arXiv subjects

P. Onorato

Publications and source records attributed to P. Onorato.

At least 19 recordsLinked to original sources

arXiv2014

Librational motion of asymmetric rolling bodies and the role of friction force

We designed a very simple asymmetric rolling spool and we studied it experimentally, in the librational regime, by using a video camera and video analysis software. Comparison of experimental results and theoretical analysis allows addressing critical aspects related both to the characteristic of the oscillatory anharmonic motion of the system and to the role of friction forces in determining it. For that reasons the asymmetric rolling spool can be presented to undergraduate students as an interesting case study where to apply their physics knowledge to understand the behavior of real objects.

physics.ed-ph↗

arXiv2007

Spin separation in a T ballistic nanojunction due to lateral-confinement-induced spin-orbit-coupling

We propose a new scheme of spin filtering employing ballistic nanostructures in two dimensional electron gases (2DEGs). The proposal is essentially based on the spin-orbit (SO) interaction arising from the lateral confining electric field. This sets the basic difference with other works employing ballistic crosses and T junctions with the conventional SO term arising from 2DEG confinement. We discuss the consequences of this different approach on magnetotransport properties of the device, showing that the filter can in principle be used not only to generate a spin polarized current but also to perform an electric measurement of the spin polarization of a charge current. We focus on single-channel transport and investigate numerically the spin polarization of the current.

cond-mat.mes-hall↗

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

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

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

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↗

arXiv2005

Integer Spin Hall Effect in Ballistic Quantum Wires

We investigate the ballistic electron transport in a two dimensional Quantum Wire under the action of an electric field ($E_y$). We demonstrate how the presence of a Spin Orbit coupling, due to the uniform electric confinement field gives a non-commutative effect as in the presence of a transverse magnetic field. We discuss how the non commutation implies an edge localization of the currents depending on the electron spins also giving a semi-classical spin dependent Hall current. We also discuss how it is possible obtain a quantized Spin Hall conductance in the ballistic transport regime by developing the Landauer formalism and show the coupling between the spin magnetic momentum and the orbital one due to the presence of a circulating current.

cond-mat.mes-hall↗

arXiv2005

Modulation of Luttinger liquid exponents in multi-walled carbon nanotubes

We develop in this paper a theoretical framework that applies to the intermediate regime between the Coulomb blockade and the Luttinger liquid behavior in multi-walled carbon nanotubes. Our main goal is to confront the experimental observations of transport properties, under conditions in which the thermal energy is comparable to the spacing between the single-particle levels. For this purpose we have devised a many-body approach to the one-dimensional electron system, incorporating the effects of a discrete spectrum. We show that, in the crossover regime, the tunneling conductance follows a power-law behavior as a function of the temperature, with an exponent that oscillates with the gate voltage as observed in the experiments. Also in agreement with the experimental observations, a distinctive feature of our approach is the existence of an inflection point in the log-log plots of the conductance vs temperature, at gate voltages corresponding to peaks in the oscillation of the exponent. Moreover, we evaluate the effects of a transverse magnetic field on the transport properties of the multi-walled nanotubes. For fields of the order of 4 T, we find changes in the band structure that may be already significant for the outer shells, leading to an appreciable variation in the power-law behavior of the conductance. We then foresee the appearance of sensible modulations in the exponent of the conductance for higher magnetic fields, as the different subbands are shifted towards the development of flat Landau levels.

cond-mat.str-el↗

arXiv2005

Transport through a double barrier for interacting quasi one-dimensional electrons in a Quantum Wire in the presence of a transverse magnetic field

We discuss the Luttinger Liquid behaviour of a semiconducting Quantum Wire. We show that the measured value of the bulk critical exponent, $α_{bulk}$, for the tunneling density of states can be easily calculated. Then, the problem of the transport through a Quantum Dot formed by two Quantum Point Contacts along the Quantum Wire, weakly coupled to spinless Tomonaga-Luttinger liquids is studied, including the action of a strong transverse magnetic field $B$. The known magnetic dependent peaks of the conductance, $G(B)$, in the ballistic regime at a very low temperature, $T$, have to be reflected also in the transport at higher $T$ and in different regimes. The temperature dependence of the maximum $G_{max}$ of the conductance peak, according to the Correlated Sequential Tunneling theory, yields the power law $G_{max}\propto T^{2α_{end}-1}$, with the critical exponent, $α_{end}$, strongly reduced by $B$. This behaviour suggests the use of a similar device as a magnetic field modulated transistor.

