Research

Technical Progress

Superconductivity

• Nobel Prize in physics awarded to Abrikosov at Stockholm ceremony: ARGONNE, Ill. (Dec. 10, 2003) — The 2003 Nobel Prize in physics was awarded to Alexei A. Abrikosov of the U.S. Department of Energy's Argonne National Laboratory at a ceremony in Stockholm. Abrikosov shared the prize with two colleagues for theories about how matter can show bizarre behavior at extremely low temperatures. The Royal Swedish Academy of Sciences cited Abrikosov, Anthony J. Leggett and Vitaly L. Ginzburg for their work concerning two phenomena called superconductivity and superfluidity. View an online photo gallery documenting the Nobel festivities.
• ARPES spectra in the superconducting state of the cuprates are characterized by a low binding energy feature (quasiparticle peak), and a higher binding energy feature (hump), with both features scaling to higher energy with reduced doping. We find that the hump feature exhibits dispersion consistent with the interaction of the electrons with a collective mode of wave vector (p,p). By a simple analysis of the data, the mode energy was extracted as a function of doping, and it agreed with the resonant mode energy directly seen by magnetic neutron scattering.
• The superconducting gap anisotropy obtained from ARPES was analyzed, and it was found that there is a monotonic change in the shape of this anisotropy with doping. The gap slope around the d-wave node was compared with values extracted from penetration depth measurements, and it was concluded that the latter are affected by a doping dependent renormalization, which has been recently confirmed by thermal conductivity measurements.
• A long existing puzzle about the large negative isotope effect on copper in YBCO has been successfully resolved. It was shown to be due to low frequency optical phonons in a d-wave superconductor. Recommendations were given for further research.
• The corrections to low-temperature conductivity due to quantum interference and to interaction of electrons were calculated for a quasi-2D metal, as functions of temperature and magnetic field. It was shown that there exist criteria distinguishing between a 3D and a 2D behavior, and these criteria differ for different corrections. The results obtained fit the experimental data on the temperature dependence of resistance in layered cuprates when their superconductivity is suppressed by strong magnetic field. The field dependent correction fits the data on the magnetic field dependence of resistance in layered manganites.
• Studies of the twist junctions in BSCCO crystals lead to the conclusion that the tunneling in the junction is strongly incoherent and the order parameter cannot have a d-wave symmetry

Electronic Structure And Properties

• An old interest is revived because of new opportunities. The studies of local excitation properties grew out of an interest focussed on ground state local character in f-electron systems and oxides. Progress thus far has only been on the base: development of codes capable of effectively dealing with the requisite low symmetry complexity, construction of a BEOWULF style computer to underlie it. That "machine'' is very nearly online. Formulation of the Fano function transformation for a real electronic structure calculation (it has been done for simple model electronic structures) and formulation of a simpler model realization to be used with a dynamic cluster formulation.
• The views of correlated electron systems and x-ray excitation properties utilize local orbitals as the basis. Those orbitals are generally not fully characterized. Some developments originated by Fano are being revived and implemented. Using formalism created to generate generalized multipole expansions on constant energy surfaces, single site expansions create functions related to, but more local than, Wannier functions. These functions give insight into the range of "local" excitations and the contributions of ionic polarization in deformations and excitations.
• Further first principles simulations of the ARPES intensity from BISCO along high symmetry lines and from the entire (k_x,k_y) plane were carried out and compared with the corresponding measurements at ANL. The comparisons yielded new insight into the importance of matrix element effects in BISCO. Further work was carried out on the electronic structure and magnetism of the Fe-V-X (where X= Si, Ga, or Al) disordered alloys within the KKR-CPA parameter free framework, including the computation of total energies.
• An extensive study of the Fermi surface (FS) and related issues in Ba-K-Bi-O (BKBO) alloys was completed with focus on the effects of Ba/K substitution within the KKR-CPA framework. The evolution of the nesting and other features of the Fermi surface of the underlying pristine cubic phase was correlated with the onset of various structural transitions with K doping. A parametrized scheme for obtaining an accurate 3D map of the FS in BKBO for an arbitrary doping level was developed.
• High resolution Compton scattering results on Al single crystals were analyzed with focus on determining the size of the break ZF in the electron momentum density at the Fermi momentum; for this purpose, a simple model for describing electron correlation effects which are missing from the independent particle band theory framework was employed. A value of ZF of 0.7-0.8 was adduced from the Compton measurements. This study shows that in sharp contrast to the case of Li, where recent Compton work indicates a ZF value of nearly zero, the standard picture of the interacting electron gas is substantially correct in Al.
• A study of the high resolution Compton profiles of Be single crystals using photon energies of 10, 29 and 56 keV was completed. The theoretically predicted Fermi surface signatures are clearly seen in the Compton data. However, subtle differences between theory and experiment remain. These discrepancies indicate that a better treatment of the electron correlation effects in the inhomogeneous electron gas is needed to develop a satisfactory description of the momentum density in Be.

