29 research outputs found
Spin waves in diluted magnetic quantum wells
We study collective spin excitations in two-dimensional diluted magnetic
semiconductors, placed into external magnetic field. Two coupled modes of the
spin waves (the electron and ion modes) are found to exist in the system along
with a number of the ion spin excitations decoupled from the electron system.
We calculate analytically the spectrum of the waves taking into account the
exchange interaction of itinerant electrons both with each other and with
electrons localized on the magnetic ions. The interplay of these interactions
leads to a number of intriguing phenomena including tunable anticrossing of the
modes and a field-induced change in a sign of the group velocity of the ion
mode
Formalism for obtaining nuclear momentum distributions by the Deep Inelastic Neutron Scattering technique
We present a new formalism to obtain momentum distributions in condensed
matter from Neutron Compton Profiles measured by the Deep Inelastic Neutron
Scattering technique. The formalism describes exactly the Neutron Compton
Profiles as an integral in the momentum variable . As a result we obtain a
Volterra equation of the first kind that relates the experimentally measured
magnitude with the momentum distributions of the nuclei in the sample. The
integration kernel is related with the incident neutron spectrum, the total
cross section of the filter analyzer and the detectors efficiency function. A
comparison of the present formalism with the customarily employed approximation
based on a convolution of the momentum distribution with a resolution function
is presented. We describe the inaccuracies that the use of this approximation
produces, and propose a new data treatment procedure based on the present
formalism.Comment: 11 pages, 8 figure
Collective modes of the massless Dirac plasma
We develop a theory for the long-wavelength plasma oscillation of a
collection of charged massless Dirac particles in a solid, as occurring for
example in doped graphene layers, interacting via the long-range Coulomb
interaction. We find that the long-wavelength plasmon frequency in such a doped
massless Dirac plasma is explicitly non-classical in all dimensions with the
plasma frequency being proportional to \hbar^{-1/2}. We also show that the long
wavelength plasma frequency of the D-dimensional superlattice made from such a
plasma does not agree with the corresponding (D + 1)-dimensional bulk plasmon
frequency. We compare and contrast such Dirac plasmons with the well-studied
regular palsmons in metals and doped semiconductors which manifest the usual
classical long wavelength plasma oscillation.Comment: 5 page
Raman scattering through a metal-insulator transition
The exact solution for nonresonant A1g and B1g Raman scattering is presented
for the simplest model that has a correlated metal-insulator transition--the
Falicov-Kimball model, by employing dynamical mean field theory. In the general
case, the A1g response includes nonresonant, resonant, and mixed contributions,
the B1g response includes nonresonant and resonant contributions (we prove the
Shastry-Shraiman relation for the nonresonant B1g response) while the B2g
response is purely resonant. Three main features are seen in the nonresonant
B1g channel: (i) the rapid appearance of low-energy spectral weight at the
expense of higher-energy weight; (b) the frequency range for this low-energy
spectral weight is much larger than the onset temperature, where the response
first appears; and (iii) the occurrence of an isosbestic point, which is a
characteristic frequency where the Raman response is independent of temperature
for low temperatures. Vertex corrections renormalize away all of these
anomalous features in the nonresonant A1g channel. The calculated results
compare favorably to the Raman response of a number of correlated systems on
the insulating side of the quantum-critical point (ranging from Kondo
insulators, to mixed-valence materials, to underdoped high-temperature
superconductors). We also show why the nonresonant B1g Raman response is
``universal'' on the insulating side of the metal-insulator transition.Comment: 12 pages, 11 figures, ReVTe
Repulsion of Single-well Fundamental Edge Magnetoplasmons in Double Quantum Wells
A {\it microscopic} treatment of fundamental edge magnetoplasmons (EMPs)
along the edge of a double quantum well (DQW) is presented for strong magnetic
