115 research outputs found
Tunable "Doniach Phase Diagram" for strongly-correlated nanoclusters
Exact diagonalization calculations reveal that the energy spacing in
the conduction band tunes the interplay between the {\it local} Kondo and {\it
non local} RKKY interactions, giving rise to a "Doniach phase diagram" for a
nanocluster with regions of prevailing Kondo or RKKY correlations. The parity
of the total number of electrons alters the competition between the Kondo and
RKKY correlations. This interplay may be relevant to experimental realizations
of small rings or quantum dots with tunable magnetic properties. Below a
critical value V of the hybridization the susceptibility exhibits a low-T
exponential activation behavior determined by the interplay of the spin gap and
.Comment: 4 pages, 5 figure
Tuning the magnetism of ordered and disordered strongly-correlated electron nanoclusters
Recently, there has been a resurgence of intense experimental and theoretical
interest on the Kondo physics of nanoscopic and mesoscopic systems due to the
possibility of making experiments in extremely small samples. We have carried
out exact diagonalization calculations to study the effect of energy spacing
in the conduction band states, hybridization, number of electrons, and
disorder on the ground-state and thermal properties of strongly-correlated
electron nanoclusters. For the ordered systems, the calculations reveal for the
first time that tunes the interplay between the {\it local} Kondo and
{\it non local} RKKY interactions, giving rise to a "Doniach phase diagram" for
the nanocluster with regions of prevailing Kondo or RKKY correlations. The
interplay of and disorder gives rise to a versus
concentration T=0 phase diagram very rich in structure. The parity of the total
number of electrons alters the competition between the Kondo and RKKY
correlations. The local Kondo temperatures, , and RKKY interactions depend
strongly on the local environment and are overall {\it enhanced} by disorder,
in contrast to the hypothesis of ``Kondo disorder'' single-impurity models.
This interplay may be relevant to experimental realizations of small rings or
quantum dots with tunable magnetic properties.Comment: 10 pages, 13 figures, to appear in Physics of Spin in Solids:
Materials, Methods, and Applications, (2004
Basis of strong change of hybridization‐induced magnetic ordering between CeSb and CeTe
A sharp change in the nature of the magnetic ordering has been observed on going from CeSb to CeTe, both of which have NaC1 structures with a small decrease in lattice parameter. This is an interesting example of the way in which hybridization of partially delocalized f electrons with band electrons gives rise to highly unusual magnetic properties which show great chemical sensitivity. In the present paper we apply our previous a b i n i t i o treatment of hybridization‐induced effects to investigate this striking change in magnetic behavior. We have performed self‐consistent warped muffin‐tin LMTO band calculations treating the Ce 4f states as resonance states that are constrained to be localized. Compared to CeSb, the anion‐derived p bands in CeTe sink well below the Fermi energy, thus strongly changing the band‐f hybridization. We have calculated the hybridization dressing of the crystal‐field levels and the anisotropic two‐ion exchange interaction and compared them with those calculated for CeSb and with experiment. A strong decrease in the two‐ion interaction explains the drastic change in observed magnetic behavior between CeSb and CeTe
Voltage Dependence of Spin Transfer Torque in Magnetic Tunnel Junctions
Theoretical investigations of spin transfer torque in magnetic tunnel
junctions using the tight-binding model in the framework of non-equilibrium
Green functions formalism are presented. We show that the behavior of the spin
transfer torque as a function of applied voltage can vary over a wide range
depending on the band parameters of the ferromagnetic electrodes and the
insulator that comprise the magnetic tunnel junction. The behavior of both the
parallel and perpendicular components of the spin torque is addressed. This
behavior is explained in terms of the spin and charge current dependence and on
the interplay between evanescent states in the insulator and the Fermi surfaces
of ferromagnetic electrodes comprising the junction. The origin of the
perpendicular (field-like) component of spin transfer torque at zero bias, i.e.
