112 research outputs found
Surface phonon propagation in topological insulators
The effect of helical Dirac states on surface phonons in a topological
insulators is investigated. Their coupling is derived in the continuum limit by
assuming displacement dependent Dirac cones. The resulting renormalisation of
sound velocity and attenuation and its dependence on chemical potential and
wave vector is calculated. At finite wave vectors a Kohn anomaly in the
renormalized phonon frequency is caused by intraband-transitions. It appears at
wave vectors q<2k_F due to a lack of backscattering for helical Dirac
electrons. The wave vector and chemical potential dependence of this anomaly is
calculated.Comment: 6 pages, 3 figures, to appear in Phys. Rev.
Magnetic torque oscillations from warped helical surface states in topological insulators
A magnetic torque method is proposed that probes the warping and mass gap of
Dirac cone surface states in topological insulators like Bi2X3 (X=Se,Te). A
rotating field parallel to the surface induces a paramagnetic moment in the
helical surface states for nonzero warping. It is non-collinear with the
applied field and therefore produces torque oscillations as function of the
field angle which are a direct signature of the surface states. The torque
dependence on field strength and angle, the chemical potential and the Dirac
cone parameters like warping strength and mass gap is calculated. It is shown
that the latter leads to a symmetry reduction in the fourfold torque
oscillations.Comment: 6 pages, 4 figure
Field induced spin exciton doublet splitting in d_{x^2-y^2}-wave 115-heavy electron superconductors
We investigate the spin-exciton modes in the superconducting d_{x^2-y^2}
state of CeMIn_5 heavy fermion compounds found at the antiferromagnetic wave
vector by inelastic neutron scattering. We present a theoretical model that
explains the field dependence for both field directions. We show that the
recently observed splitting of the spin exciton doublet in CeCoIn_5 into two
non-degenerate modes for in-plane field appears naturally in this model. This
is due to the spin anisotropy of g- factors and quasiparticle interactions
which lead to different resonant conditions for the dynamic susceptibility
components. We predict that the splitting of the spin resonance doublet becomes
strongly nonlinear for larger fields when the energy of both split components
decreases. For field along the tetragonal axis no splitting but only a
broadening of the resonance is found in agreement with experiment.Comment: 8 pages, 5 figure
Thermodynamics of anisotropic triangular magnets with ferro- and antiferromagnetic exchange
We investigate thermodynamic properties like specific heat and
susceptibility in anisotropic - triangular quantum spin
systems (). As a universal tool we apply the finite temperature Lanczos
method (FTLM) based on exact diagonalization of finite clusters with periodic
boundary conditions. We use clusters up to sites where the thermodynamic
limit behavior is already stably reproduced. As a reference we also present the
full diagonalization of a small eight-site cluster. After introducing model and
method we discuss our main results on and . We show the
variation of peak position and peak height of these quantities as function of
control parameter . We demonstrate that maximum peak positions and
heights in N\'eel phase and spiral phases are strongly asymmetric, much more
than in the square lattice - model. Our results also suggest a
tendency to a second side maximum or shoulder formation at lower temperature
for certain ranges of the control parameter. We finally explicitly determine
the exchange model of the prominent triangular magnets CsCuCl and
CsCuBr from our FTLM results.Comment: 13 pages, 12 figure
Collective spin resonance excitation in the gapped itinerant multipole hidden order phase of URu2Si2
An attractive proposal for the hidden order (HO) in the heavy electron
compound URu2Si2 is an itinerant multipole order of high rank. It is due to the
pairing of electrons and holes centered on zone center and boundary,
respectively in states that have maximally different total angular momentum
components. Due to the pairing with commensurate zone boundary ordering vector
the translational symmetry is broken and a HO quasiparticle gap opens below the
transition temperature T_HO. Inelastic neutron scattering (INS) has
demonstrated that for T<T_HO the collective magnetic response is dominated by a
sharp spin exciton resonance at the ordering vector Q that is reminiscent of
spin exciton modes found inside the gap of unconventional superconductors and
Kondo insulators. We use an effective two-orbital tight binding model
incorporating the crystalline electric field effect to derive closed
expressions for quasiparticle bands reconstructed by the multipolar pairing
terms. We show that the magnetic response calculated within that model exhibits
the salient features of the resonance found in INS. We also use the calculated
dynamical susceptibility to explain the low temperature NMR relaxation rate.Comment: 13 pages, 8 figure
Gap function of hexagonal pnictide superconductor SrPtAs from quasiparticle interference spectrum
The pnictide superconductor SrPtAs has a hexagonal layered structure
containing inversion symmetry. It is formed by stacking two inequivalent PtAs
layers separated by Sr layers. The former have no local (in-plane) inversion
symmetry and therefore a (layer-) staggered Rashba spin orbit coupling appears
which splits the three Kramers degenerate bands into six quasi-2D bands. The
symmetry of the superconducting state of SrPtAs is unknown. Three candidates,
spin-singlet and as well as triplet states have been
proposed. We predict the quasiparticle interference (QPI) spectrum for these
gap functions in t-matrix Born approximation. We show that distinct differences
in the pattern of characteristic QPI wave vectors appear. These results may be
important to determine the gap symmetry of SrPtAs by STM-QPI method.Comment: 5 pages, 4 figure
Inelastic magnetic scattering effect on LDOS of topological insulators
Magnetic ions such as Fe, Mn and Co with localized spins may be adsorbed on
the surface of topological insulators like \Bi. They form scattering centers
for the helical surface states which have a Dirac cone dispersion as long as
the local spins are disordered. However, the local density of states (LDOS) may
be severely modified by the formation of bound states. Commonly only elastic
scattering due to normal and exchange potentials of the adatom is assumed.
