Searcharxiv⌕ Search

arXiv subjects

H. Berger

Publications and source records attributed to H. Berger.

At least 199 records · Page 11Linked to original sources
arXiv2003

Pressure induced suppression of the singlet insulator phase in BaVS$_3$: infrared optical study

The metal-insulator (MI) transition in BaVS$_3$ has been studied at ambient pressure and under hydrostatic pressure up to $p=26 $kbar in the frequency range of $20-3000 $cm$^{-1}$. The charge gap determined from the optical reflectivity is enhanced, $Δ_{ch}(p)/k_BT_{MI}(p)\sim 12$. This ratio is independent of pressure indicating that the character of the transition does not vary along the $p-T$ phase boundary. Above the critical pressure, $p_{cr}\sim20$ kbar, metallic spectra were recorded in the whole temperature range, as expected from the shape of the phase diagram. Our results exclude the opening of a pseudogap above $T_{MI}$ at any pressure. Below $T_{MI}$ an unusually strong temperature dependence of the charge gap was observed, resulting in a $Δ_{ch}(T)$ deviating strongly from the mean field-like variation of the structural order parameter.

cond-mat.str-el↗
arXiv2003

Circular dichroism in angle-resolved photoemission spectra of under- and overdoped Pb-Bi2212

We use angle-resolved photoemission with circularly polarized excitation to demonstrate that in the 5x1 superstructure-free Pb-Bi2212 material there are no signatures of time-reversal symmetry breaking in the sense of the criteria developed earlier (Kaminski et al. Nature {\bf 416}, 610 (2002)). In addition to the existing technique, we suggest and apply an independent experimental approach to prove the absence of the effect in the studied compounds. The dichroic signal retains reflection antisymmetry as a function of temperature and doping and in all mirror planes, precisely defined by the experimental dispersion at low energies. The obtained results demonstrate that the signatures of time-reversal symmetry violation in pristine Bi2212, as determined by ARPES, are not a universal feature of all cuprate superconductors.

cond-mat.supr-con↗
arXiv2003

Unusual magnetic field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity κin the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c-axis, at several temperatures below T_c. With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H_c2, an unexpected hysteretic behavior of κ(H) with all the generic features of a first order phase transition. The transition is not manifest in the κ(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.

cond-mat.supr-con↗
arXiv2003

Doping dependence of the mass enhancement in (Pb,Bi)_2 Sr_2 Ca Cu_2 O_8 at the antinodal point in the superconducting and normal state

Angle-resolved photoemission spectroscopy (ARPES) is used to study the mass renormalization of the charge carriers in the high-T_c superconductor (Pb,Bi)_2Sr_2CaCu_2O_8 in the vicinity of the (pi,0) point in the superconducting and the normal state. Using matrix element effects at different photon energies and due to a high momentum and energy resolution the bonding and the antibonding bands could be separated in the whole dopant range. A huge anisotropic coupling to a bosonic collective mode is observed below T_c for both bands in particular for the underdoped case. Above T_c, the more isotropic coupling to a continuum or a mode at much higher energy is significantly weaker.

cond-mat.supr-con↗
arXiv2002

Fractional power-law susceptibility and specific heat in low temperature insulating state of o-TaS_{3}

Measurements of the magnetic susceptibility and its anisotropy in the quasi-one-dimensional system o-TaS_{3} in its low-T charge density wave (CDW) ground state are reported. Both sets of data reveal below 40 K an extra paramagnetic contribution obeying a power-law temperature dependence χ(T)=AT^{-0.7}. The fact that the extra term measured previously in specific heat in zero field, ascribed to low-energy CDW excitations, also follows a power law C_{LEE}(0,T)=CT^{0.3}, strongly revives the case of random exchange spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but do not change essentially the exponent. Within the two-level system (TLS) model, we estimate from the amplitudes A and C that there is one TLS with a spin s=1/2 localized on the chain at the lattice site per cca 900 Ta atoms. We discuss the possibility that it is the charge frozen within a soliton-network below the glass transition T_{g}~40 K determined recently in this system.

