arXiv subjects
Publications and source records attributed to A. A. Kordyuk.
In a recent preprint Campuzano et al.(cond-mat/0309402), have questioned the validity of our ARPES results (cond-mat/0305179) contradicting the interpretation of earlier photoemission experiments in terms of the time-reversal symmetry breaking in Bi2212. Here we highlight the principal results of our study and refute all the criticism.
One of the mysteries of modern condenced-matter physics is the nature of the pseudogap state of the superconducting cuprates. Kaminski et al.1 claimed to have observed signatures of time-reversal symmetry breaking in the pseudogap regime in underdoped Bi2Sr2CaCu2O8+d (Bi2212). Here we argue that the observed dichroism is due to the 5x1 superstructure replica of the electronic bands and therefore cannot be considered as evidence for the spontaneous time-reversal symmetry breaking in cuprates.
We report an ARPES investigation of the circular dichroism in the first Brillouin zone (BZ) of under- and overdoped Pb-Bi2212 samples. We show that the dichroism has opposite signs for bonding and antibonding components of the bilayer-split CuO-band and is antisymmetric with respect to reflections in both mirror planes parallel to the c-axis. Using this property of the energy and momentum intensity distributions we prove the existence of the bilayer splitting in the normal state of the underdoped compound and compare its value with the splitting in overdoped sample. In agreement with previous studies the magnitude of the interlayer coupling does not depend significantly on doping. We also discuss possible origins of the observed dichroism.
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.
Based on DC magnetization measurements, the temperature dependencies of the trapped magnetic field have been calculated for two MgB2 samples prepared by two different techniques: the high-pressure sintering and the hot pressing. Experimentally measured trapped field values for the first sample coincide remarkably well with calculated ones in the whole temperature range. This proves, from one side, the validity of the introduced calculation approach, and demonstrates, from another side, the great prospects of the hot pressing technology for large scale superconducting applications of the MgB2.
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.
Recent angle-resolved photoemission and neutron scattering data have provided new ingredients for the interpretation of scanning tunneling spectra on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We analyze the low-temperature tunneling spectra, from oxygen overdoped to underdoped samples, including details about the bilayer splitting and the neutron resonance peak. Two van Hove singularities are identified: the first is integrated in the coherence peaks, the second is heavily broadened at higher binding energy. The shape of the tunneling spectra suggests a strong coupling of the quasiparticles with a collective mode, and a comparison with photoemission shows that the scattering rate in tunneling is an order of magnitude smaller than in ARPES. Finally, the theoretical spectra calculated with an isotropic tunneling matrix element are in better agreement with the experimental data than those obtained with anisotropic matrix elements.
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.
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.
Angle-resolved photoemission spectroscopy (ARPES) is considered as the only experimental tool from which the momentum distribution of both the superconducting and pseudo-gap can be quantitatively derived. The binding energy of the leading edge of the photoemission spectrum, usually called the leading edge gap (LEG), is the model-independent quantity which can be measured in the modern ARPES experiments with the very high accuracy--better than 1 meV. This, however, may be useless as long as the relation between the LEG and the real gap is unknown. We present a systematic study of the LEG as a function of a number of physical and experimental parameters. The absolute gap values which have been derived from the numerical simulation prove, for example that the nodal direction in the underdoped Bi-2212 in superconducting state is really the node--the gap is zero. The other consequences of the simulations are discussed.
The discovery of high temperature superconductors (HTS) has led to understanding that, in order to explain and utilize the phenomenon, completely new physical approaches should be introduced at all scales: microscopic, mesoscopic, macroscopic. Leaving first two scales beyond the scope of the present paper we focus in the upper limit of the last one, the study of the magnetic flux dynamics in HTS bulks - the dynamics of the 'compact vortex structures'. A new direction in the experimental superconducting physics, the investigation of HTS bulks with levitation techniques, which has been elaborated during last years to effectively explore the subject, as well as the new fundamental and applied results obtained therefrom are overviewed here.
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.
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.
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.
We propose an empirical approach to estimate the quality of superconducting joints (welds) between blocks of bulk high temperature superconductors (HTS). As a measuring value, we introduce a joint's quality factor and show its natural correlation with joint's critical current density. Being simple and non-destructive, this approach is considered to be quite important to solve the problem of utilization of HTS in large scale applications. The approach has been applied to characterize the joint's quality of melt-processed Y-123 joined by Tm-123 solder.
The influence of surface treatment on AC loss in melt-processed quasi-single crystal HTS was investigated with resonance oscillations technique. We have found that amplitude dependencies of AC loss on magnetic field amplitude become rather complicated after surface polishing. The experimental data show well distinguished dynamic crossover from absence of barrier at low rates of field variation to its appearence at higher rates. An explaination of such a dynamic surface barrier appearance based on consideration of along surface vortex propagation was suggested.
We have developed a set of experimental techniques with levitation to investigate the macroscopic magnetic properties of bulk HTS: (i) resonance oscillation technique; (ii) high speed magnetic rotor technique; (iii) the method of viscous motion of a permanent magnet through a HTS aperture under a steady force. Early, we used these techniques to investigate the magnetic flux dynamics in granular HTS. Now, we have extended and modified these techniques to study the large grain melt-textured HTS. With resonance oscillations technique we obtain the energy losses vs ac magnetic field amplitude at HTS surface and can derive the values of critical current density with high accuracy. We extract information about distribution of critical current in a thin undersurface layer and, in this way, check the HTS degradation processes. The treatment of the HTS surface that has influence on the surface pinning was investigated with this technique.
A simple approach to describe the levitation force measurements on melt-processed HTS bulks was developed. A couple of methods to determine the critical current density $J_c$ were introduced. The averaged $ab$-plane $J_c$ values for the field parallel to this plane were determined. The first and second levitation force hysteresis loops calculated with these $J_c$ values coincide remarkably well with the experimental data.