arXiv subjects

J. Caro

Publications and source records attributed to J. Caro.

15 recordsLinked to original sources

arXiv2007

Sub-threshold channels at the edges of nanoscale triple-gate silicon transistors

We investigate by low-temperature transport experiments the sub-threshold behavior of triple-gate silicon field-effect transistors. These three-dimensional nano-scale devices consist of a lithographically defined silicon nanowire surrounded by a gate with an active region as small as a few tens of nanometers, down to 50x60x35 nm^3. Conductance versus gate voltage show Coulomb-blockade oscillations with a large charging energy due to the formation of a small potential well below the gate. According to dependencies on device geometry and thermionic current analysis, we conclude that sub-threshold channels, a few nanometers wide, appear at the nanowire edges, hence providing an experimental evidence for the corner-effect.

cond-mat.mes-hall↗

arXiv2006

Transport spectroscopy of a single dopant in a gated silicon nanowire

We report on spectroscopy of a single dopant atom in silicon by resonant tunneling between source and drain of a gated nanowire etched from silicon on insulator. The electronic states of this dopant isolated in the channel appear as resonances in the low temperature conductance at energies below the conduction band edge. We observe the two possible charge states successively occupied by spin-up and spin-down electrons under magnetic field. The first resonance is consistent with the binding energy of the neutral $D^0$ state of an arsenic donor. The second resonance shows a reduced charging energy due to the electrostatic coupling of the charged $D^-$ state with electrodes. Excited states and Zeeman splitting under magnetic field present large energies potentially useful to build atomic scale devices.

cond-mat.mes-hall↗

arXiv2004

Current-induced magnetization changes in a spin valve due to incoherent emission of non-equilibrium magnons

We describe spin transfer in a ferromagnet/normal metal/ferromagnet spin-valve point contact. Spin is transferred from the spin-polarized device current to the magnetization of the free layer by the mechanism of incoherent magnon emission by electrons. Our approach is based on the rate equation for the magnon occupation, using Fermi's golden rule for magnon emission and absorption and the non-equilibrium electron distribution for a biased spin valve. The magnon emission reduces the magnetization of the free layer. For anti-parallel alignment of the magnetizations of the layers and at a critical bias a magnon avalanche occurs, characterized by a diverging effective magnon temperature. This critical behavior can result in magnetization reversal and consequently to suppression of magnon emission. However, magnon-magnon scattering can lead to saturation of the magnon concentration at a large but finite value. The further behavior depends on the parameters of the system. In particular, gradual evolution of the magnon concentration followed by a magnetization reversal is possible. Another scenario corresponds to a step-like increase of the magnon concentration followed by a slow decrease. In the latter case a spike in the differential resistance is expected due to a contribution of electron-magnon scattering. A comparison of the obtained results to existing experimental data and theoretical approches is given.

cond-mat.mes-hall↗

arXiv2003

Stark effect of shallow impurities in Si

We have theoretically studied the effect of an electric field on the energy levels of shallow donors and acceptors in silicon. An analysis of the electric field dependence of the lowest energy states in donors and acceptors is presented, taking the bandstructure into account. A description as hydrogen-like impurities was used for accurate computation of energy levels and lifetimes up to large (several MV/m) electric fields. All results are discussed in connection with atomic scale electronics and solid state quantum computation.

cond-mat↗

arXiv2003

Gate-induced ionization of single dopant atoms

Gate-induced wave function manipulation of a single dopant atom is a possible basis of atomic scale electronics. From this perspective, we analyzed the effect of a small nearby gate on a single dopant atom in a semiconductor up to field ionization. The dopant is modelled as a hydrogen-like impurity and the Schrodinger equation is solved by a variational method. We find that--depending on the separation of the dopant and the gate--the electron transfer is either gradual or abrupt, defining two distinctive regimes for the gate-induced ionization process.

cond-mat↗

arXiv2003

Conductance distribution in nanometer-sized semiconductor devices due to dopant statistics

We show that individual dopant atoms dominate the transport characteristics of nanometer sized devices, by investigating metal semiconductor diodes down to 15 nm diameter. Room temperature measurements reveal a strongly increasing scatter in the device-to-device conductance towards smaller device sizes. The low-temperature measurements exhibit pronounced features, caused by resonant tunneling through electronic states of individual dopant atoms. We demonstrate by a statistical analysis that this behavior can be explained by the presence of randomly distributed individual dopant atoms in the space charge region.

cond-mat↗

arXiv2003

Group theoretical analysis of double acceptors in a magnetic field: identification of the Si:B^+ ground state

A boron impurity in silicon binding an extra hole is known to have only one bound state at an energy of just below 2 meV. The nature of the Si:B^+ ground state is however not well established. We qualitatively analyze the behavior in a magnetic field of isolated acceptors in a tetrahedral lattice binding two holes using group theory. Applying these results, we analyze recent measurements and conclude that the ground state of B^+ is most compatible with a non-degenerate Gamma_1 state.

cond-mat↗

arXiv2003

Direct observation by resonant tunneling of the B^+ level in a delta-doped silicon barrier

