arXiv subjects

M. Nunez-Regueiro

Publications and source records attributed to M. Nunez-Regueiro.

10 recordsLinked to original sources

arXiv2026

Topological flat bands emerging at the inversion of stacking order in rhombohedral graphite

Motivated by the indications of high-Tc superconductivity in natural graphite enriched in the rhombohedral phase, we study the band structure of several stacking configurations that combine two of the three graphite structures as well as modifications of the rhombohedral sequence (from ABCABC... to CBACBA...), using first-principles calculations. We focus in particular on the possible emergence of flat bands near the Fermi level. When the two different rhombohedral orderings are combined, flat bands of topological origin emerge at the interface between the two domains, near the K and K' points of the Brillouin zone. Mapping a simple tight-binding model of a rhombohedral slab along the direction perpendicular to the graphene layers onto a Su-Schrieffer-Heeger chain provides a transparent understanding of the underlying physics.

cond-mat.mes-hall↗

arXiv2018

High-pressure polymorphism of BaFe2Se3

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is intrinsic to the BaFe2Se3 phase and not to FeSe with Tc > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder X-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFe2Se3 transforms into δ-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transitions is characterized by a significant shortening of the b lattice parameter of γ-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

cond-mat.supr-con↗

arXiv2009

High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe

We have studied the structural and superconducting properties of tetragonal FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller than the rest of Fe based superconductors. At 12GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high pressure orthorhombic phase has a higher Tc reaching 34K at 22GPa.

cond-mat.supr-con↗

arXiv2008

Correlated pressure effects on structure and superconductivity in LaFeAsO0.9F0.1

We have studied the structural and superconductivity properties of the compound LaFeAsO0.9F0.1 under pressures up to 32GPa using synchrotron radiation and diamond anvil cells. We obtain an ambient pressure bulk modulus K_0 = 78(2)GPa, compressibility comparable to some cuprates. At high pressures, the sample is in the overdoped region, with a linear decrease with pressure variation of the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2006

Tomonaga-Luttinger liquids and Coulomb blockade in multiwall carbon nanotubes under pressure

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger Liquids (LL). Contrary to individual MWNT, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, ~2.4, can only be accounted for by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.

cond-mat.mes-hall↗

arXiv2006

Pressure variation of Luttinger liquids parameters in single wall carbon nanotubes networks

We measure electrical transport on networks of single wall nanotube of different origin as a function of temperature T, voltage V and pressure P . We observe Luttinger liquid (LL) behavior, a conductance T^alpha and a dynamic conductance V^alpha . We observe a sample dependent P variation of the alpha parameters, interpreted as fermi level changes due to pressure induced charge transfer. We show how, through standard four-leads and crossed configuration methods, it is possible to determine alpha_bulk and alpha_end, respectively. We study and discuss the pressure and doping level dependences of the number of channels N, the LL parameter g and the intra-rope tube-tube coupling constant U .

cond-mat.mes-hall↗

arXiv2004

Pressure control of conducting channels in singlewall nanotube networks

We measure electrical transport on networks of single wall nanotube ropes as a function of temperature T, voltage V and pressure up to 22GPa. We observe Luttinger liquid (LL) behavior, a conductance ~T^alpha and a dynamic conductance ~V^alpha. With pressure conductance increases while alpha decreases, enabling us to test the theoretical prediction for LL on the alpha dependence of the T and V independent coefficient of the tunneling conductance, and to obtain the high frequency cut-off of LL modes. The possible transition to a fermi liquid at alpha -> 0 is unattainable, as nanotubes collapse to an insulating state at high pressures.

cond-mat.mes-hall↗

arXiv2004

High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)

We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K +/- 1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic $a$-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the $a$ axes is one of the keys that controls the Tc of high temperature superconductors.

cond-mat.supr-con↗

arXiv2003

Electrical resistivity of the Ti4O7 Magneli phase under high pressure

We have measured resistivity as a function of temperature and pressure of Ti4O7 twinned crystals using different contact configurations. Pressures over 4kbar depress the localization of bipolarons and allow the study of the electrical conduction of the bipolaronic phase down to low temperatures. For pressures P > 40 kbar the bipolaron formation transition is suppressed and a nearly pressure independent behavior is obtained for the resistivity. We observed an anisotropic conduction. When current is injected parallel to the principal axis, a metallic conduction with interacting carrier effects is predominant. A superconducting state was not obtained down to 1.2 K, although evidences of the proximity of a quantum critical point were noticed. While when current is injected non-parallel to the crystal's principal axis, we obtained a logarithmic divergence of the resistivity at low temperatures. For this case, our results for the high pressure regime can be interpreted in the framework of interacting carriers (polarons or bipolarons) scattered by Two Level Systems.

cond-mat.str-el↗

arXiv2001

Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry

We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a monotonous decrease of the lattice parameters and of the c/a ratio, but no structural transition down to parameters smaller than those of AlB2. The transition superconducting temperature also decreases with temperature in a sample dependent way. The results are explained by an increase of the filling of the 2D pxy bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.

cond-mat.supr-con↗