arXiv subjects

A. I. Lichtenstein

Publications and source records attributed to A. I. Lichtenstein.

At least 127 records · Page 7Linked to original sources

arXiv2008

Midgap states in corrugated graphene: Ab-initio calculations and effective field theory

We investigate the electronic properties of corrugated graphene and show how rippling-induced pseudomagnetic fields alter graphene's low-energy electronic properties by combining first principles calculations with an effective field theory. The formation of flat bands near the Fermi level corresponding to pseudo-Landau levels is studied as a function of the rippling parameters. Quenched and relaxed ripples turn out to be fundamentally different is this respect: It is demonstrated, both numerically and analytically, that annealing of quenched ripples can destroy the flat bands.

cond-mat.mes-hall↗

arXiv2008

Non-quasiparticle states in Co$_2$MnSi evidenced through magnetic tunnel junction spectroscopy measurements

We investigate the effects of electronic correlations in the full-Heusler Co$_2$MnSi, by combining a theoretical analysis of the spin-resolved density of states with tunneling-conductance spectroscopy measurements using Co$_2$MnSi as electrode. Both experimental and theoretical results confirm the existence of so-called non-quasiparticle states and their crucial contribution to the finite-temperature spin polarisation in this material.

cond-mat.str-el↗

arXiv2008

Dual Fermion Approach to Susceptibility of Correlated Lattice Fermions

In this paper, we show how the two-particle Green function (2PGF) can be obtained within the framework of the Dual Fermion approach. This facilitates the calculation of the susceptibility in strongly correlated systems where long-ranged non-local correlations cannot be neglected. We formulate the Bethe-Salpeter equations for the full vertex in the particle-particle and particle-hole channels and introduce an approximation for practical calculations. The scheme is applied to the two-dimensional Hubbard model at half filling. The spin-spin susceptibility is found to strongly increase for the wavevector $\vc{q}=(π,π)$, indicating the antiferromagnetic instability. We find a suppression of the critical temperature compared to the mean-field result due to the incorporation of the non-local spin-fluctuations.

cond-mat.str-el↗

arXiv2008

Correlation effects in the electronic structure of Mn$_4$ molecular magnet

We present joint theoretical-experimental study of the correlation effects in the electronic structure of (pyH)$_3$[Mn$_4$O$_3$Cl$_7$(OAc)$_3$]$\cdot$2MeCN molecular magnet (Mn$_4$). Describing the many-body effects by cluster dynamical mean-field theory, we find that Mn$_4$ is predominantly Hubbard insulator with strong electron correlations. The calculated electron gap (1.8 eV) agrees well with the results of optical conductivity measurements, while other methods, which neglect many-body effects or treat them in a simplified manner, do not provide such an agreement. Strong electron correlations in Mn$_4$ may have important implications for possible future applications.

cond-mat.mtrl-sci↗

arXiv2008

Phonon mediated tunneling into graphene

Recent scanning tunneling spectroscopy experiments [V. W. Brar et. al., Appl. Phys. Lett. 91, 122102 (2007), Y. Zhang et. al., arXiv:0802.4315 (2008)] on graphene reported an unexpected gap of about $\pm 60$meV around the Fermi level. Here, we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab-initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasi free electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated and its effects on tunneling into graphene are discussed. In particular, good agreement of the tunneling density of states within our model and the experimental[V. W. Brar et. al., Appl. Phys. Lett. 91, 122102 (2007), Y. Zhang et. al., arXiv:0802.4315 (2008)] dI/dU spectra is found.

cond-mat.mes-hall↗

arXiv2008

Orbital magnetism in transition-metal systems: The role of local correlation effects

The influence of correlation effects on the orbital moments for transition metals and their alloys is studied by first-principle relativistic Density Functional Theory in combination with the Dynamical Mean-Field Theory. In contrast to the previous studies based on the orbital polarization corrections we obtain an improved description of the orbital moments for wide range of studied systems as bulk Fe, Co and Ni, Fe-Co disordered alloys and 3$d$ impurities in Au. The proposed scheme can give simultaneously a correct dynamical description of the spectral function as well as static magnetic properties of correlated disordered metals.

cond-mat.str-el↗

arXiv2008

Plane-wave based electronic structure calculations for correlated materials using dynamical mean-field theory and projected local orbitals