cond-mat↗
arXiv2002

What the resonance peak does

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the ($π$,0)-point of differently doped (Pb,Bi)$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. In contrast to overdoped samples, where intrinsic effects at the ($π$,0)point are virtually absent, we find in underdoped samples \textit{intrinsic} effects in the superconducting-state ($π$,0) spectra of the antibonding band. This intrinsic effect is present only below the critical temperature and weakens considerably with doping. Our results give strong support for models which involve a strong coupling of electronic excitations with the resonance mode seen in inelastic neutron scattering experiments.

cond-mat.supr-con↗
arXiv2002

Superconducting gap in the presence of bilayer splitting in underdoped Bi(Pb)2212

The clearly resolved bilayer splitting in ARPES spectra of the underdoped Pb-Bi2212 compound rises the question of how the bonding and antibonding sheets of the Fermi surface are gapped in the superconducting state. Here we compare the superconducting gaps for both split components and show that within the experimental uncertainties they are identical. By tuning the relative intensity of the bonding and antibonding bands using different excitation conditions we determine the precise {\bf k}-dependence of the leading edge gap. Significant deviations from the simple cos($k_{x}$)-cos($k_{y}$) gap function for the studied doping level are detected.

cond-mat.supr-con↗
arXiv2002

Observation of an unconventional metal-insulator transition in overdoped CuO_2 compounds

The electron dynamics in the normal state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ is studied by inelastic light scattering over a wide range of doping. A strong anisotropy of the electron relaxation is found which cannot be explained by single-particle properties alone. The results strongly indicate the presence of an unconventional quantum-critical metal-insulator transition where "hot" (antinodal) quasiparticles become insulating while "cold" (nodal) quasiparticles remain metallic. A phenomenology is developed which allows a quantitative understanding of the Raman results and provides a scenario which links single- and many-particle properties.

cond-mat.supr-con↗
arXiv2002

Phonon anomaly at the charge ordering transition in 1T-TaS2

The infrared reflectance of the transition metal chalcogenide 1T-TaS2 has been measured at temperatures from 30K to 360K over 30-45,000cm^-1 (4meV-5.5eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudogap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1eV. As the temperature decreases, the low frequency conductivity also decreases, phonon modes become more pronounced and pseudogap develops into a gap at 800cm^-1 (100meV). We observe an anomalous frequency dependence of the 208cm^-1 infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180K metal-to-insulator transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc-resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes associated with the metal-to-insulator transition.

cond-mat↗
arXiv2002

Femtosecond "snapshots "of gap-forming charge-density-wave correlations in quasi-two-dimensional dichalcogenides 1T-TaS2 and 2H-TaSe2

Time-resolved optical spectroscopy of collective and single-particle excitations of 1T-TaS2 and 2H-TaSe2 reveals the presence of a large gap in the excitation spectrum on the femtosecond timescale, associated with the formation of various degrees of CDW order. In common with superconducting cuprates, excitations with energies less than the full gap show much slower relaxation. This separation of timescales cannot be explained in a quasi-2D Fermi-Liquid picture with an anisotropic gap but rather suggests the formation of a fluctuating spatially inhomogeneous state eventually forming a long-range ordered state at low temperatures.

cond-mat.str-el↗
arXiv2001

Doping dependence of the electronic Raman spectra in cuprates

We report electronic Raman scattering measurements on Bi$_2$Sr$_2$(Y$_{1-x}$Ca$_x$)Cu$_2$O$_{8+δ}$ single crystals at different doping levels. The dependence of the spectra on doping and on incoming photon energy is analyzed for different polarization geometries, in the superconducting and in the normal state. We find the scaling behavior of the superconductivity pair-breaking peak with the carrier concentration to be very different in B$_{1g}$ and B$_{2g}$ geometries. Also, we do not find evidence of any significant variation of the lineshape of the spectra in the overdoped region in both symmetries, while we observe a reduction of the intensity in B$_{2g}$ upon decreasing photon energies. The normal state data are analyzed in terms of the memory-function approach. The quasiparticle relaxation rates in the two symmetries display a dependence on energy and temperature which varies with the doping level.