Magnetism

• An extensive study elucidating the more complicated long-distance repeat distance in Cr-V alloy spacer layers has been competed. This particular alloy system probes the mechanisms generating this repeat distance --- which is especially important both because it is one of the few examples of anomalously long repeat distances and because its explanation has proven illusive. It is particularly sensitive to the details of the calculations and thus has reopened the question of deficiencies in the density functional theory functionals used for these materials --- a problem, which has been quiescent for over a decade.
• Linear quantum magnetoresistance in strong fields was predicted for semimetals with a band structure similar to graphite.
• It was demonstrated that the large linear magnetoresistance in Bi, discovered in 1928 and repeated recently, is the quantum magnetoresistance predicted by A. Abrikosov in 1969. It was shown also that a small pocket of the Fermi surface having a small effective mass could make a major contribution to the magnetoresistance of a good metal with a large Fermi surface in a high magnetic field. This provides an explanation of the linear magnetoresistance observed in some good metals.

Future Accomplishments

Superconductivity

• We plan to calculate the dynamic susceptibility for high Tc superconductors, using ARPES dispersion information as input, and thus gain insight into several issues revealed by recent neutron scattering data: (1) the origin of incommensurability and its variation with energy, (2) the nature of the magnetic resonance mode, (3) the question of whether the exchange energy is lowered in the superconducting state, and (4) the variation of the spin gap with momentum. This project will help to correlate the only two momentum resolved experimental probes in this field.
• Formally, the condensation energy associated with superconductivity can be derived if the occupied part of the spectral function is known. This is exactly the information obtained by ARPES measurements. Our plan is to take such data and by analysis shed light on the origin of superconductivity in the cuprates.
• By detailed analysis of ARPES data, we plan to extract the momentum variation of both the momentum distribution function, and the electron self-energy. This work will be of use in deciding between various microscopic theories of the cuprates.
• The origin of the non-Fermi-liquid behavior observed by ARPES in underdoped cuprates and quasi-one-dimensional metals above the transition point will be examined on the basis of two possible ideas: a) proximity to one-dimensionality, b) proximity to the spin-, or charge-density-wave phase transition.
• Theoretical studies are planned of intercalation compounds of the Bi-based HTSC. It is already known that intercalation of Bi2212 with HgBr2 leads to a pseudogap in the tunneling density of states. The idea is to connect this phenomenon with formation of charge- and spin density waves in these materials.
• The behavior of the order parameter at interfaces, such as twist junctions and boundaries with a cubic metal, will be studied with the goal to establish whether a "subdominant" s-type order parameter can appear.
• The temperature dependence of the order parameter within the ELU model will be studied with the goal to explain the unusual temperature dependence of the Josephson penetration depth in YBCO found by the Vancouver Group.