The description of realistic strongly correlated systems has recently advanced through the combination of density functional theory in the local density approximation (LDA) and dynamical mean field theory (DMFT). This LDA+DMFT method is able to treat both strongly correlated insulators and metals. Several interfaces between LDA and DMFT have been used, such as (N-th order) Linear Muffin Tin Orbitals or Maximally localized Wannier Functions. Such schemes are however either complex in use or additional simplifications are often performed (i.e., the atomic sphere approximation). We present an alternative implementation of LDA+DMFT, which keeps the precision of the Wannier implementation, but which is lighter. It relies on the projection of localized orbitals onto a restricted set of Kohn-Sham states to define the correlated subspace. The method is implemented within the Projector Augmented Wave (PAW) and within the Mixed Basis Pseudopotential (MBPP) frameworks. This opens the way to electronic structure calculations within LDA+DMFT for more complex structures with the precision of an all-electron method. We present an application to two correlated systems, namely SrVO3 and beta-NiS (a charge-transfer material), including ligand states in the basis-set. The results are compared to calculations done with Maximally Localized Wannier functions, and the physical features appearing in the orbitally resolved spectral functions are discussed.

cond-mat.str-el↗

arXiv2008

Weak ferromagnetism in Mn nanochains on the CuN surface

We investigate electronic and magnetic structures of the Mn chains supported on the CuN surface using first-principle LSDA and LDA+U calculations. The isotropic exchange integrals and anisotropic Dzyaloshinskii-Moriya interactions between Mn atoms are calculated using Green function formalism. It is shown that the account of lattice relaxation and on-site Coulomb interaction are important for accurate description of magnetic properties of the investigated nanosystems. We predict a weak ferromagnetism phenomenon in the Mn antiferromagnetic nanochains on the CuN surface. The value of a net magnetic moment and direction of spin canting are calculated. We show that some experimental features may be explained using anisotropic exchange interactions.

cond-mat.mtrl-sci↗

arXiv2007

Hydrogen on graphene: Electronic structure, total energy, structural distortions, and magnetism from first-principles calculations

Density functional calculations of electronic structure, total energy, structural distortions, and magnetism for hydrogenated single-layer, bilayer, and multi-layer graphene are performed. It is found that hydrogen-induced magnetism can survives only at very low concentrations of hydrogen (single-atom regime) whereas hydrogen pairs with optimized structure are usually nonmagnetic. Chemisorption energy as a function of hydrogen concentration is calculated, as well as energy barriers for hydrogen binding and release. The results confirm that graphene can be perspective material for hydrogen storage. Difference between hydrogenation of graphene, nanotubes, and bulk graphite is discussed.

cond-mat.mtrl-sci↗

arXiv2007

Half-metallic ferromagnets: From band structure to many-body effects

A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.

cond-mat.mtrl-sci↗

arXiv2007

Magnetism and local distortions near carbon impurity in $γ$-iron

Local perturbations of crystal and magnetic structure of $γ$-iron near carbon interstitial impurity is investigated by {\it ab initio} electronic structure calculations. It is shown that the carbon impurity creates locally a region of ferromagnetic ordering with substantial tetragonal distortions. Exchange integrals and solution enthalpy are calculated, the latter being in a very good agreement with experimental data. Effect of the local distortions on the carbon-carbon interactions in $γ$-iron is discussed.

cond-mat.mtrl-sci↗

arXiv2007

Cluster Dual Fermion Approach to Nonlocal Correlations

We formulate a general cluster Dual Fermion Approach to nonlocal correlations in crystals. The scheme allows the treatment of long-range correlations beyond cluster DMFT and nonlocal effects in realistic calculations of multiorbital systems. We show that the the simplest approximation exactly corresponds to free cluster DMFT. We further consider the relation between the two-particle Green functions in real and dual variables. We apply this approach by calculating the Green function of the Hubbard model in one dimension starting from the two-site cluster DMFT solution. The result agrees well with the Green function obtained from a DMRG calculation.

