arXiv subjects

S. Bellucci

Publications and source records attributed to S. Bellucci.

At least 91 records · Page 5Linked to original sources

arXiv2007

Filtering of spin currents based on ballistic ring

Quantum interference effects in rings provide suitable means for controlling spin at mesoscopic scales. Here we apply such a control mechanism to the spin-dependent transport in a ballistic quasi one dimensional ring patterned in two dimensional electron gases (2DEGs). The study is essentially based on the {\it natural} spin-orbit (SO) interactions, one arising from the laterally confining electric field {($β$ term) and the other due to to the quantum-well potential that confines electrons in the 2DEG (conventional Rashba SO interaction or $α$ term).} We focus on single-channel transport and solve analytically the spin polarization of the current. As an important consequence of the presence of spin splitting, we find the occurrence of spin dependent current oscillations. We analyze %the effects of disorder by discussing the transport in the presence of one non-magnetic obstacle in the ring. We demonstrate that a spin polarized current can be induced when an unpolarized charge current is injected in the ring, by focusing on the central role that the presence of the obstacle plays.

cond-mat.mes-hall↗

arXiv2007

Spin Hall Effect and Spin Orbit coupling in Ballistic Nanojunctions

We propose a new scheme of spin filtering based on nanometric crossjunctions in the presence of Spin Orbit interaction, employing ballistic nanojunctions patterned in a two-dimensional electron gas. We demonstrate that the flow of a longitudinal unpolarized current through a ballistic X junction patterned in a two-dimensional electron gas with Spin Orbit coupling (SOC) induces a spin accumulation which has opposite signs for the two lateral probes. This spin accumulation, corresponding to a transverse pure spin current flowing in the junction, is the main observable signature of the spin Hall effect in such nanostructures. We benchmark the effects of two different kinds of Spin Orbit interactions. The first one ($α$-SOC) is due to the interface electric field that confines electrons to a two-dimensional layer, whereas the second one ($β$-SOC) corresponds to the interaction generated by a lateral confining potential.

cond-mat.mes-hall↗

arXiv2007

Transport through a double barrier in Large Radius Carbon Nanotubes in the presence of a transverse magnetic field

We discuss the Luttinger Liquid behaviour of Large Radius Carbon Nanotube e.g. the Multi Wall ones (MWNT), under the action of a transverse magnetic field $B$. Our results imply a reduction with $B$ in the value of the $bulk$ critical exponent, $α_{bulk}$, for the tunneling density of states, which is in agreement with that observed in transport experiments. Then, the problem of the transport through a Quantum Dot formed by two intramolecular tunneling barriers along the MWNT, weakly coupled to Tomonaga-Luttinger liquids is studied, including the action of a strong transverse magnetic field $B$. {We predict the presence of some peaks in the conductance G versus $B$, related to the magnetic flux quantization in the ballistic regime (at a very low temperature, $T$) and also at higher $T$, where the Luttinger behaviour dominates}. The temperature dependence of the maximum $G_{max}$ of the conductance peak according to the Sequential Tunneling follows a power law, $G\propto T^{γ_e-1}$ with $γ_e$ linearly dependent on the critical exponent, $α_{end}$, strongly reduced by $B$.

cond-mat.mes-hall↗

arXiv2007

Quantum Hall effect in carbon nanotubes and curved graphene strips

We develop a long wavelength approximation in order to describe the low-energy states of carbon nanotubes in a transverse magnetic field. We show that, in the limit where the square of the magnetic length $l = \sqrt{\hbar c /e B}$ is much larger than the $C$-$C$ distance times the nanotube radius $R$, the low-energy theory is given by the linear coupling of a two-component Dirac spinor to the corresponding vector potential. We investigate in this regime the evolution of the band structure of zig-zag nanotubes for values of $R/l > 1$, showing that for radius $R \approx 20$ nm a clear pattern of Landau levels start to develop for magnetic field strength $B \gtrsim 10$ T. The levels tend to be four-fold degenerate, and we clarify the transition to the typical two-fold degeneracy of graphene as the nanotube is unrolled to form a curved strip. We show that the dynamics of the Dirac fermions leads to states which are localized at the flanks of the nanotube and that carry chiral currents in the longitudinal direction. We discuss the possibility to observe the quantization of the Hall conductivity in thick carbon nanotubes, which should display steps at even multiples of $2 e^2/h$, with values doubled with respect to those in the odd-integer quantization of graphene.

