Aldo H. Brunetti

Anomalous Josephson current, incipient time-reversal symmetry breaking, and Majorana bound states in interacting multilevel dots

We study the combined effects of spin-orbit interaction, magnetic field, and Coulomb charging on the Josephson current-phase relation, I(\varphi), for a multi-level quantum dot tunnel-contacted by two conventional s-wave superconductors with phase difference \varphi. A general model is formulated and analyzed in the cotunneling regime (weak tunnel coupling) and in the deep subgap limit, fully taking into account interaction effects. We determine the conditions for observing a finite anomalous supercurrent I_a=I(\varphi=0). For a two-level dot with spin-orbit coupling and arbitrarily weak Zeeman field B, we find the onset behavior I_a\propto {\rm sgn}(B) in the presence of interactions, suggesting the incipient spontaneous breakdown of time-reversal symmetry. We also provide conditions for realizing spatially separated (but topologically unprotected) Majorana bound states in this system, which have a clear signature in the 2\pi-periodic current-phase relation.

X-ray diffraction study of polycrystalline p-chloronitrobenzene

The X-ray diffraction pattern of powdered 1-Cl-4-NO2-benzene was measured at several temperatures in order to characterize the structure of two different phases. For T < 282 K, the crystal array is ordered and exhibits P21 symmetry with two molecules in the unit cell; the observed lattice parameters at T = 190 K are a = 5.838 (4), b = 5.218 (3), c = 10.716 (5) A and β = 96.43 (5)°. A molecular arrangement inside the unit cell which minimizes the crystalline packing energy was calculated and, in combination with the Rietveld method, was used to reproduce the observed diffraction pattern. The refined structure yields an excellent agreement with the experimental results and confirms a qualitative model previously suggested to explain the measured low-frequency Raman spectrum. The high-temperature phase is monoclinic, Z = 2 with a = 3.84 (1), b = 6.80 (1), c = 13.37 (1) A and β = 97.4 (1)° at T = 290 K. As confirmed by a Rietveld refinement, this phase exhibits an orientationally disordered arrangement in which dipoles of equivalent molecules point along opposite directions. This leads to a statistically centrosymmetrical molecule and enables P21/c symmetry, in agreement with previous studies. The X-ray diffraction pattern also reveals an important increase of the background radiation which, in turn, exhibits a marked modulation of its intensity. On this basis, a simple analytical model has been developed to predict the angular dependence of diffuse scattering due to orientational disorder. The scope and limitations of this model are exhaustively discussed through a detailed comparison with the experimental results.

Dynamic orientational disorder of the nitro group in 2-chloro-nitrobenzene revealed through 35 Cl nuclear quadrupole resonance

In the present paper we analyse the contributions of nitro group movements in 2-chloro-nitrobenzene to the nuclear quadrupole resonance (NQR) parameters of the chlorine nucleus in the molecule. We found two contributions to the spin–lattice relaxation time (T1) and the NQR frequency (νQ) due to the onset of nitro group movements in the molecule. One of these contributions is the well-known semirotation of the nitro group around the N–C axis. The other one is attributed to some tilting or tipping of the nitro plane away from the benzene ring introducing some dynamic orientational disorder of this group in the crystal only observed as a contribution from the temperature dependence of T1 and νQ. Its activation energy is similar to that of the nitro group reorientation (21.9 and 23.6 kJ mol−1 for the two processes) and may arise from competing crystalline and steric chlorine nucleus effects. The present investigation shows that in chloronitrobenzenes the NO2 group dynamic orientational disorder can produce modulation effects on the chlorine T1 which are large enough to be observed by means of the NQR.

The Ordered Phase of p-Chloronitrobenzene Studied by Raman Spectroscopy

p-Chloronitrobenzene exhibit an order-disorder phase transition at 282 K. The high temperature disordered phase has P2 1 /c symmetry with two molecules per unit cell. A statistically centrosymmetric array results from the superposition of molecules occupying regular lattice sites but facing randomly opposite directions. The ordered low temperature phase structure could not be resolved yet. In the present work, Raman spectroscopy measurements were carried out as a function of temperature in the range 5 K–300 K in ordered and disordered phases of p-chloronitrobenzene. Symmetry information of the ordered unknown phase was obtained. A comparison of experimental results with Raman spectra of p-dihalobenzenes and p-dinitrobenzene has been carried out and possible molecular arrangements are discussed through lattice dynamics calculations.

