All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

We have measured and analyzed the dielectric constant of the dimer Mott insulator κ−(BEDT-TTF)2Cu2(CN)3, which is known as a playground for a spin-liquid state. Most unexpectedly, this particular organic salt has nontrivial charge degrees of freedom, being characterized by a relaxor-like dielectric relaxation below around 60 K. This is ascribed to the charge disproportionation within the dimer due to the intersite Coulomb repulsion. A possible microscopic model is suggested and discussed.

T he metallic state of high-temperature copper-oxide superconductors, characterized by unusual and distinct temperature dependences in the transport properties 1-4 , is markedly different from that of textbook metals. Despite intense theoretical efforts 5-11 , our limited understanding is impaired by our inability to determine experimentally the temperature and momentum dependence of the transport scattering rate. Here, we use a powerful magnetotransport probe to show that the resistivity and the Hall coefficient in highly doped Tl 2 Ba 2 CuO 6+δ originate from two distinct inelastic scattering channels. One channel is due to conventional electronelectron scattering; the other is highly anisotropic, has the same symmetry as the superconducting gap and a magnitude that grows approximately linearly with temperature. The observed form and anisotropy place tight constraints on theories of the metallic state. Moreover, in heavily doped non-superconducting La 2−x Sr x CuO 4 , this anisotropic scattering term is absent 12 , suggesting an intimate connection between the origin of this scattering and superconductivity itself.The in-plane properties of layered metals can sometimes be obtained from measurements of out-of-plane quantities. For example, angular magnetoresistance oscillations (AMRO), which are angular variations in the interlayer resistivity ρ ⊥ induced by rotating a magnetic field H in a polar plane relative to the conducting layers, can provide detailed information on the shape of the in-plane Fermi surface (FS) in layered metals. Here we resolve for the first time, the momentum (k) and energy (ω or T) dependence of the in-plane transport lifetime τ in an overdoped cuprate Tl 2 Ba 2 (Ca 0 )Cu 1 O 6+δ (Tl2201) through advances, both experimental and theoretical, in the AMRO technique. Experimentally, we extend the temperature range of previous AMRO measurements on overdoped Tl2201 13 (with a superconducting transition temperature T c = 15 K) by more than one order of magnitude. Theoretically, we derive a new general analytical expression for the interlayer conductivity σ ⊥ in a tilted H that incorporates basal-plane anisotropy. For T > 4 K, the AMRO can only be explained by inclusion of an anisotropic scattering rate 1/τ whose anisotropy grows with T. Significantly, the anisotropy in 1/τ and its T dependence up to 55 K can quantitatively account for both the robust linear-in-T component to the in-plane resistivity ρ ab and the T-dependent Hall coefficient R H over the same temperature range 14,15 . These anomalous behaviours are not characteristic of a simple Fermi liquid, which is often the starting point for modelling overdoped cuprates. We discuss the consequences of these findings for our understanding of the normal-state transport in cuprates.As described in the Supplementary Information, detailed azimuthal and polar-angle-dependent AMRO data were taken at 4.2 K and 45 T and fitted to the Shockley-Chambers tube integral form of the Boltzmann transport equation, modified for a quasitwo-dimensional (quasi...

Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl2Ba2CuO 6+δ for a range of Tc values between 15K and 35K. The size of the anisotropic scattering term is found to scale linearly with Tc, establishing a link between the superconducting and normal state physics. Comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.PACS numbers: 74.72. Jt, Understanding the normal state is regarded as a key step in resolving the problem of high temperature superconductivity (HTSC) yet establishing any clear correlation between the two has proved difficult. Most empirical correlations to date, such as the Uemura plot [1] and the linear scaling of the magnetic resonance mode energy with T c [2], are associated with the superconductivity. Homes' law, linking T c with the product of the superfluid density and the dc conductivity (at T c ) [3], is a rare example of a correlation linking the two states, though this also could be viewed as a consequence of superconducting (SC) gap formation rather than a normal state property [4]. Transport properties, particularly for current in-plane, appear very much tied to the T c parabola (for a review see [4]) but any direct correlation between transport and superconductivity has not yet been found. Finally, the precise doping dependence of the pseudogap and its relation to T c remains a controversy [5].Here we identify a new correlation between the normal and superconducting states using a bulk transport probe, namely interlayer angle-dependent magnetoresistance (ADMR).[6] ADMR have provided detailed Fermi surface (FS) information for a variety of one-and twodimensional (2D) metals [7,8,9]. Recently the technique was extended to incorporate basal-plane anisotropy and to reveal the temperature T -and momentum (k-) dependence of the scattering rate Γ(T ,k) in heavily overdoped (OD) Tl 2 Ba 2 CuO 6+δ (Tl2201) [10]. There Γ(T ,k) was found to consist of two components, one isotropic and quadratic in T , the other anisotropic, maximal near the saddle points at (π, 0) and proportional to T .In this Letter, ADMR measurements at T = 40K and magnetic field µ 0 H = 45Tesla are compared for a number of OD Tl2201 crystals with T c values between 15K and 35K. The strength of the anisotropic scattering extracted from the analysis is found to scale linearly with T c , appearing to extrapolate to zero at the doping level where superconductivity vanishes. This finding implies that the anisotropic scattering mechanism is intimately related to the mechanism of HTSC. In marked contrast to recent results from angle resolved photoemission spectroscopy (ARPES) [11,12], no sign reversal of the anisotropy in the quasiparticle lifetime is inferred. Finally our results shed new light on the doping evolution of both ρ ab (T ) and R H (T ) in OD cuprates.For this study a total of six tetragonal self-flux grown crystals (typical dime...