cond-mat.mes-hall↗

arXiv2007

Attractors with Vanishing Central Charge

We consider the Attractor Equations of particular $\mathcal{N}=2$, d=4 supergravity models whose vector multiplets' scalar manifold is endowed with homogeneous symmetric cubic special Kähler geometry, namely of the so-called $st^{2}$ and $stu$ models. In this framework, we derive explicit expressions for the critical moduli corresponding to non-BPS attractors with vanishing $\mathcal{N}=2$ central charge. Such formulæhold for a generic black hole charge configuration, and they are obtained without formulating any \textit{ad hoc} simplifying assumption. We find that such attractors are related to the 1/2-BPS ones by complex conjugation of some moduli. By uplifting to $\mathcal{N}=8$, d=4 supergravity, we give an interpretation of such a relation as an exchange of two of the four eigenvalues of the $\mathcal{N}=8$ central charge matrix $Z_{AB}$. We also consider non-BPS attractors with non-vanishing $\mathcal{Z}$; for peculiar charge configurations, we derive solutions violating the Ansatz usually formulated in literature. Finally, by group-theoretical considerations we relate Cayley's hyperdeterminant (the invariant of the stu model) to the invariants of the st^{2} and of the so-called t^{3} model.

hep-th↗

arXiv2007

Mirror Fermat Calabi-Yau Threefolds and Landau-Ginzburg Black Hole Attractors

We study black hole attractor equations for one-(complex structure)modulus Calabi-Yau spaces which are the mirror dual of Fermat Calabi-Yau threefolds (CY_{3}s). When exploring non-degenerate solutions near the Landau-Ginzburg point of the moduli space of such 4-dimensional compactifications, we always find two species of extremal black hole attractors, depending on the choice of the Sp(4,Z) symplectic charge vector, one 1/2-BPS (which is always stable, according to general results of special Kahler geometry) and one non-BPS. The latter turns out to be stable (local minimum of the ``effective black hole potential'' V_{BH}) for non-vanishing central charge, whereas it is unstable (saddle point of V_{BH}) for the case of vanishing central charge. This is to be compared to the large volume limit of one-modulus CY_{3}-compactifications (of Type II A superstrings), in which the homogeneous symmetric special Kahler geometry based on cubic prepotential admits (beside the 1/2-BPS ones) only non-BPS extremal black hole attractors with non-vanishing central charge, which are always stable.

hep-th↗

arXiv2007

Influence of dimensionality on superconductivity in carbon nanotubes

We investigate the electronic instabilities in carbon nanotubes (CNs), looking for the break-down of the one dimensional Luttinger liquid regime due to the strong screening of the long-range part of the Coulomb repulsion. We show that such a breakdown is realized both in ultra-small single wall CNs and multi wall CNs, while a purely electronic mechanism could explain the superconductivity (SC) observed recently in ultra-small (diameter $ \sim 0.4 nm$) single wall CNs ($T_c\sim 15 ^{o}K$) and entirely end-bonded multi-walled ones ($T_c\sim 12 ^{o}K$). We show that both the doping and the screening of long-range part of the electron-electron repulsion, needed to allow the SC phase, are related to the intrinsically 3D nature of the environment where the CNs operate.

cond-mat.supr-con↗

arXiv2006

Quenching of Spin Hall Effect in Ballistic nano-junctions

We show that a nanometric four-probe ballistic junction can be used to check the presence of a transverse spin Hall current in a system with a Spin Orbit coupling not of the Rashba type, but rather due to the in-plane electric field. Indeed, the spin Hall effect is due to the presence of an effective small transverse magnetic field corresponding to the Spin Orbit coupling generated by the confining potential. The strength of the field and the junction shape characterize the quenching Hall regime, usually studied by applying semi-classical approaches. We discuss how a quantum mechanical relativistic effect, such as the Spin Orbit one, can be observed in a low energy system and explained by using classical mechanics techniques.