cond-mat.mes-hall↗

arXiv2005

Magnetic field effects and renormalization of the long-range Coulomb interaction in Carbon Nanotubes

We develop two theoretical approaches for dealing with the low-energy effects of the repulsive interaction in one-dimensional electron systems. Renormalization Group methods allow us to study the low-energy behavior of the unscreened interaction between currents of well-defined chirality in a strictly one-dimensional electron system. A dimensional regularization approach is useful, when dealing with the low-energy effects of the long-range Coulomb interaction. This method allows us to avoid the infrared singularities arising from the long-range Coulomb interaction at $D = 1$. We can also compare these approaches with the Luttinger model, in order to analyze the effects of the short range term in the interaction. Thanks to these methods, we are able to discuss the effects of a strong magnetic field $B$ in quasi one-dimensional electron systems, by focusing our attention on Carbon Nanotubes. Our results imply a variation with $B$ in the value of the critical exponent $α$ for the tunneling density of states, which is in fair agreement with that observed in a recent transport experiment involving carbon nanotubes. The dimensional regularization allows us to predict the disappearance of the Luttinger liquid, when the magnetic field increases, with the formation of a chiral liquid with $α=0$.

cond-mat.str-el↗

arXiv2005

Spin-orbit coupling in a Quantum Dot at high magnetic field

We describe the simultaneous effects of the spin-orbit (SO) perturbation and a magnetic field $B$ on a disk shaped quantum dot (QD). {As it is known the} combination of electrostatic forces among the $N$ electrons confined in the QD and the Pauli principle can induce a spin polarization when $B$ (applied in the direction orthogonal to the QD) is above a threshold value. In the presence of an electric field parallel to $B$, coupled to the spin $ S $ by a Rashba term, we demonstrate that a symmetry breaking takes place: we can observe it by analyzing the splitting of the levels belonging to an unperturbed multiplet. We also discuss the competitive effects of the magnetic field, the SO perturbation and the electron electron interaction, in order to define the hierarchy of the states belonging to a multiplet. We demonstrate how this hierarchy depends on the QD's size. We show the spin texture due to the combined effects of the Rashba effect and the interaction responsible for the polarization.

cond-mat.mes-hall↗

arXiv2005

Magnetic field effects on low dimensional electron systems: Luttinger liquid behaviour in a Quantum Wire

We discuss the effects of a strong magnetic field in Quantum Wires. We show how the presence of a magnetic field modifies the role played by % which %coefficients corresponding to electron electron interaction % and the Fermi velocity producing a strong reduction of the backward scattering corresponding to the Coulomb repulsion. We discuss the consequences of this and other effects of magnetic field on the Tomonaga-Luttinger liquids and especially on their power-law behaviour in all correlation functions. The focal point is the rescaling of all the repulsive terms of the interaction between electrons with opposite momenta, due to the edge localization of the electrons and to the reduction of the length scale. Because of the same two reasons there are some interesting effects of the magnetic field concerning the backward scattering due to the presence of one impurity and the corresponding conductance. As an effect of the magnetic field we find also a spin polarization induced by a combination of electrostatic forces and the Pauli principle, quite similar to the one observed in large Quntum Dots.

cond-mat.str-el↗

arXiv2005

Crossover from Luttinger liquid to Coulomb blockade regime in carbon nanotubes

We develop a theoretical approach to the low-energy properties of 1D electron systems aimed to encompass the mixed features of Luttinger liquid and Coulomb blockade behavior observed in the crossover between the two regimes. For this aim we extend the Luttinger liquid description by incorporating the effects of a discrete single-particle spectrum. The intermediate regime is characterized by a power-law behavior of the conductance, but with an exponent oscillating with the gate voltage, in agreement with recent experimental observations. Our construction also accounts naturally for the existence of a crossover in the zero-bias conductance, mediating between two temperature ranges where the power-law behavior is preserved but with different exponent.

cond-mat.str-el↗