We observe a resonance in the conductance of silicon tunneling devices with a delta-doped barrier. The position of the resonance indicates that it arises from tunneling through the B^+ state of the boron atoms of the delta-layer. Since the emitter Fermi level in our devices is a field-independent reference energy, we are able to directly observe the diamagnetic shift of the B^+ level. This is contrary to the situation in magneto-optical spectroscopy, where the shift is absorbed in the measured ionization energy.

cond-mat↗

arXiv1999

Nanoconstriction Microscopy of the Giant Magnetoresistance in Cobalt/Copper Spin Valves

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

cond-mat.mes-hall↗

arXiv1999

Impediments to mixing classical and quantum dynamics

The dynamics of systems composed of a classical sector plus a quantum sector is studied. We show that, even in the simplest cases, (i) the existence of a consistent canonical description for such mixed systems is incompatible with very basic requirements related to the time evolution of the two sectors when they are decoupled. (ii) The classical sector cannot inherit quantum fluctuations from the quantum sector. And, (iii) a coupling among the two sectors is incompatible with the requirement of physical positivity of the theory, i.e., there would be positive observables with a non positive expectation value.

quant-ph↗

arXiv1998

Double $Λ$ Hypernuclei and the Nuclear Medium Effective $ΛΛ$ Interaction

We fit the $^1 S_0 ΛΛ$ interaction in the nuclear medium to the masses of the experimentally known double-$Λ$ hypernuclei: $^{\phantom{6}6}_{ΛΛ}$He, $^{10}_{ΛΛ}$Be and $^{13}_{ΛΛ}$B. We derive this effective interaction from OBE Jülich $ΛΛ$-type potentials and using both Hartree-Fock and variational approaches. We find that the inclusion of $ΛΛ$ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double-$Λ$ hypernuclei to the $ωΛΛ$ coupling and the form factor at the $σΛΛ$ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double-$Λ$ hypernuclei, not discovered yet. Finally, we discard the existence of $^1 S_0$ $ΛΛ$ bound states provided the $ΛΛ-ΞN$ coupling can be neglected.

nucl-th↗

arXiv1997

Double $Λ$ and the $Λ-Λ$ Interaction

The $Λ$-$Λ$ effective interaction, in the channel $L=S=0$, in the nuclear medium is fitted to the available binding energies, $B_{ΛΛ}$, of double $Λ$ hypernuclei: $^{6}_{ΛΛ}$He, $^{10}_{ΛΛ}$Be and $^{13}_{ΛΛ}$B. The mesonic decay of these hypernuclei is also investigated. Finally, this effective interaction is used to predict the binding energies and mesonic decays widths of heavier double $Λ$ hypernuclei.

nucl-th↗

arXiv1996

Elimination of the vacuum instability for finite nuclei in the relativistic $σ$-$ω$ model

The $σ$-$ω$ model of nuclei is studied at leading order in the $1/N$ expansion thereby introducing the self consistent Hartree approximation, the Dirac sea corrections and the one fermion loop meson self energies in a unified way. For simplicity, the Dirac sea is further treated within a semiclassical expansion to all orders. The well-known Landau pole vacuum instability appearing in this kind of theories is removed by means of a scheme recently proposed in this context. The effect of such removal on the low momentum effective parameters of the model, relevant to describe nuclear matter and finite nuclei, is analyzed. The one fermion loop meson self energies are found to have a sizeable contribution to these parameters. However, such contribution turns out to come mostly from the Landau poles and is thus spurious. We conclude that the fermionic loop can only be introduced consistently in the $σ$-$ω$ nuclear model if the Landau pole problem is dealt with properly.

nucl-th↗

arXiv1996

Semiclassical treatment of the Dirac sea contribution for finite nuclei

Dirac sea corrections for bulk properties of finite nuclei are computed within a self-consistent scheme in the $σ$-$ω$ model. The valence part is treated in the Hartree approximation whereas the sea contribution is evaluated semiclassically up to fourth order in $\hbar$. Numerically, we find a quick convergence of the semiclassical expansion; the fourth order contributing much less than one percent to the binding energy per nucleon.

nucl-th↗

arXiv1994

Semiclassical expansions up to $\hbar^4$-order in relativistic nuclear physics

We present the first calculation of the $\hbar^4$-Wigner--Kirkwood corrections to a relativistic system of fermions in the presence of external scalar and vector potentials. The method we propose allows to compute efficiently semiclassical corrections to one body operators such as mean energies and local densities. It also preserves gauge invariance and produces explicitly convergent results despite some apparent divergencies at the classical turning points. As a byproduct we obtain the $\hbar^4$ corrections stemming from the polarization of the Dirac sea. We apply our results to the relativistic $σ$-$ω$ Lagrangian in the Hartree valence approximation. We compare the semiclassical expansion with the exact result and with the Strutinsky average whenever it can be obtained. We find that the $\hbar^4 $ corrections are much smaller than typical shell effects. Our results provide convincing arguments to neglect higher than second order $\hbar$ effects in the Wigner--Kirkwood scheme to relativistic nuclear physics in the mean field approximation.

nucl-th↗