1,494 research outputs found
The Realization of Artificial Kondo Lattices in Nanostructured Arrays
The interplay of magnetic energies in a Kondo lattice is the underlying
physics of a heavy fermion system. Creating an artificial Kondo lattice system
by localizing the moments in an ordered metallic array provides a prototype
system to tune and study the energetic interplay while avoiding the
complications introduced by random alloying of the material. In this article,
we create a Kondo lattice system by fabricating a hexagonally ordered
nanostructured array using niobium as the host metal and cobalt as the magnetic
constituent. Electrical transport measurements and magnetoresistivity
measurements of these artificial lattices show that the competing exchange
coupling properties can be easily tuned by controlling the impurity percentage.
These artificial Kondo lattice systems enable the exploration of an artificial
superconductor which should lead to a deep understanding of the role of
magnetism in unconventional superconductors.Comment: Artificial Magnetic Crystal
Numerical renormalization-group study of the Bose-Fermi Kondo model
We extend the numerical renormalization-group method to Bose-Fermi Kondo
models (BFKMs), describing a local moment coupled to a conduction band and a
dissipative bosonic bath.
We apply the method to the Ising-symmetry BFKM with a bosonic bath spectral
function , of interest in connection with
heavy-fermion criticality. For , an interacting critical point,
characterized by hyperscaling of exponents and -scaling, describes a
quantum phase transition between Kondo-screened and localized phases.
Connection is made to other results for the BFKM and the spin-boson model.Comment: 4 pages, 4 figure
Entanglement of Two Impurities through Electron Scattering
We study how two magnetic impurities embedded in a solid can be entangled by
an injected electron scattering between them and by subsequent measurement of
the electron's state. We start by investigating an ideal case where only the
electronic spin interacts successively through the same unitary operation with
the spins of the two impurities. In this case, high (but not maximal)
entanglement can be generated with a significant success probability. We then
consider a more realistic description which includes both the forward and back
scattering amplitudes. In this scenario, we obtain the entanglement between the
impurities as a function of the interaction strength of the electron-impurity
coupling. We find that our scheme allows us to entangle the impurities
maximally with a significant probability
Spectral properties of locally correlated electrons in a BCS superconductor
We present a detailed study of the spectral properties of a locally
correlated site embedded in a BCS superconducting medium. To this end the
Anderson impurity model with superconducting bath is analysed by numerical
renormalisation group (NRG) calculations. We calculate one and two-particle
dynamic response function to elucidate the spectral excitation and the nature
of the ground state for different parameter regimes with and without
particle-hole symmetry. The position and weight of the Andreev bound states is
given for all relevant parameters. We also present phase diagrams for the
different ground state parameter regimes. This work is also relevant for
dynamical mean field theory extensions with superconducting symmetry breaking.Comment: 22 pages, 12 figure
Gate-voltage dependence of Kondo effect in a triangular quantum dot
We study the conductance through a triangular triple quantum dot, which are
connected to two noninteracting leads, using the numerical renormalization
group (NRG). It is found that the system shows a variety of Kondo effects
depending on the filling of the triangle. The SU(4) Kondo effect occurs at
half-filling, and a sharp conductance dip due to a phase lapse appears in the
gate-voltage dependence. Furthermore, when four electrons occupy the three
sites on average, a local S=1 moment, which is caused by the Nagaoka mechanism,
is induced along the triangle. The temperature dependence of the entropy and
spin susceptibility of the triangle shows that this moment is screened by the
conduction electrons via two separate stages at different temperatures. The
two-terminal and four-terminal conductances show a clear difference at the gate
voltages, where the SU(4) or the S=1 Kondo effects occurring.Comment: 4 pages, 4 figs: typos just below (4) are corrected, results are not
affecte
Spontaneous interlayer coherence in bilayer Kondo systems
Bilayer Kondo systems present interesting models to illustrate the
competition between the Kondo effect and intermoment exchange. Such bilayers
can exhibit two sharply distinct Fermi liquid phases which are distinguished by
whether or not the local moments participate in the Fermi sea. We study these
phases and the evolution from one to the other upon changing Kondo coupling. We
argue that an ordered state with spontaneous interlayer phase coherence
generically intervenes between the two Fermi liquids. Such a condensate phase
breaks a U(1) symmetry and is bounded by a finite-temperature
Kosterlitz-Thouless transition. Based on general arguments and mean-field
calculations we investigate the phase diagram and associated quantum phase
transitions.Comment: 4 pages, 3 figs, (v2) misprints in eqs corrected, final version as
publishe
Sum-rule Conserving Spectral Functions from the Numerical Renormalization Group
We show how spectral functions for quantum impurity models can be calculated
very accurately using a complete set of ``discarded'' numerical renormalization
group eigenstates, recently introduced by Anders and Schiller. The only
approximation is to judiciously exploit energy scale separation. Our derivation
avoids both the overcounting ambiguities and the single-shell approximation for
the equilibrium density matrix prevalent in current methods, ensuring that
relevant sum rules hold rigorously and spectral features at energies below the
temperature can be described accurately.Comment: 4 pages + 1 page appendix, 2 figure
Conductance of deformable molecules with interaction
Zero temperature linear response conductance of molecules with Coulomb
interaction and with various types of phonon modes is analysed together with
local occupation, local moment, charge fluctuations and fluctuations of
molecular deformation. Deformation fluctuations are quantitatively related to
charge fluctuations which exhibit similarity also to static charge
susceptibility.Comment: 4 pages, color figure
Infrared study of valence transition compound YbInCu4 using cleaved surfaces
Optical reflectivity R(w) of YbInCu4 single crystals has been measured across
its first-order valence transition at T_v ~ 42 K, using both polished and
cleaved surfaces. R(w) measured on cleaved surfaces Rc(w) was found much lower
than that on polished surface Rp(w) over the entire infrared region. Upon
cooling through T_v, Rc(w) showed a rapid change over a temperature range of
less than 2 K, and showed only minor changes with further cooling. In contrast,
Rp(w) showed much more gradual and continuous changes across T_v, similarly to
previously reported data on polished surfaces. The present result on cleaved
surfaces demonstrates that the microscopic electronic structures of YbInCu4
observed with infrared spectroscopy indeed undergo a sudden change upon the
valence transition. The gradual temperature-evolution of Rp(w) is most likely
due to the compositional and/or Yb-In site disorders caused by polishing.Comment: 4 pages, 4 figures, Fig.1(a) correcte
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