cond-mat.mes-hall↗

arXiv2006

Electronic screening and correlated superconductivity in carbon nanotubes

A theoretical analysis of the superconductivity observed recently in Carbon nanotubes is proposed. We argue that ultra-small (diameter $ \sim 0.4 nm$) single wall carbon nanotubes (with transition temperature $T_c\sim 15 ^{o}K$) and entirely end-bonded multi-walled ones ($T_c\sim 12 ^{o}K$) can superconduct by an electronic mechanism, basically the same in both cases. By a Luttinger liquid -like approach, one finds enhanced superconducting correlations due to the strong screening of the long-range part of the Coulomb repulsion. Based on this finding, we perform a detailed analysis on the resulting Hubbard-like model, and calculate transition temperatures of the same order of magnitude as the measured ones.

cond-mat.supr-con↗

arXiv2006

Generic N=4 supersymmetric hyper-Kähler sigma models in D=1

We analyse the geometry of four-dimensional bosonic manifolds arising within the context of $N=4, D=1$ supersymmetry. We demonstrate that both cases of general hyper-Kähler manifolds, i.e. those with translation or rotational isometries, may be supersymmetrized in the same way. We start from a generic N=4 supersymmetric three-dimensional action and perform dualization of the coupling constant, initially present in the action. As a result, we end up with explicit component actions for $N=4, D=1$ nonlinear sigma-models with hyper-Kähler geometry (with both types of isometries) in the target space. In the case of hyper-Kähler geometry with translational isometry we find that the action possesses an additional hidden N=4 supersymmetry, and therefore it is N=8 supersymmetric one.

hep-th↗

arXiv2006

Geometry of N=4, d=1 nonlinear supermultiplet

We construct the general action for $N=4, d=1$ nonlinear supermultiplet including the most general interaction terms which depend on the arbitrary function $h$ obeying the Laplace equation on $S^3$. We find the bosonic field $B$ which depends on the components of nonlinear supermultiplet and transforms as a full time derivative under N=4 supersymmetry. The most general interaction is generated just by a Fayet-Iliopoulos term built from this auxiliary component. Being transformed through a full time derivative under $N=4, d=1$ supersymmetry, this auxiliary component $B$ may be dualized into a fourth scalar field giving rise to a four dimensional $N=4, d=1$ sigma-model. We analyzed the geometry in the bosonic sector and find that it is not a hyper-Kähler one. With a particular choice of the target space metric $g$ the geometry in the bosonic sector coincides with the one which appears in heterotic $(4,0)$ sigma-model in $d=2$.

hep-th↗

arXiv2006

N=4, d=3 nonlinear electrodynamics

We construct a new off-shell $\mathcal{N}{=}4$, $d{=}3$ nonlinear vector supermultiplet. The irreducibility constraints for the superfields leave in this supermultiplet the same component content as in the ordinary linear vector supermultiplet. We present the most general sigma-model type action for the $\mathcal{N}{=}4$, $d{=}3$ electrodynamics with the nonlinear vector supermultiplet, which despite the nonlinearity of the supermultiplet may be written as an integral over a chiral superspace. This action share the most important properties with its linear counterpart. We also perform the dualization of the vector component into a scalar one and find the corresponding $\mathcal{N}{=}4$, $d{=}3$ supersymmetric action which describes new hyper-Kähler sigma-model in the bosonic sector.

hep-th↗

arXiv2006

A new N = 8 nonlinear supermultiplet

We construct a new off-shell $\mathcal{N}{=}8$, $d{=}1$ nonlinear supermultiplet $(\mathbf{4,8,4})$ proceeding from the nonlinear realization of the $\mathcal{N}{=}8$, $d{=}1$ superconformal group $OSp(4^{\star}|4)$ in its supercoset $\frac{OSp(4^{\star}|4)}{SU(2)_{\mathcal{R}}\otimes \left\{D,K\right\} \otimes SO(4)}$. The irreducibility constraints for the superfields automatically follow from appropriate covariant conditions on the $osp(4^{\star}|4)$-valued Cartan superforms. We present the most general sigma-model type action for $(\mathbf{4,8,4})$ supermultiplet. The relations between linear and nonlinear $(\mathbf{4,8,4})$ supermultiplets and linear $\mathcal{N}{=}8$ $(\mathbf{5,8,3})$ vector supermultiplet are discussed.