Nuclear quadrupole resonance in 2,4-dinitrochlorobenzene

Abstract A study of the spin lattice relaxation (T1) and the nuclear quadrupole resonance frequency (νQ) gives an important information about the dynamics of molecular groups in molecular solids. In the present paper, we analyze the contributions of the reorientational motion of nitro groups of 2,4-dinitrochlorobenzene to the NQR parameters of the chlorine nucleus in the molecule. We found two contributions to T1 and νQ due to the onset of the reorientation of nitro groups in the molecule; one of these contributions is mostly due to intermolecular effects in the crystal. For the chlorine nuclei, the efficiency of the modulation mechanism is usually provided by the change of the electric field gradient due to the moving molecular group; this gives us a way of how to assign each contribution to T1 from the ortho and para positions of the NO2 groups in the molecule. It is observed that there are two different potential barriers depending on the position of the nitro groups in the molecule. The behavior in the temperature dependence of the line width shows a thermal history dependence of the molecular crystal.

NQR and Raman spectroscopy study of -dichlorobiphenyl

Pulsed nuclear quadrupole resonance (NQR) and Raman spectroscopy studies were performed to get information about the structural and dynamic thermal behaviour of the dichlorobiphenyl. NQR measurements of the line-shape and the spin - lattice relaxation time were obtained in the temperature range 80 K - 320 K. Raman spectra from powder samples were recorded for temperatures between 4.5 K and 300 K. Although the morphology of the dichlorobiphenyl molecule is very close to the biphenyl one, there is no evidence of any structural instability, such as the incommensurate phase transitions in biphenyl. Up to the lowest temperatures scanned, the compound seems to remain in the same crystalline ordered phase reported by x-ray diffraction at room temperature. The and Raman measurements suggest that there are no significant dynamical changes (for example, the softening of a mode) in the scanned temperature range. Some considerations are made on the size of the para-substituents of biphenyl, the crystal packing and the stability of the periodic phases.

A new ordered phase in p-chloronitrobenzene detected by NQR studies

A study of NQR frequency and line width as a function of temperature has revealed the existence of a new ordered phase in pure p-chloronitrobenzene (PCNB). This phase is stable below K. For temperatures above the disordered phase is the stable one.

A study of the bis(4-chlorophenyl) sulphide by 35Cl nuclear quadrupole resonance and ab initio calculations

As an extension of previous works in sulphur-substituted biphenyls, a complete pulsed 35Cl nuclear quadrupole resonance (NQR) study of the bis(4-chlorophenyl) sulphide compound is presented. In order to characterize the structural thermal behaviour of the solid phase, the NQR spectra and spin-lattice relaxation times were recorded from 80 K to 330 K. In addition, ab initio calculations at HF/3-21G* and 6-31G* levels were performed in order to obtain information about the equilibrium geometry and the torsional potential of the free bis(4-chlorophenyl) sulphide molecule. The NQR study results show that the title compound presents a unique stable crystalline phase in the scanned temperature range. This phase diagram contrasts notably with those observed at the same temperatures in bis(4-chlorophenyl) sulphone and bis(4-chlorophenyl) sulphoxide, which show an incommensurate phase transition and two-phase coexistence, respectively. The narrow NQR line observed suggests that the 35Cl atoms in the unit cell are in crystallographic equivalent positions. Ab initio calculations for the free molecule show that the symmetry point group is C2 and the dihedral angle is 60°. The results are discussed and compared with the known information about the sulphone and sulphoxide derivatives. The effect of the interphenyl linking group is also discussed, in order to understand the evolution of the phase diagram in this group of compounds.

NQR study of bis(4-chlorophenyl)sulphoxide

A pulsed nuclear quadrupolar resonance (NQR) study was performed on bis(4-chlorophenyl)sulphoxide. In order to characterize the structural thermal behaviour, the NQR spectra were recorded from 80 to 350 K. The results suggest the coexistence of two crystalline phases over the whole scanned temperature range. In spite of the very close molecular structure of the compound with the bis(4-chlorophenyl)sulphone, there is no evidence of any incommensurate phase transition. This fact demonstrates the fundamental role of the molecular group linking phenyl rings in the appearance of a normal - incommensurate phase transition in biphenyl-type compounds.