82 research outputs found
Orbital symmetry fingerprints for magnetic adatoms in graphene
In this paper, we describe the formation of local resonances in graphene in
the presence of magnetic adatoms containing localized orbitals of arbitrary
symmetry, corresponding to any given angular momentum state. We show that
quantum interference effects which are naturally inbuilt in the honeycomb
lattice in combination with the specific orbital symmetry of the localized
state lead to the formation of fingerprints in differential conductance curves.
In the presence of Jahn-Teller distortion effects, which lift the orbital
degeneracy of the adatoms, the orbital symmetries can lead to distinctive
signatures in the local density of states. We show that those effects allow
scanning tunneling probes to characterize adatoms and defects in graphene.Comment: 15 pages, 11 figures. Added discussion about the multi-orbital case
and the validity of the single orbital picture. Published versio
Theory of Scanning Tunneling Spectroscopy of Magnetic Adatoms in Graphene
We examine theoretically the signatures of magnetic adatoms in graphene
probed by scanning tunneling spectroscopy (STS). When the adatom hybridizes
equally with the two graphene sublattices, the broadening of the local adatom
level is anomalous and can scale with the cube of the energy. In contrast to
ordinary metal surfaces, the adatom local moment can be suppressed by the
proximity of the probing scanning tip. We propose that the dependence of the
tunneling conductance on the distance between the tip and the adatom can
provide a clear signature for the presence of local magnetic moments. We also
show that tunneling conductance can distinguish whether the adatom is located
on top of a carbon atom or in the center of a honeycomb hexagon.Comment: 4.1 pages, 4 figure
Theoretical Aspects of the Fractional Quantum Hall Effect in Graphene
We review the theoretical basis and understanding of electronic interactions
in graphene Landau levels, in the limit of strong correlations. This limit
occurs when inter-Landau-level excitations may be omitted because they belong
to a high-energy sector, whereas the low-energy excitations only involve the
same level, such that the kinetic energy (of the Landau level) is an
unimportant constant. Two prominent effects emerge in this limit of strong
electronic correlations: generalised quantum Hall ferromagnetic states that
profit from the approximate four-fold spin-valley degeneracy of graphene's
Landau levels and the fractional quantum Hall effect. Here, we discuss these
effects in the framework of an SU(4)-symmetric theory, in comparison with
available experimental observations.Comment: 12 pages, 3 figures; review for the proceedings of the Nobel
Symposium on Graphene and Quantum Matte
Magnetic structure and critical behavior of GdRhIn: resonant x-ray diffraction and renormalization group analysis
The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by
resonant x-ray diffraction at the Gd LII and LIII edges, followed by a
renormalization group analysis for this and other related Gd-based compounds,
namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the
isostructural Ce-based heavy-fermion superconductors. The ground state of
GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation
vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the
a-direction and antiparallel alignment along b and c. A comparison between this
magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n
family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is
determined by a competition between first-(J1) and second-neighbor(J2)
antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an
antiparallel alignment along the three directions (the so-called G-AFM
structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM).
In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is
in a frontier between these two ground states, which may explain its
non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close
to the paramagnetic transition at TN = 39 K was also studied in detail. A
typical second-order transition with the ordered magnetization critical
parameter beta = 0.35 was experimentally found, and theoretically investigated
by means of a renormalization group analysis.Comment: 22 pages, 4 figure
Surface superconductivity in multilayered rhombohedral graphene: Supercurrent
The supercurrent for the surface superconductivity of a flat-band
multilayered rhombohedral graphene is calculated. Despite the absence of
dispersion of the excitation spectrum, the supercurrent is finite. The critical
current is proportional to the zero-temperature superconducting gap, i.e., to
the superconducting critical temperature and to the size of the flat band in
the momentum space
Nodal liquid and s-wave superconductivity in transition metal dichalcogenides
We explore the physical properties of a unified microscopic theory for the
coexistence of superconductivity and charge density waves in two-dimensional
transition metal dichalcogenides. In the case of particle-hole symmetry the
elementary particles are Dirac fermions at the nodes of the charge density wave
gap. When particle-hole symmetry is broken electron (hole) pockets are formed
around the Fermi surface. The superconducting ground state emerges from the
pairing of nodal quasi-particles mediated by acoustic phonons via a
piezoelectric coupling. We calculate several properties in the s-wave
superconducting phase, including specific heat, ultra-sound absorption, nuclear
magnetic relaxation, thermal, and optical conductivities. In the case with
particle-hole symmetry, the specific heat jump at the transition deviates
strongly from ordinary superconductors. The nuclear magnetic response shows an
anomalous anisotropy due to the broken time-reversal symmetry of the
superconducting gap, induced by the triple charge density wave state. The loss
of lattice inversion symmetry in the charge density wave phase leads to
anomalous coherence factors in the optical conductivity and to the appearance
of an absorption edge at the optical gap energy. Furthermore, optical and
thermal conductivities display anomalous peaks in the infrared when
particle-hole symmetry is broken.Comment: 23 pages, 16 figures. Published versio
The Physics of Kondo Impurities in Graphene
This article summarizes our understanding of the Kondo effect in graphene,
primarily from a theoretical perspective. We shall describe different ways to
create magnetic moments in graphene, either by adatom deposition or via
defects. For dilute moments, the theoretical description is in terms of
effective Anderson or Kondo impurity models coupled to graphene's Dirac
electrons. We shall discuss in detail the physics of these models, including
their quantum phase transitions and the effect of carrier doping, and confront
this with existing experimental data. Finally, we point out connections to
other quantum impurity problems, e.g., in unconventional superconductors,
topological insulators, and quantum spin liquids.Comment: 27 pages, 8 figs. Review article prepared for Rep. Prog. Phys. ("key
issues" section). (v2) Final version as publishe
Electron-Electron Interactions in Graphene: Current Status and Perspectives
We review the problem of electron-electron interactions in graphene. Starting
from the screening of long range interactions in these systems, we discuss the
existence of an emerging Dirac liquid of Lorentz invariant quasi-particles in
the weak coupling regime, and strongly correlated electronic states in the
strong coupling regime. We also analyze the analogy and connections between the
many-body problem and the Coulomb impurity problem. The problem of the magnetic
instability and Kondo effect of impurities and/or adatoms in graphene is also
discussed in analogy with classical models of many-body effects in ordinary
metals. We show that Lorentz invariance plays a fundamental role and leads to
effects that span the whole spectrum, from the ultraviolet to the infrared. The
effect of an emerging Lorentz invariance is also discussed in the context of
finite size and edge effects as well as mesoscopic physics. We also briefly
discuss the effects of strong magnetic fields in single layers and review some
of the main aspects of the many-body problem in graphene bilayers. In addition
to reviewing the fully understood aspects of the many-body problem in graphene,
we show that a plethora of interesting issues remain open, both theoretically
and experimentally, and that the field of graphene research is still exciting
and vibrant.Comment: Review Article to appear in Reviews of Modern Physics. 62 pages, 44
figure
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Spin-half paramagnetism in graphene induced by point defects
Using magnetization measurements, we show that point defects in graphene -
fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments
with spin 1/2. Both types of defects lead to notable paramagnetism but no
magnetic ordering could be detected down to liquid helium temperatures. The
induced paramagnetism dominates graphene's low-temperature magnetic properties
despite the fact that maximum response we could achieve was limited to one
moment per approximately 1000 carbon atoms. This limitation is explained by
clustering of adatoms and, for the case of vacancies, by losing graphene's
structural stability.Comment: 14 pages, 14 figure
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