Investigation of the nonlocal coherent-potential approximation

Recently the nonlocal coherent-potential approximation (NLCPA) has been introduced by Jarrell and Krishnamurthy for describing the electronic structure of substitutionally disordered systems. The NLCPA provides systematic corrections to the widely used coherent-potential approximation (CPA) whilst preserving the full symmetry of the underlying lattice. Here an analytical and systematic numerical study of the NLCPA is presented for a one-dimensional tight-binding model Hamiltonian, and comparisons with the embedded cluster method (ECM) and molecular coherent potential approximation (MCPA) are made.Comment: 18 pages, 5 figure

Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate

Determination of the orbital symmetry of a state in spin triplet Sr$_2$RuO$_4$ superconductor is a challenge of considerable importance. Most of the experiments show that the chiral state of the $\hat{z} (k_x \pm ik_y)$ type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr$_2$RuO$_4$ in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has $=0$ and is therefore not affected to linear order in the coupling parameter $\lambda$. We found that stability of the chiral state requires spin dependent pairing interactions. This imposes strong constraint on the pairing mechanism.Comment: 4 pages, 4 figure

Friedel oscillations induced surface magnetic anisotropy

We present detailed numerical studies of the magnetic anisotropy energy of a magnetic impurity near the surface of metallic hosts (Au and Cu), that we describe in terms of a realistic tight-binding surface Green's function technique. We study the case when spin-orbit coupling originates from the d-band of the host material and we also investigate the case of a strong local spin-orbit coupling on the impurity itself. The splitting of the impurity's spin-states is calculated to leading order in the exchange interaction between the impurity and the host atoms using a diagrammatic Green's function technique. The magnetic anisotropy constant is an oscillating function of the separation d from the surface: it asymptotically decays as 1/d2 and its oscillation period is determined by the extremal vectors of the host's Fermi Surface. Our results clearly show that the host-induced magnetic anisotropy energy is by several orders of magnitude smaller than the anisotropy induced by the local mechanism, which provides sufficiently large anisotropy values to explain the size dependence of the Kondo resistance observed experimentally.Comment: 11 pages, 7 figures, submitted to PR

Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition

We have studied the effects of strong impurity scattering on disordered superconductors beyond the low impurity concentration limit. By applying the full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations of the local order parameters $\Delta_{A}, \Delta_{B}$ and charge densities, $n_{A}, n_{B}$ for weak and strong on site disorder. We find that for narrow band alloy s-wav e superconductors the conditions for Anderson's theorem are satisfied in general only for the case of particle-hole symmetry. In this case it is satisfied regardless whether we are in the weak or strong scattering regimes. Interestingly, we find that strong scattering leads to band splitting and in this regime for any band filling we have a critical concentration where a superconductor-insulator quantum phase transition occurs at T=0.Comment: 28 pages, 13 figure

Van Hove Singularity and D-Wave Pairing in Disordered Superconductors

We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d-wave pairing. We demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T$_c$ is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T$_c$ vs. doping curve with increasing disorder and compare our results with experiments on the Y_{0.8}Ca_{0.2}Ba_2(Cu_{1-c}Zn_c)_{3}O_{7-\delta} alloys.Comment: 4 pages of text and 7 postscript file

Spin currents and spin dynamics in time-dependent density-functional theory

We derive and analyse the equation of motion for the spin degrees of freedom within time-dependent spin-density-functional theory (TD-SDFT). Results are (i) a prescription for obtaining many-body corrections to the single-particle spin currents from the Kohn-Sham equation of TD-SDFT, (ii) the existence of an exchange-correlation (xc) torque within TD-SDFT, (iii) a prescription for calculating, from TD-SDFT, the torque exerted by spin currents on the spin magnetization, (iv) a novel exact constraint on approximate xc functionals, and (v) the discovery of serious deficiencies of popular approximations to TD-SDFT when applied to spin dynamics.Comment: now includes discussion of OEP and GGA; to appear in Phys. Rev. Let

PSMB7 is associated with anthracycline resistance and is a prognostic biomarker in breast cancer

BACKGROUND: To date individual markers have failed to correctly predict resistance against anticancer agents in breast cancer. We used gene expression patterns attributable to chemotherapy-resistant cells to detect potential new biomarkers related to anthracycline resistance. One of the genes, PSMB7, was selected for further functional studies and clinical validation. METHODS: We contrasted the expression profiles of four pairs of different human tumour cell lines and of their counterparts resistant to doxorubicin. Observed overexpression of PSMB7 in resistant cell lines was validated by immunohistochemistry. To examine its function in chemoresistance, we silenced the gene by RNA interference (RNAi) in doxorubicin-resistant MCF-7 breast cancer cells, then cell vitality was measured after doxorubicin treatment. Microarray gene expression from GEO raw microarray samples with available progression-free survival data was downloaded, and expression of PSMB7 was used for grouping samples. RESULTS: After doxorubicin treatment, 79.8+/-13.3% of resistant cells survived. Silencing of PSMB7 in resistant cells decreased survival to 31.8+/-6.4% (P>0.001). A similar effect was observed after paclitaxel treatment. In 1592 microarray samples, the patients with high PSMB7 expression had a significantly shorter survival than the patients with low expression (P<0.001). CONCLUSION: Our findings suggest that high PSMB7 expression is an unfavourable prognostic marker in breast cancer

Magnetic field induced rotation of the d-vector in the spin triplet superconductor Sr2_2RuO4_4

In zero magnetic field the superconductor Sr$_2$RuO$_4$ is believed to have a chiral spin triplet pairing state in which the gap function d-vector is aligned along the crystal c-axis. Using a phenomenological but orbital specific description of the spin dependent electron-electron attraction and a realistic quantitative account of the electronic structure in the normal state we analyze the orientation of the spin triplet Cooper pair d-vector in response to an external c-axis magnetic field. We show that for suitable values of the model parameters a c-axis field of only 20 mT is able to cause a reorientation phase transition of the d-vector from along $c$ to the $a-b$ plane, in agreement with recent experiments.Comment: 6 pages, 7 figures. Submitted Phys Rev

Pressure Induced Charge Disproportionation in LaMnO3_{3}

We present a total energy study as a function of volume in the cubic phase of LaMnO$_{3}$. A charge disproportionated state into planes of Mn$^{3+}$O$_{2}$/Mn$^{4+}$O$_{2}$ was found. It is argued that the pressure driven localisation/delocalisation transition might go smoothly through a region of Mn$^{3+}$ and Mn$^{4+}$ coexistence.Comment: 3 pages, 1 figure, Conference Proceedings: Nanospintronics: Design and Realization (Kyoto, Japan 24-28 May, 2004

Magnetic properties of Quantum Corrals from first principles calculations

We present calculations for electronic and magnetic properties of surface states confined by a circular quantum corral built of magnetic adatoms (Fe) on a Cu(111) surface. We show the oscillations of charge and magnetization densities within the corral and the possibility of the appearance of spin--polarized states. In order to classify the peaks in the calculated density of states with orbital quantum numbers we analyzed the problem in terms of a simple quantum mechanical circular well model. This model is also used to estimate the behaviour of the magnetization and energy with respect to the radius of the circular corral. The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method.Comment: 14 pages, 9 figures, submitted to J. Phys. Cond. Matt. special issue on 'Theory and Simulation of Nanostructures