arXiv subjects

H. Berger

Publications and source records attributed to H. Berger.

At least 163 records · Page 9Linked to original sources

arXiv2006

Electric and magnetic characterization of NbSe2 single crystals: anisotropic superconducting fluctuations above Tc

Electric and magnetic characterization of NbSe2 single crystals is first presented in detail. Then, some preliminary measurements of the fluctuation-diamagnetism (FD) above the transition temperature Tc are presented. The moderate uniaxial anisotropy of this compound allowed us to observe the fluctuation effects for magnetic fields H applied in the two main crystallographic orientations. The superconducting parameters resulting from the characterization suggest that it is possible to do a reliable analysis of the FD in terms of the Ginzburg-Landau (GL) theory.

cond-mat.supr-con↗

arXiv2006

X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2

Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cluster calculation we are able to interpret the Co L_{23}-edge absorption spectrum and find a doping dependent energy splitting between the t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.

cond-mat.str-el↗

arXiv2006

Doping Dependence of the Redistribution of Optical Spectral Weight in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

We present the ab-plane optical conductivity of four single crystals of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) with different carrier doping levels from the strongly underdoped to the strongly overdoped range with $T_c$=66, 88, 77, and 67 K respectively. We focus on the redistribution of the low frequency optical spectral weight (SW) in the superconducting and normal states. The temperature dependence of the low-frequency spectral weight in the normal state is significantly stronger in the overdoped regime. In agreement with other studies, the superconducting order is marked by an increase of the low frequency SW for low doping, while the SW decreases for the highly overdoped sample. The effect crosses through zero at a doping concentration $δ$=0.19 which is slightly to the right of the maximum of the superconducting dome. This sign change is not reproduced by the BCS model calculations, assuming the electron-momentum dispersion known from published ARPES data. Recent Cluster Dynamical Mean Field Theory (CDMFT) calculations based on the Hubbard and t-J models, agree in several relevant respects with the experimental data.

cond-mat.str-el↗

arXiv2006

Unadulterated spectral function of low energy quasiparticles: Bi-2212, nodal direction

Fitting the momentum distribution photoemission spectra to the Voigt profile appears to be a robust procedure to purify the interaction effects from the experimental resolution. In application to Bi-2212 high-Tc cuprates, the procedure reveals the true scattering rate at low binding energies and temperatures, and, consequently, the true value of the elastic scattering. Reaching the minimal value ~ 16 meV, the elastic scattering does not reveal a systematic dependence on doping level, but is rather sensitive to impurity concentration, and can be explained by the forward scattering on out-of-plane impurities. The inelastic scattering is found to form well-defined quasiparticles with the quadratic and cubic energy dependence of the scattering rate above and below Tc, respectively. The observed energy-temperature asymmetry of the scattering rate is also discussed.

cond-mat.str-el↗

arXiv2006

A reevaluation of the coupling to a bosonic mode of the charge carriers in (Bi,Pb)$_2$Sr$_2$CaCu$_2$O$_{8+δ} $ at the antinodal point

Angle-resolved photoemission spectroscopy (ARPES) is used to study the spectral function of the optimally doped high-T$_c$ superconductor (Bi,Pb)$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ in the vicinity of the antinodal point in the superconducting state. Using a parameterized self-energy function, it was possible to describe both the coherent and the incoherent spectral weight of the bonding and the antibonding band. The renormalization effects can be assigned to a very strong coupling to the magnetic resonance mode and at higher energies to a bandwidth renormalization by a factor of two, probably caused by a coupling to a continuum. The present reevaluation of the ARPES data allows to come to a more reliable determination of the value of the coupling strength of the charge carriers to the mode. The experimental results for the dressing of the charge carriers are compared to theoretical models.

cond-mat.supr-con↗

arXiv2006

Three-dimensional band structure of layered TiTe2: Photoemission final-state effects