Electronic Structure and Properties

• The surface component of interlayer coupling will be examined. The code developed by the Freeman group at Northwestern University will be used. However, the project to bring it on-line for us will be an oxide problem examining the occurrence of oxygen deficiencies at twin boundaries in YBCO that has been posed by B. Veal. The interface properties of the multilayers need a more detailed examination than most investigators have been willing to invest. We will only be positioned to address those problems when we have dealt with the complexity posed by the twin-boundary problem and plan to serially order the effort.
• The initial application of the local orbitals will be to a dynamical cluster many body algorithm. It is expected that this will significantly improve the realism of applications to the many body aspects in correlated electron systems.
• The special local functions when coupled with LAPW codes capable of resolving small distortions will be used to examine hopping behavior and ionic polarization contributions in oxides exhibiting small distortions.
• These projects are also stepping stones to the problems for which the functions were actually designed: local excitations. Two additional features are the need to use less-well-understood more advanced aspects of density functional theory and the need to incorporate possible modifications of the orbitals by the excitation process.
• Extensive studies of angle-scanned spectra will be started on materials where suitable high quality ARPES data are available. Interesting possibilities are: La-Sr-Cu-O, Bi2201, Sr-Cu-O-Cl and Ca-Cu-O-Cl, and the ruthenates.
• A first-principles methodology will be developed and implemented for modeling the inelastic x-ray scattering profiles in metals and alloys. This involves computation of the S(q,w) and represents a reasonable extension of the ongoing and previous extensive ARPES, Compton and other work in metals and alloys.
• Methodology for computing first-principles ARPES intensities will be further developed in order to simulate a variety of effects that are important in understanding the spectra of complex materials; some work in this connection has already been completed during the last year. In particular, we will simulate the effects on the ARPES spectra of a reduction in the overlap between the CuO2 plane band states (compared to the LDA-based predictions). The effects of lattice modulation and of possible stripe-type orderings in the cuprates will also be simulated.
• The aforementioned development will be used to gain insight into a number of issues concerning the ARPES spectra of BISCO that have become the subject of intense interest during the last year. The ARPES data from the ANL group will be used in our work as far as possible. Some of the questions of inerest are: (i) Connectivity of the Fermi surface of Bi2212 and Bi2201 near the M-point; (ii) Nature of the ARPES signature associated with the two CuO2 bands at and near the M-point; (iii) Photon energy and polarization dependence of the ARPES intensity; (iv) Signature of the lattice modulation in the spectra; (v) Signature of stripe-type modulations on the ARPES spectra. With the availability of the ARPES data on La-Sr-Cu-O (LSCO) single crystals during the last year, we will start ARPES computations on LSCO. Many of the basic issues of interest are similar to those noted above in the case of BISCO and would be addressed appropriately. The simpler crystal structure of LSCO will make this an easier system to treat than BISCO.
• Highly accurate magnetic Compton profiles (MCP's) will be computed for the three high symmetry directions in ferromagnetic Ni and NiCu disordered alloys within the parameter-free charge and spin self-consistent KKR-CPA framework. The results will be compared and contrasted with corresponding MCP measurements currently underway at APS in Synchrotron Radiation Studies (58926).
• The analysis of the high resolution Compton data on Al from ESRF and independently from Spring-8 will be completed in the light of our corresponding theoretical calculations with focus on fermiology and correlation related effects. Study of momentum density and Compton profiles of Al-rich Al-Li alloys where high quality Compton data has become available from Spring-8 will be undertaken. Initial analysis indicates very exciting changes in electron correlation effects in the vicinity of the Li impurities in the Al matrix.
• The work towards incorporating electron-electron correlation effects in the momentum density beyond the LDA framework will be continued with the purpose of developing a practical scheme of applicability to wide classes of materials. In this connection, significant progress was made during the last year in developing the methodology for first-principles computation of the dielectric function e(w). This development will also serve as a basis for our effort to compute the dynamic structure function S(q,w) which is central for investigating the signatures of electronic transitions via inelastic x-ray scattering.

Magnetism

• In connection with experiments being designed based on Huesler and/or Laves phase materials with the objecive of minimizing strain effects and to control the magnetic enhancements in the spacer layer, the requisite calculations will be performed.
• A theory of antiferromagnetism in insulating layered copper oxides will be constructed on the basis of the idea of a spin-Peierls transition in a model with localized electron states at the copper sites and delocalized electrons spanning the rows of oxygen atoms. This model will be studied also above TN with the goal of establishing the electron interaction due to spin fluctuations.
• The theory of the linear magnetoresistance, observed recently in underdoped YBCO, with both, the current and magnetic field, along the c-axis will be constructed on the basis of the idea that the magnetic field quantization of the transverse momentum tends to reduce coherence of resonant tunneling through different localized centers.