Magnetization measurements show, however, that considerable magnetic single ion
anisotropies exist which lead to a splitting of the local impurity spin states
resulting in a singlet ground state. Therefore inelastic scattering processes
of helical Dirac electrons become possible as described by a dynamical local
self energy of second order in the exchange interaction. The self energy
influences bound state formation and leads to significant new anomalies in the
LDOS at low energies and low temperatures which we calculate within T-matrix
approach. We propose that they may be used for spectroscopy of local impurity
spin states by appropriate tuning of chemical potential and magnetic field.Comment: 10 pages, 9 figures; published version; Fig.9b and five references
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Multiorbital and hybridization effects in the quasiparticle interference of triplet superconductor Sr2RuO4
The tetragonal compound Sr2RuO4 exhibits a chiral p-wave superconducting (SC)
state of its three t2g-type conduction bands. The characteristics of
unconventional gap structure are known from experiment, in particular
field-angle resolved specific heat measurements and from microscopic theories.
A rotated extremal structure on the main active SC band with respect to the
nodal gaps on the passive bands was concluded. We propose that this gap
structure can be further specified by applying the STM- quasiparticle
interference (QPI) method. We calculate the QPI spectrum within a three band
and chiral three gap model and give closed analytical expressions. We show that
as function of bias voltage the chiral three gap model will lead to
characteristic changes in QPI that may be identified and may be used for more
quantitative gap determination of the chiral gap structure.Comment: 9 pages, 5 figure
Magnetotransport and thermoelectricity in disordered graphene
We have studied the electric and thermal response of two-dimensional
Dirac-fermions in a quantizing magnetic field in the presence of localized
disorder. The electric and heat current operators in the presence of magnetic
field are derived. The self-energy due to impurities is calculated
self-consistently, and depends strongly on the frequency and field strength,
resulting in asymmetric peaks in the density of states at the Landau level
energies, and small islands connecting them. The Shubnikov-de Haas oscillations
remain periodic in 1/B, in spite of the distinct quantization of quasiparticle
orbits compared to normal metals. The Seebeck coefficient depends strongly on
the field strength and orientation. For finite field and chemical potential,
the Wiedemann-Franz law can be violated.Comment: 10 pages, 6 figure
Frustrated two dimensional quantum magnets
We overview physical effects of exchange frustration and quantum spin
fluctuations in (quasi-) two dimensional (2D) quantum magnets () with
square, rectangular and triangular structure. Our discussion is based on the
- type frustrated exchange model and its generalizations. These
models are closely related and allow to tune between different phases,
magnetically ordered as well as more exotic nonmagnetic quantum phases by
changing only one or two control parameters. We survey ground state properties
like magnetization, saturation fields, ordered moment and structure factor in
the full phase diagram as obtained from numerical exact diagonalization
computations and analytical linear spin wave theory. We also review finite
temperature properties like susceptibility, specific heat and magnetocaloric
effect using the finite temperature Lanczos method. This method is powerful to
determine the exchange parameters and g-factors from experimental results. We
focus mostly on the observable physical frustration effects in magnetic phases
where plenty of quasi-2D material examples exist to identify the influence of
quantum fluctuations on magnetism.Comment: 78 pages, 54 figure
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