Three-dimensional band structure of unoccupied and occupied states of the prototype layered material TiTe2 is determined focusing on the GammaA line of the Brillouin zone. Dispersions and lifetimes of the unoccupied states, acting as the final states in the photoemission process, are determined from a very-low-energy electron diffraction experiment supported by first-principles calculations based on a Bloch waves treatment of multiple scattering. The experimental unoccupied states of TiTe2 feature dramatic non-free-electron effects such as multiband composition and non-parabolic dispersions. The valence band layer-perpendicular dispersions are then determined from a photoemission experiment consistently interpreted on the basis of the experimental final states to achieve control over the 3-dimensional wavevector. The experimental results demonstrate the absence of the Te 4pz* Fermi surface pocket at the Gamma point and significant self-energy renormalization of the valence band dispersions. Photoemission calculations based on a novel Bloch waves formalism within the one-step theory reveal limitations of understanding photoemission from layered materials such as TiTe2 in terms of direct transitions.

cond-mat.str-el↗

arXiv2006

Separation of orbital contributions to the optical conductivity of BaVS$_3$

The correlation-driven metal-insulator transition (MIT) of BaVS$_3$ was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: The quasi 1D metallic transport of $A_{1g}$ electrons is superimposed on the isotropic hopping conduction of localized $E_g$ electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass $m_{eff}\approx7m_e$ and scattering rate $Γ\geq160$ meV of the quasi-particles in the $A_{1g}$ band. There is a pseudo-gap above $T_{MI}=69$ K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with $Δ_{ch}\approx42$ meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.

cond-mat.str-el↗

arXiv2006

Quantum critical phase in BaVS$_3$

We study the high-pressure metallic phase of high-purity single crystals of BaVS$_3$ by measuring the temperature, pressure, and magnetic field dependence of the resistivity. Above the critical pressure of $p_{\rm cr}=1.97$GPa an extended non-Fermi liquid $p-T$ regime emerges with resistivity exponent $1.5 \le n<2$, crossing over to a FL only around $p=2.7$GPa. A hysteretic feature indicates that close to the insulator--metal boundary, the system is magnetically ordered. Our findings reveal a close analogy between the extended partially ordered NFL state of non-conventional itinerant magnets and the corresponding state of BaVS$_3

cond-mat.str-el↗

arXiv2006

Experimental proof of a structural origin for the shadow Fermi surface in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ with structural information from LEED and {\it ab initio} one-step photoemission simulations, we prove that the shadow Fermi surface in these systems has structural origin, being due to orthorhombic distortions from tetragonal symmetry present in both surface and bulk. Consequently, one of the longest standing open issues in the fermiology of these widely studied systems finally meets its resolution.

cond-mat.supr-con↗

arXiv2006

Spin-lattice interaction in the quasi one-dimensional helimagnet LiCu_2O_2

The field dependence of the electron spin resonance in a helimagnet, LiCu_2O_2, was investigated for the first time. In the paramagnetic state a broad resonance line was observed corresponding to a g-factor of 2.3. In the critical regime around the paramagnetic to helimagnetic phase transition the resonance broadens and shifts to higher frequencies. A narrow signal is recovered at low temperature, corresponding to a spin gap of 1.5 meV in zero field. A comprehensive model of the magnons is presented, using exchange parameters from neutron scattering (T. Masuda et al, Phys. Rev. B 72, 014405), and the spin anisotropy determined here. The role of the quantum fluctuations is discussed.

cond-mat.str-el↗

arXiv2006

Band Mapping in One-Step Photoemission Theory: Multi-Bloch-Wave Structure of Final States and Interference Effects

A novel Bloch-waves based one-step theory of photoemission is developed within the augmented plane wave formalism. Implications of multi-Bloch-wave structure of photoelectron final states for band mapping are established. Interference between Bloch components of initial and final states leads to prominent spectral features with characteristic frequency dispersion experimentally observed in VSe_2 and TiTe_2. Interference effects together with a non-free-electron nature of final states strongly limit the applicability of the common direct transitions band mapping approach, making the tool of one-step analysis indispensable.

cond-mat.mtrl-sci↗

arXiv2006

The effect of externally applied pressure on the magnetic behavior of Cu2Te2O5(BrxCl{1-x})2