cond-mat.str-el↗

arXiv2007

Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals

The crossover between a free magnetic moment phase and a Kondo phase in low dimensional disordered metals with dilute magnetic impurities is studied. We perform a finite size scaling analysis of the distribution of the Kondo temperature as obtained from a numerical renormalization group calculation of the local magnetic susceptibility and from the solution of the self-consistent Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic moments when the exchange coupling falls below a disorder-dependent critical value $J_{\rm c}$. Our numerical results show that between the free moment phase due to Anderson localization and the Kondo screened phase there is a phase where free moments occur due to the appearance of random local pseudogaps at the Fermi energy whose width and power scale with the elastic scattering rate $1/τ$.

cond-mat.str-el↗

arXiv2007

Dual fermion approach to nonlocal correlations in the Hubbard model

A new diagrammatic technique is developed to describe nonlocal effects (e.g., pseudogap formation) in the Hubbard-like models. In contrast to cluster approaches, this method utilizes an exact transition to the dual set of variables, and it therefore becomes possible to treat the irreducible vertices of an effective {\it single-impurity} problem as small parameters. This provides a very efficient interpolation between weak-coupling (band) and atomic limits. The antiferromagnetic pseudogap formation in the Hubbard model is correctly reproduced by just the lowest-order diagrams.

cond-mat.str-el↗

arXiv2007

Local impurity effects in superconducting graphene

We study the effect of impurities in superconducting graphene and discuss their influence on the local electronic properties. In particular, we consider the case of magnetic and non-magnetic impurities being either strongly localized or acting as a potential averaged over one unit cell. The spin dependent local density of states is calculated and possibilities for visualizing impurities by means of scanning tunneling experiments is pointed out. A possibility of identifying magnetic scatters even by non spin-polarized scanning tunneling spectroscopy is explained.

cond-mat.mes-hall↗

arXiv2007

Molecular Doping of Graphene

Graphene, a one-atom thick zero gap semiconductor [1, 2], has been attracting an increasing interest due to its remarkable physical properties ranging from an electron spectrum resembling relativistic dynamics [3-12] to ballistic transport under ambient conditions [1-4]. The latter makes graphene a promising material for future electronics and the recently demonstrated possibility of chemical doping without significant change in mobility has improved graphene's prospects further [13]. However, to find optimal dopants and, more generally, to progress towards graphene-based electronics requires understanding the physical mechanism behind the chemical doping, which has been lacking so far. Here, we present the first joint experimental and theoretical investigation of adsorbates on graphene. We elucidate a general relation between the doping strength and whether or not adsorbates have a magnetic moment: The paramagnetic single NO2 molecule is found to be a strong acceptor, whereas its diamagnetic dimer N2O4 causes only weak doping. This effect is related to the peculiar density of states of graphene, which provides an ideal situation for model studies of doping effects in semiconductors. Furthermore, we explain recent results on its "chemical sensor" properties, in particular, the possibility to detect a single NO2 molecule [13].

cond-mat.mes-hall↗

arXiv2007

Half-Metallic Ferromagnetism and the spin polarization in CrO$_2$

We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO$_2$ as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO$_2$.

cond-mat.str-el↗

arXiv2007

An dynamical-mean-field-theory investigation of specific heat and electronic structure of $α$ and $δ$-plutonium

We have carried out a comparative study of the electronic specific heat and electronic structure of $α$ and $δ$-plutonium using dynmical mean field theory (DMFT). We use the perturbative T-matrix and fluctuating exchange (T-matrix FLEX) as a quantum impurity solver. We considered two different physical pictures of plutonoium. In the first, $5{f^5}+$, the perturbative treatment of electronic correlations has been carried out around the non-magnetic (LDA) Hamiltonian, which results in an f occupation around a bit above $n_f = 5 $. In the second, $5{f^6}-$, plutonium is viewed as being close to an $5{f^6}$ configuration, and perturbation theory is carried out around the (LDA+U) starting point bit below $n_f = 6 $. In the latter case the electronic specific heat coefficient $γ$ attains a smaller value in $γ$-Pu than in $α$-Pu, in contradiction to experiment, while in the former case our calculations reproduce the experimentally observed large increase of $γ$ in $δ$-Pu as compared to the $α$ phase. This enhancement of the electronic specific heat coefficient in $δ$-Pu is due to strong electronic correlations present in this phase, which cause a substantial increase of the electronic effective mass, and high density of states at $E_F$. The densities of states of $α$ and $δ$-plutonium obtained starting from the open-shell configuration are also in good agreement with the experimental photoemission spectra.

cond-mat.str-el↗