exchange coupling through the barrier between ferromagnetic electrodes is
discussed.Comment: 5 pages,4 figure
Novel Family of Chiral-Based Topological Insulators: Elemental Tellurium under Strain
Employing ab initio electronic structure calculations, we predict that trigonal tellurium consisting of weakly interacting helical chains undergoes a trivial insulator to strong topological insulator (metal) transition under shear (hydrostatic or uniaxial) strain. The transition is demonstrated by examining the strain evolution of the band structure, the topological Z_2 invariant and the concomitant band inversion. The underlying mechanism is the depopulation of the lone-pair orbitals associated with the valence band via proper strain engineering. Thus, Te becomes the prototype of a novel family of chiral-based three-dimensional topological insulators with important implications in spintronics, magneto-optics, and thermoelectrics
Strongly Correlated Cerium Systems: Non-Kondo Mechanism for Moment Collapse
We present an ab initio based method which gives clear insight into the
interplay between the hybridization, the coulomb exchange, and the
crystal-field interactions, as the degree of 4f localization is varied across a
series of strongly correlated cerium systems. The results for the ordered
magnetic moments, magnetic structure, and ordering temperatures are in
excellent agreement with experiment, including the occurence of a moment
collapse of non-Kondo origin. In contrast, standard ab initio density
functional calculations fail to predict, even qualitatively, the trend of the
unusual magentic properties.Comment: A shorter version of this has been submitted to PR
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Electronic Structure Calculations of an Oxygen Vacancy in KH2PO4
We present first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of an oxygen vacancy in KH{sub 2}PO{sub 4} (KDP). Even though the overall DOS profiles for the defective KDP are quite similar to those of the perfect KDP, the oxygen vacancy in the neutral and +1 charge states induces defect states in the band gap. For the neutral oxygen vacancy, the gap states are occupied by two electrons. The difference between the integral of the total density of states (DOS) and the sum of the DOS projected on the atoms of 0.98 |e|, indicates that one of the two electrons resulting from the removal of the oxygen atom is trapped in the vacancy, while the other tends to delocalize in the neighboring atoms. For the +1 charge oxygen vacancy, the addition of the hole reduces the occupation of the filled gap-states in the neutral case from two to one electron and produces new empty states in the gap. The new empty gap states are very close to the highest occupied states, leading to a dramatic decrease of the band gap. The difference between the integral of the total DOS and the sum of the DOS projected on the atoms is 0.56 |e|, which implies that more than 56% of the redundant electron is trapped in the oxygen vacancy, and 44% spreads over the neighboring atoms. In sharp contrast, no defect states appear in the energy gap for the +2 charge O vacancy. Thus, the addition of the two holes completely compensates the two redundant electrons, and removes in turn the occupied gap states in the neutral case
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Novel Family of Chiral-Based Topological Insulators: Elemental Tellurium under Strain
Employing ab initio electronic structure calculations, we predict that trigonal tellurium consisting of weakly interacting helical chains undergoes a trivial insulator to strong topological insulator (metal) transition under shear (hydrostatic or uniaxial) strain. The transition is demonstrated by examining the strain evolution of the band structure, the topological Z(2) invariant and the concomitant band inversion. The underlying mechanism is the depopulation of the lone-pair orbitals associated with the valence band via proper strain engineering. Thus, Te becomes the prototype of a novel family of chiral-based three-dimensional topological insulators with important implications in spintronics, magneto-optics, and thermoelectrics. DOI: 10.1103/PhysRevLett.110.17640
Generalized stacking fault energy surfaces and dislocation properties of aluminum
We have employed the semidiscrete variational generalized Peierls-Nabarro
model to study the dislocation core properties of aluminum. The generalized
stacking fault energy surfaces entering the model are calculated by using
first-principles Density Functional Theory (DFT) with pseudopotentials and the
embedded atom method (EAM). Various core properties, including the core width,
splitting behavior, energetics and Peierls stress for different dislocations
have been investigated. The correlation between the core energetics and
dislocation character has been explored. Our results reveal a simple
relationship between the Peierls stress and the ratio between the core width
and atomic spacing. The dependence of the core properties on the two methods
for calculating the total energy (DFT vs. EAM) has been examined. The EAM can
give gross trends for various dislocation properties but fails to predict the
finer core structures, which in turn can affect the Peierls stress
significantly (about one order of magnitude).Comment: 25 pages, 12 figure
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