The effect of externally applied pressure on the magnetic behavior of Cu2Te2O5(BrxCl{1-x})2 with x = 0, 0.73 and 1, is investigated by a combination of magnetic susceptibility, neutron diffraction and neutron inelastic scattering measurements. The magnetic transition temperatures of the x = 0 and 0.73 compositions are observed to increase linearly with increasing pressure at a rate of 0.23(2) K/kbar and 0.04(1) K/kbar respectively. However, the bromide shows contrasting behavior with a large suppression of the transition temperature under pressure, at a rate of -0.95(9) K/kbar. In neutron inelastic scattering measurements of Cu2Te2O5Br2 under pressure only a small change to the ambient pressure magnetic excitations were observed. A peak in the density of states was seen to shift from ~5 meV in ambient pressure to ~6 meV under an applied pressure of 11.3 kbar, which was associated with an increase in the overall magnetic coupling strength.

cond-mat.mtrl-sci↗

arXiv2005

Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl, Br) studied by single crystal neutron diffraction

Polarized and unpolarized neutron diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k'Cl ~ -0.150,0.422,1/2 and k'Br ~ -0.172,0.356,1/2 and stable below TN=18 K for X=Cl and TN=11 K for X=Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the 4 Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+magnetic moment determined for X=Br, 0.395(5)muB, is significantly less than for X=Cl, 0.88(1)muB at 2K. The magnetic structure of the chloride associated with the wave-vector k' differs from that determined previously for the wave vector k~0.150,0.422,1/2 [O. Zaharko et.al. Phys. Rev. Lett. 93, 217206 (2004)].

cond-mat.mtrl-sci↗

arXiv2005

Probing the superfluid velocity with a superconducting tip: the Doppler shift effect

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing Scanning Tunneling Spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals both the vortex cores, on the scale of the SC coherence length $ξ$, and the supercurrents, on the scale of the London penetration length $λ$. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.

cond-mat.supr-con↗

arXiv2005

Stripe correlations in Na$_{0.75}$CoO$_2$

We unambiguously demonstrate, based on high-energy x-ray diffraction data and LDA calculations, that sodium-density stripes are formed in Na0.75CoO2 at low temperatures and rule out the previously proposed Na-ordering models. The LDA calculations prove, that the sodium-density stripes lead to a sizeable dip in the density of the Co-states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripe-like charge correlations within the CoO2 layers, leading to an astonishing similarity between the doped cuprates and the NaxCoO2 compounds.

cond-mat.supr-con↗

arXiv2005

Constituents of the "kink" in high-Tc cuprates

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in details the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below T* and sharpening below Tc, and is the main responsible for the formation of the kink in question.

cond-mat.supr-con↗

arXiv2005

Optical evidence for a magnetically driven structural transition in the spin web $Cu_3TeO_6$

$Cu_3TeO_6$ is a modest frustrated $S=1/2$ spin system, which undergoes an anti-ferromagnetic transition at $T_N\sim61$ $K$. The anti-ferromagnetic spin alignment in $Cu_3TeO_6$ below $T_N$ is supposed to induce a magneto-elastic strain of the lattice. The complete absorption spectrum of $Cu_3TeO_6$ is obtained through Kramers-Kronig transformation of the optical reflectivity, measured from the far-infrared up to the ultraviolet spectral range as a function of temperature ($T$). Below $T^*\sim 50$ $K$, we find a new mode at 208 $cm^{-1}$. The spectral weight associated to this additional mode increases as $\propto (T^*-T)^{1/2}$ with decreasing $T$ below $T^*$. The implication of the optical findings will be discussed in relation to the magnetic phase transition at $T_N$.

cond-mat.str-el↗

arXiv2005

Life of the nodal quasiparticles in Bi-2212 as seen by ARPES

While the pronounced doping dependence of the quasiparticle spectral weight in the antinodal region of the superconducting cuprates, as seen by ARPES, unambiguously points to the magnetic origin of the strong electron-boson coupling there, the nature of the electron scattering in the nodal direction remained unclear. Here we present a short review of our recent detailed investigations of the nodal direction of Bi-2212. Our findings prove the existence of well defined quasiparticles even in the pseudogap state and show that the essential part of the quasiparticle scattering rate, which appears on top of Auger-like electron-electron interaction, also implies a magnetic origin.

cond-mat.supr-con↗