Piotr Chudzinski

Effects of thermal and spin fluctuations on the band structure of purple bronze Li 2 Mo 12 O 34

The band structures of ordered and thermally disordered Li2Mo12O34 are calculated by use of the ab initio density-functional-theory–linear-muffin-tin (DFT-LMTO) method. The unusual, very one-dimensional band dispersion obtained in previous band calculations is confirmed for the ordered structure, and the overall band structure agrees reasonably well with existing photoemission data. Dispersion and band structure perpendicular to the main dispersive direction are obtained. A temperature-dependent band broadening is calculated from configurations with thermal disorder of the atomic positions within the unit cell. This leads to band broadening of the two bands at the Fermi energy which can become comparable to their energy separation. The bands are particularly sensitive to in-plane movements of Mo sites far from the Li sites, where the density of states (DOS) is highest. The latter fact makes the effect of Li vacancies on the two bands relatively small. Spin-polarized band results for the ordered structure show a surprisingly large exchange enhancement on the high DOS Mo sites. Consequences for spin fluctuations associated with a cell doubling along the conducting direction are discussed.

Luttinger-liquid theory of purple bronze Li 0.9 Mo 6 O 17 in the charge regime

Molybdenum purple bronze Li0.9Mo6O17 is an exceptional material known to exhibit one-dimensional (1D) properties for energies down to a few meV. This fact seems to be well established both in experiments and in band structure theory. We use the unusual, very 1D band dispersion obtained in ab initio DFT-LMTO band calculations as our starting point to study the physics emerging below 300 meV. A dispersion perpendicular to the main dispersive direction is obtained and investigated in detail. Based on this, we derive an effective low-energy theory within the Tomonaga-Luttinger liquid (TLL) framework. We estimate the strength of the possible interactions and from this deduce the values of the TLL parameters for charge modes. Finally, we investigate possible instabilities of TLL by deriving renormalization group equations which allow us to predict the size of potential gaps in the spectrum. While 2kF instabilities strongly suppress each other, the 4kF instabilities cooperate, which paves the way for a possible charge-density wave at the lowest energies. The aim of this work is to understand the experimental findings, in particular the ones which are certainly lying within the 1D regime. We discuss the validity of our 1D approach and further perspectives for the lower-energy phases.

Magnetic phases in the one-dimensional Kondo chain on a metallic surface

A.M.L. and P.C. acknowledge support from the Swiss National Science Foundation under MaNEP and Division II. A.M.L. acknowledges support from DARPA QuEST, JQI-NSFPFC. M.A.C. acknowledges the hospitality of D.-W. Wang at NCTS (Taiwan) and A. H. Castro-Neto at the Graphene Research Center of the National University of Singapore, and the support of Spanish MEC through Grant No. FIS2010-19609-C02-02.

Phase diagram of hole doped two-leg Cu-O ladders

In the weak coupling limit, we establish the phase diagram of a two-leg ladder with a unit cell containing both Cu and O atoms, as a function of doping. We use bosonization and design a specific renormalization group procedure to handle the additional degrees of freedom. Significant differences are found with the single orbital case; for purely repulsive interactions, a completely massless quantum critical region is obtained at intermediate carrier concentrations (well inside the bands) where the ground state consists of an incommensurate pattern of orbital currents plus a spin density wave structure.

Spin-orbit coupling and proximity effects in metallic carbon nanotubes

We study the spin-orbit coupling in metallic carbon nanotubes (CNTs) within the many-body Tomonaga-Luttinger liquid framework. For a well-defined subclass of metallic CNTs, that contains both achiral zigzag as well as a subset of chiral tubes, an effective low-energy field theory description is derived. We aim to describe systems at finite dopings, but close to the charge neutrality point (commensurability). A new regime is identified where the spin-orbit coupling leads to an inverted hierarchy of minigaps of bosonic modes. We then add a proximity coupling to a superconducting (SC) substrate and show that the only order parameter that is supported within the spin-orbit induced phase is a topologically trivial s-SC.

Magnetoplasmon resonances in polycrystalline bismuth as seen via time-domain terahertz spectroscopy

We report the magnetic-field-dependent far-infrared reflectivity of polycrystalline bismuth. We observe four distinct absorptions that we attribute to magnetoplasmon resonances, which are collective modes of an electron-hole liquid in a magnetic field and become optical and acoustic resonances of the electron-hole system in the small-field limit. Acoustic mode are only when the masses of distinct components are very different, which is the case in bismuth. In a polycrystal, where the translational symmetry is broken, a big shift of spectral weight to an acoustic plasmon is possible. This enables us to detect an associated plasma edge. Although the polycrystal sample has grains of randomly distributed orientations, our reflectivity results can be explained by invoking only two, clearly distinct, series of resonances. In the limit of zero field, the optical modes of these two series converge onto plasma frequencies measured in a monocrystal along the main optical axes.

Orbital current patterns in doped two-leg Cu-O Hubbard ladders

In the weak-coupling limit, we investigate two-leg ladders with a unit cell containing both Cu and O atoms as a function of doping. For purely repulsive interactions, using bosonization, we find significant differences with the single-orbital case: a completely massless quantum critical regime is obtained for a finite range of carrier concentration. In a broad region of the phase diagram, the ground state consists of a pattern of orbital currents plus a density wave. NMR properties of the Cu and O nuclei are presented for the various phases.

Spin rotational symmetry breaking by orbital current patterns in two-leg ladders

We investigate the physical consequences of orbital current patterns (OCP) in doped two-leg Cu-O Hubbard ladders. The internal symmetry of the pattern, in the case of the ladder structure, differs slightly from that suggested so far for cuprates. We focus on this OCP and look for measurable signatures of its existence. We compute the magnetic field produced by the OCP at each lattice site and estimate its value in view of a possible experimental detection. Using a renormalization-group (RG) analysis, we determine the changes that are caused by the SU(2) spin rotational symmetry breaking which occurs when the OCP is present in the ground-state phase diagram. The most significant one is an in-plane spin-density wave gap opening in an otherwise critical phase, at intermediate dopings. We estimate the value of this gap, give an analytic expression for the correlation functions and examine some of the magnetic properties of this new phase which can be revealed in measurements. We compute the conductance in the presence of a single impurity using an RG analysis. A discussion of the various sources of SU(2) symmetry breaking underscores the specificity of the OCP-induced effects.

Influence of non-magnetic impurities on hole-doped two-leg Cu–O Hubbard ladders

We study the influence of non-magnetic impurities on the phase diagram of doped two-leg Hubbard Cu–O ladders. In the absence of impurities, this system possesses d-wave superconducting states and orbital current states depending on the doping. A single, strong, scatterer modifies its environment locally and this effect is assessed using a renormalization group (RG) analysis. At high doping, disorder causes intraband instabilities and at low doping it promotes interband instabilities. In the former case, we extend the boundary conformal field theory method—developed in the context of single chains—to handle the ladder problem, and we find exact closed-form analytical expressions for the correlation functions. This allows us to compute experimentally measurable local quantities such as the nuclear magnetic resonance line broadenings and scanning tunneling microscope profiles. We also discuss the low-doping regime where the Kondo physics is at play, making qualitative predictions about its nature. Insight into collective effects is also given in the many weak impurities case, based on an RG approach. In this regime, one sees the interplay between interactions and disorder. We emphasize the influence of the O atoms on disorder effects both for the single- and for the many-defect situations.