should show strong angular-dependent magnetoresistance within this model and would provide a robust way of determining the in-plane scattering anisotropy in the cuprates.

Ground state inSr3Ru2O7:Fermi liquid close to a ferromagnetic instability

We show that single-crystalline Sr3Ru2O7 grown by a floating-zone technique is an isotropic paramagnet and a quasi-two dimensional metal as spin-triplet superconducting Sr2RuO4 is. The ground state is Fermi liquid with very low residual resistivity (3 micro ohm cm for in-plane currents) and a nearly ferromagnetic metal with the largest Wilson ratio Rw>10 among paramagnets so far. This contrasts with the ferromagnetic order at Tc=104 K reported on single crystals grown by a flux method [Cao et al., Phys. Rev. B 55, R672 (1997)]. We have also found a dramatic changeover from paramagnetism to ferromagnetism under applied pressure. This suggests the existence of a substantial ferromagnetic instability on the verge of a quantum phase transition in the Fermi liquid state.

Structural Phase Transitions of the Quasi Two-Dimensional Metal (K1-xRbx)3Cu8S6: X-Ray Scattering Studies

A quasi two-dimensional metal K 3 Cu 8 S 6 has two phase transitions at T 1 (153 K) and at T 2 (55 K), accompanied by an incommensurate (between T 1 and T 2 ) and a commensurate (below T 2 ) superstructures, respectively. On the other hand, the resistivity of Rb 3 Cu 8 S 6 , which is isostructural with K 3 Cu 8 S 6 , suggests that Rb 3 Cu 8 S 6 has only the T 1 transition. In order to investigate the superstructures of Rb 3 Cu 8 S 6 , X-ray diffuse scattering studies were carried out. The results show that an incommensurate superstructure with a wave vector of (0 η 0), η≃0.45, appears between T 1 and the measuring lowest temperature (about 15 K). The temperature dependence of the intensity of the superstructure is that of an ordinary charge-density wave and there is no evidence for the T 2 transition in Rb 3 Cu 8 S 6 . The X-ray scattering results on the mixed crystals (K 1- x Rb x ) 3 Cu 8 S 6 are also reported.

Study on carriers and charge-density wave transitions of two-dimensional metals (K 1−xRb x) 3Cu 8S 6, (Rb 1−xCs x) 3Cu 8S 6, K 3Cu 8(S 1−xSe x) 6, and Rb 3Cu 8(S 1−xSe x) 6

A quasi two-dimensional metal K 3Cu 8S 6 undergoes two kinds of phase transitions at T 1 (153K) and at T 2 (55K), attended by an incommensurate (IC) and a commensurate superstructures, respectively. In order to investigate these phase transitions, the mixed crystals (K 1−xRb x) 3Cu 8S 6, K 3Cu 8(S 1−xSe x), and Rb 3Cu 8(S 1−xSe x) have been synthesized and their transport properties have been studied. Their resistivities show that the T 1 transition of K 3Cu 8S 6, assigned as onset transition of an IC charge-density wave (CDW), is slightly affected by the substitution of alkali atoms but strongly suppressed by the substitution of chalcogen atoms. Their thermoelectric powers suggest that the existence of n-type carriers is essential for the CDW transition. On the other hand, the T 2 transition of K 3Cu 8S 6, whose origin is not clear, is suppressed by Rb doping and by Se doping.

New organic superconductors K- and θ-(BEDT-TTF) 2I 3: Transport property

Transport properties of newly synthesized organic conductors K - and θ-(BEDT-TTF) 2I 3 are investigated. We find that these materials are quasi-two dimensional metals. Both materials exhibit ambient pressure superconductivity with T c around 3.6K. The upper critical magnetic field is determined as a function of temperature and the coherence length is estimated. We find that the coherence lengths for k -type in the two dimensional plane and normal to it are about 360A and 13.5A, respectively.