cond-mat.supr-con↗
arXiv2001

Origin of the peak-dip-hump structure in the photoemission spectra of Bi2212

The famous peak-dip-hump lineshape of the (π,0) photoemission spectrum of the bilayer Bi HTSC in the superconducting state is shown to be a superposition of spectral features originating from different electronic states which reside at different binding energies, but are each describable by essentially identical single-particle spectral functions. The 'superconducting' peak is due to the antibonding Cu-O-related band, while the hump is mainly formed by its bonding counterpart, with a c-axis bilayer coupling induced splitting of about 140 meV.

cond-mat.supr-con↗
arXiv2001

Ruthenocuprates RuSr2(Eu,Ce)2Cu2O10: Intrinsic magnetic multilayers

We report ac susceptibility data on RuSr_2(Eu,Ce)_2Cu_2O_(10-y) (Ru-1222, Ce content x=0.5 and 1.0), RuSr_2GdCu_2O_8 (Ru-1212) and SrRuO_3. Both Ru-1222 (x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic fields: logarithmic time relaxation, switching behavior, and `inverted' hysteresis loops. Neither Ru-1212 nor SrRuO_3 exhibit such magnetic dynamics. The results are interpreted as evidence of the complex magnetic order in Ru-1222. We propose a specific multilayer model to explain the data, and note that superconductivity in the ruthenocuprate is compatible with both the presence and absence of the magnetic dynamics.

cond-mat.supr-con↗
arXiv2001

Estimation of matrix element effects and determination of the Fermi surface in BSCCO systems using angle-scanned photoemission spectroscopy

The strong dependence of the momentum distribution of the photoelectrons on experimental conditions raises the question as to whether angle-resolved photoemission spectroscopy (ARPES) is able to provide an accurate reflection of the Fermi surface in Bi-based cuprate superconductors. In this paper we experimentally prove that the main contribution to the intensity variation comes from matrix elements effects and develop an approach to overcome this problem. We introduce a concept of 'self-normalization' which makes the spectra essentially independent of both the matrix elements and particular experimental parameters. On the basis of this concept we suggest a simple and precise method of Fermi surface determination in quasi-2D systems.

cond-mat.supr-con↗
arXiv2001

Surprises in the doping dependence of the Fermi surface in Bi(Pb)-2212

A detailed and systematic ARPES investigation of the doping-dependence of the normal state Fermi surface (FS) of modulation-free (Pb,Bi)-2212 is presented. The FS does not change in topology away from hole-like at any stage. The data reveal, in addition, a number of surprises. Firstly the FS area does not follow the usual curve describing Tc vs x for the hole doped cuprates, but is down-shifted in doping by ca. 0.05 holes per Cu site, indicating either the break-down of Luttinger's theorem or the consequences of a significant bi-layer splitting of the FS. Secondly, the strong k-dependence of the FS width is shown to be doping independent. Finally, the relative strength of the shadow FS has a doping dependence mirroring that of Tc.

cond-mat.supr-con↗
arXiv2000

Effect of controlled disorder on quasiparticle thermal transport in Bi$_2$Sr$_2$CaCu$_2$O$_8$

Low temperature thermal conductivity, $κ$, of optimally-doped Bi2212 was studied before and after the introduction of point defects by electron irradiation. The amplitude of the linear component of $κ$ remains unchanged, confirming the universal nature of heat transport by zero-energy quasiparticles. The induced decrease in the absolute value of $κ$ at finite temperatures allows us to resolve a nonuniversal term in $κ$ due to conduction by finite-energy quasiparticles. The magnitude of this term provides an estimate of the quasiparticle lifetime at subkelvin temperatures.

cond-mat.supr-con↗
arXiv2000

Pressure dependence of the spin gap in BaVS_3

We carried out magnetotransport experiments under hydrostatic pressure in order to study the nature of the metal-insulator transition in BaVS$_3$. Scaling relations for $ρ(T,H,p)$ are established and the pressure dependence of the spin gap is determined. Our new results, in conjunction with a re-analysis of earlier specific heat and susceptibility data, demonstrate that the transition is weakly second order. The nature of the phase diagram in the $T$--$p$--$H$ space is discussed.

cond-mat.str-el↗