fields, low temperatures, and total filling factor \nu=2. It is valid for
lateral confining potentials that Landau level (LL) flattening can be
neglected. The cyclotron and Zeeman energies are assumed larger than the DQW
energy splitting \sqrt{\Delta^2 +4T^2}, where \Delta is the splitting of the
isolated wells and T the tunneling matrix element. %hen calculated unperturbed
density profile is sharp at the edge. Using a random-phase approximation (RPA),
which includes local and nonlocal contributions to the current density, it is
shown that for negligible tunnel coupling 2T << \Delta the inter-well Coulomb
coupling leads to two DQW fundamental EMPs which are strongly renormalized in
comparison with the decoupled, single-well fundamental EMP. These DQW modes can
be modified further upon varying the inter-well distance d, along the z axis,
and/or the separation of the wells' edges \Delta y along the y axis. The charge
profile of the {\it fast} and {\it slow} DQW mode varies, respectively, in an
{\it acoustic} and {\it optical} manner along the y axis and is not smooth on
the \ell_{0} scale. For strong tunneling \Delta\alt 2T these DQW modes are
essentially modified when \Delta is changed by applying a transverse electric
field to the DQW.Comment: Text 18 pages in Latex/Revtex/Preprint format, 2 Postscript figure
Random-phase Approximation Treatment Of Edge Magnetoplasmons: Edge-state Screening And Nonlocality
A random-phase approximation (RPA) treatment of edge magnetoplasmons (EMP) is
presented for strong magnetic fields, low temperatures, and integer filling
factors \nu. It is valid for negligible dissipation and lateral confining
potentials smooth on the scale of the magnetic length \ell_{0} but sufficiently
steep that the Landau-level (LL) flattening can be neglected. LL coupling,
screening by edge states, and nonlocal contributions to the current density are
taken into account. In addition to the fundamental mode with typical dispersion
relation \omega\sim q_x \ln(q_{x}), fundamental modes with {\it acoustic}
dispersion relation \omega\sim q_x are obtained for \nu>2. For \nu=1,2 a {\bf
dipole} mode exists, with dispersion relation \omega\sim q_x^3, that is
directly related to nonlocal responses.Comment: Text 12 pages in Latex/Revtex format, 4 Postscript figure
Role of electronic excitations in magneto-Raman spectra of graphene
We investigate the signature of the low-energy electronic excitations in the Raman spectrum of monolayer and bilayer graphenes. The dominant contribution to the Raman spectra is due to the interband electron-hole (e-h) pairs, which belong to the irreducible representation A(2) of the point group C-6v of the graphene lattice, and are characterized by crossed polarization of incoming and outgoing photons. At high magnetic fields, this is manifested by the excitation of e-h inter-Landau-level (LL) transitions with selection rule n(-) -> n(+). Weaker Raman-active inter-LL modes also exist. One of those has a selection rule similar to the infrared absorption process, n(-) -> (n +/- 1)(+), but the created e-h excitation belongs to the irreducible representation E-2 (rather than E-1) and couples to the optical phonon mode, thus undergoing an anticrossing with the optical phonon G-line in Raman in a strong magnetic field. The fine structure acquired by the G-line due to such anticrossing depends on the carrier density, inhomogeneity of doping and presence of inhomogeneous strain in the sample
Direct observation of tunneling in KDP using neutron Compton scattering
Neutron Compton Scattering measurements presented here of the momentum
distribution of hydrogen in (KDP) just above and well below the
ferroelectric transition temperature show clearly that the proton is coherent
over both sites in the in the high temperature phase, a result that invalidates
the commonly accepted order-disorder picture of the transition.
The Born-Oppenheimer potential for the hydrogen, extracted directly from data
for the first time, is consistent with neutron diffraction data, and the
vibrational spectrum is in substantial agreement with infrared absorption
measurements. The measurements are sensitive enough to detect the effect of
surrounding ligands on the hydrogen bond, and can be used to study the
systematic effect of the variation of these ligands in other hydrogen bonded
systems.Comment: 5 pages, 3 figure