Photoemission Study of Quasi-Two Dimensional Metal; Ag2F

Ultraviolet (UPS) and X-ray (XPS) photoemission spectra of quasi-two dimensional metal Ag 2 F are presented. Three prominent bands are found at about 2.0 eV, 4.6 eV and 6.0 eV from the Fermi level in the UPS spectrum. They are assigned to the F 2 p state, the Ag 4 d state and the mixture of the two states, respectively. The comparison of the photoemission spectra with a recent LAPW band calculation shows that the calculated 4 d band is shifted toward the lower binding energy by about 1 eV relative to that of the experiment. This discrepancy suggests that the 16th band which forms a small ellipsoidal Fermi surface in the LAPW calculation may be above the Fermi level.

Charge density wave instability in the quasi two-dimensional metal γ-Mo 40 11

Transport and specific heat properties have been studied on the orthorhombic molybdenum oxide γ-Mo 40 11. The anisotropy of the electrical resistivity establishes that this compound is a quasi two-dimensional metal, as expected from crystal structure data. Both the resistivity and the thermopower show that an electronic transition, probably due to a charge density wave instability takes place at T c = 100 K. Low temperature specific heat data provide an estimation of the Debye temperature and of the electronic density of states in the low temperature metallic phase.

Structure cristalline d'un conducteur métallique bidimensionnel: Le bronze violet de potassium et molybdene K 0.9Mo 6O 17

Single crystals of the purple bronze K 0.9Mo 6O 17 have been grown by a electrolytic reduction technique. Measurements of the electrical resistivity show that this compound is a quasi two-dimensional metal. A crystal structure determination is used to explain the strongly anisotropic resistivity. K 0.9Mo 6O 17 is found to be trigonal with lattice parameters: a = 5.538 Å and c = 13.656 Å; Z = 1. The space group is P 3 . The K cations are surrounded by 12 oxygen anions which form icosahedral sites. One third of the Mo cations (Mo(1)) occupy tetrahedral sites, while the Mo(2) and Mo(3) cations have octahedral surroundings. The structure can be described as slabs of ReO 3-type connected by KO 12 icosahedra. Each slab is built up of 4 layers, parallel to the (001) plane, of Mo(2) and Mo(3) octahedra sharing corners. Two slabs are connected by K icosahedra which share 2 faces with the upper and lower neighboring Mo(3) octahedra. Each K icosahedron is edge linked to 3 upper and 3 lower Mo(1) tetrahedra. Each Mo(1) tetrahedron is itself corner linked to 3 Mo(3) octahedra of the neighboring layer. The deformations of the occupied polyhedra are a good illustration of the Pauling's third rule. There is no Mo(1)OMo(1) bonding, so that the MoOMo bonding, infinite in the a and b directions, is disrupted in the c direction. The Zachariasen's bond length-bond strength relation has been applied to the MoO bonds. The computed effective mean Mo valences are +6 for Mo(1) on tetrahedral sites and about 5.1 and 5.8, respectively, for Mo(2) and Mo(3) on octahedral sites. The 4 d electrons of Mo atoms are so located in the two-dimensional infinite slabs of octahedra. The conduction band is expected to be an antibonding π∗ band resulting from the hybridization of the Mo 4 d t 2 g and oxygen p π states. Thus, the structural properties should lead to a very anisotropic Fermi surface and thus, to a quasi two-dimensional electrical conductivity.

Dimensionality inbis-Tetramethylselenafulvalenium Hexafluorophosphate,(TMTSF)2PF6

The dimensionality of the spin dynamics in bis-tetramethylselenafulvalenium hexafluorophosphate ${(\mathrm{TMTSF})}_{2}$P${\mathrm{F}}_{6}$, is studied in the high-pressure metallic phase at low temperatures via the field dependence of proton relaxation rates. Two regimes are identified: a low-field regime (1-12 kOe) where the relaxation rate is independent of field and a highfield regime (12-120 kOe) where the relaxation rate depends logarithmically on field. The high-field behavior is interpreted as spin dynamics in a quasi two-dimensional metal with a crossover to three-dimensional spin dynamics at low field.