Sher Alam

Temperature dependent polarized XANES spectra for Zn-doped LSCO system

Abstract The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations (i.e. quasiparticle or collective), spin-charge separation, stripes (static and dynamics), inhomogeneities, pseudogap, effect of impurity dopings (e.g. Zn, Ni) and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal, were grown by travelling solvent floating zone technique. Temperature dependent polarized XANES (near edge local structure) spectra were measured at the BL13-B1 (Photon Factory) in the fluorescence mode from 10 to 300 K. Since both stripes and nonmagnetic Zn impurities substituted for Cu give rise to inhomogeneous charge and spin distribution it is interesting to understand the interplay of Zn impurities and stripes. To understand these points we have used Zn-doping and some of the results obtained are as follows: The spectra show a strong dependence with respect to the polarization angle, θ , as is evident at any temperature by comparing the spectra where the electric field vector is parallel to the ab -plane to the one where it is parallel to the c -axis. By using the XANES (temperature) difference spectra we have determined T * (experimentally we find, T * ≈ 160–170 K) for this sample. The XANES difference spectra shows that the changes in XANES features are larger in the ab -plane than the c -axis, this trend is expected since zinc is doped in the ab -plane at the copper site. Our study also complements the results in literature namely that zinc doping does not affect the c -axis transport.

Quantum Group Based Theory for Antiferromagnetism and Superconductivity: Proof and Further Evidence

Abstract Previously one of us presented a conjecture to model antiferromagnetism and high temperature superconductivity and their ‘unification’ by quantum group symmetry rather than the corresponding classical symmetry in view of the critique by Baskaran and Anderson of Zhang’s classical SO(5) model. This conjecture was further sharpened, experimental evidence and the important role of 1-d systems (stripes) was emphasized and moreover the relationship between quantum groups and strings via WZWN models were given in an earlier paper. In this brief note we give and discuss mathematical proof of this conjecture, which completes an important part of this idea, since previously an explicit simple mathematical proof was lacking. It is important to note that in terms of physics that the arbitrariness (freedom) of the d-wave factor g2(k) is tied to quantum group symmetry whereas in order to recover classical SO(5) one must set it to unity in an adhoc manner. We comment on the possible connection between this freedom and the pseudogap behaviour in the cuprates.

SET-based experiments for HTSC materials

Cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations (i.e. quasiparticle or collective), spin-charge separation, stripes (static and dynamics), inhomogeneities, pseudogap, effect of impurity dopings (e.g. Zn, Ni) and any other phenomenon in these materials must be consistently understood. In this paper we further discuss our original suggestion of using single electron tunnelling transistor (SET)-based experiments to understand the role of charge dynamics in these systems. Assuming that SET operates as an efficient charge detection system, we can expect to understand the underlying physics of charge transport and charge fluctuations in these materials for a range of doping. Experiments such as these can be classified in a general sense as mesoscopic and nano characterization of cuprates and related materials. In principle such experiments can show whether an electron is fractionalized in cuprates as indicated by angle-resolved photoemission spectroscopy data. In contrast to flux trapping experiments, SET-based experiments are more direct in providing evidence about spin-charge separation. In addition a detailed picture of nano charge dynamics in cuprates may be obtained.

X-ray absorption near edge structure of small FePt atomic clusters

Abstract X-ray absorption near edge structure of small FePt atomic clusters has been studied using a full multiple scattering, a self-consistent field, and the real-space Green’s function approach realized via the ab initio FEFF8.20 code. We show the μ difference spectra of the Pt L3 edge with respect to the size and shape of the small FePt atomic clusters. The difference spectra at the Fe L3 edge also shows variation, but is less pronounced as compared with the results from the Pt L3 edge. Calculations are made with and without core-hole. It is shown that the white line intensity is reduced for both the Pt and Fe L3 edges with the core-hole. In the case of Pt L3 edge the shape of the spectra without the core-hole agrees much more closely with the experimental results.

Parameters for systems exhibiting local lattice distortions, charge and spin ordering

Keeping in mind the experimental results that indicate local lattice distortions, charge and spin orderings, we have developed a phenomenological approach which allows us to describe the electronic phase diagram of cuprates and related systems in terms of few parameters. In the present work we consider a third-order parameter theory which characterize charge, spin and superconductivity orderings. We are thus led to a theory of three scalar fields. By coupling these scalars to gauge fields we are naturally led to string-like solutions, which we interpret as stripes. This ties nicely with our quantum group conjecture that 1d systems play an important role in the physics of cuprates and related materials. We show that this simple approach can give rough values for two-order parameters which can be naively be interpreted as charge and spin orderings. We also report our attempt to understand how local lattice distortions are involved and what role they play in terms of these two order parameters.

Edge radiation in short-wavelength (extreme-UV and x-ray) free-electron lasers using classical and quantum interference

Ordinary Free-Electron Lasers (FELs) can be found in successful operation in the spectral range from millimeters to ultraviolet wavelengths. However the operation of the common FELs in the extreme ultraviolet and X-ray wavelength regimes faces certain adverse effects. Some of the main obstacles in the way of the realization of X-ray FEL are electron momentum spread and angular divergence. Another point to keep in mind is that ordinary FELs work on the principle of `momentum population inversion. By this one means that electrons with momenta larger than the resonant value contribute to the gain whereas electrons with momenta smaller than the resonant value contribute to the loss. Thus to ensure a net gain we need more electrons with momenta lying in the upper momentum domain than in the lower one, i.e. a `momentum population inversion. To bypass these difficulties the idea of Lasing Without Inversion to achieve the successful operation of short-wavelength (extreme UV and X-ray) FELs has been suggested by Scully and co-workers, which is an ingenious suggestion.

Optical coherent control in materials with stripe phase

Recently experimental evidence for static spin/charge order or stripes in Mn-Ni and Cu-oxides has been emerging. One may view the stripe order in doped antiferromagnets asa new nanophase.