hep-th↗

arXiv2006

Universal Superfield Action for $N=8 \to N=4$ Partial Breaking of Global Supersymmetry in D=1

We explicitly construct N=4 worldline supersymmetric minimal off-shell actions for five options of 1/2 partial spontaneous breaking of $N=8, d=1$ Poincaré supersymmetry. We demonstrate that the action for the N=4 Goldstone supermultiplet with four fermions and four auxiliary components is a universal one. The remaining actions for the Goldstone supermultiplets with physical bosons are obtained from the universal one by off-shell duality transformations.

hep-th↗

arXiv2006

Hyper-Kaehler geometry and dualization

We demonstrate that in N=8 supersymmetric mechanics with linear and nonlinear chiral supermultiplets one may dualize two auxiliary fields into physical ones in such a way that the bosonic manifold will be a hyper-Kaehler one. The key point of our construction is about different dualizations of the two auxiliary components. One of them is turned into a physical one in the standard way through its replacement by the total time derivative of some physical field. The other auxiliary field is dualized through a Lagrange multiplier. We clarify this choice of dualization by presenting the analogy with a three-dimensional case.

hep-th↗

arXiv2006

Quantum Hall effect in carbon nanotubes

We investigate the effects of a transverse magnetic field on the transport properties of carbon nanotubes, making use of a long-wavelength description in terms of Dirac fermion fields. For values of the magnetic length smaller than the nanotube radius, we observe that the electronic states organize into incipient Landau subbands, with a highly degenerate level at zero energy. We show that only the states in dispersive branches, localized at the flanks of the nanotube, are able to transport current in the longitudinal direction. This is at the origin of the quantization of the Hall conductivity, that turns out to be given by even multiples of 2 e^2 /h. We also analyze the effects of the electron-electron interaction, showing that the magnetic field induces a suppression of the electronic correlations, reflected in particular in the enhancement of the tunneling density of states near the Fermi level.

cond-mat.mes-hall↗

arXiv2006

N=8 Mechanics in SU(2)xSU(2) Harmonic Superspace

The ${\cal N}{=}8, 1D$ analytic bi-harmonic superspace is shown to provide a natural setting for ${\cal N}{=}8$ supersymmetric mechanics associated with the off-shell multiplet ${\bf (4, 8, 4)}$ . The latter is described by an analytic superfield $q^{1,1}$, and we construct the general superfield and component actions for any number of such multiplets. The set of transformations preserving the flat superspace constraints on $q^{1,1}$ constitutes ${\cal N}{=}8$ extension of the two-dimensional Heisenberg algebra {\bf h}(2), with an operator central charge. The corresponding invariant $q^{1,1}$ action is constructed. It is unique and breaks 1D scale invariance. We also find a one-parameter family of scale-invariant $q^{1,1}$ actions which, however, are not invariant under the full ${\cal N}{=}8$ Heisenberg supergroup. Based on preserving the bi-harmonic Grassmann analyticity, we formulate ${\cal N}{=}8, 1D$ supergravity in terms of the appropriate analytic supervielbeins. For its truncated version we construct, both at the superfield and component levels, the first example of off-shell $q^{1,1}$ action with local ${\cal N}{=}8, 1D$ supersymmetry. This construction can be generalized to any number of self-interacting $q^{1,1}$ .

hep-th↗

arXiv2006

Suppression of electron-electron repulsion and superconductivity in Ultra Small Carbon Nanotubes

Recently, ultra-small-diameter Single Wall Nano Tubes with diameter of $ \sim 0.4 nm$ have been produced and many unusual properties were observed, such as superconductivity, leading to a transition temperature $T_c\sim 15^oK$, much larger than that observed in the bundles of larger diameter tubes. By a comparison between two different approaches, we discuss the issue whether a superconducting behavior in these carbon nanotubes can arise by a purely electronic mechanism. The first approach is based on the Luttinger Model while the second one, which emphasizes the role of the lattice and short range interaction, is developed starting from the Hubbard Hamiltonian. By using the latter model we predict a transition temperature of the same order of magnitude as the measured one.

cond-mat.supr-con↗