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M. Bianchi

Publications and source records attributed to M. Bianchi.

At least 19 recordsLinked to original sources
arXiv2026

Integrable models of inflation beyond slow-roll

We propose a novel analytic approach to the study of multi-component FLRW cosmologies and their perturbations. The dynamics is triggered by a single scalar field with a scalar potential codifying the energy density and pressure of the multi-component fluid. This description unifies standard Big Bang cosmologies, models of inflation and dark energy under a unique framework. The key to integrability is to express the scalar potential in terms of the Hubble function $H(\phi)$ that plays the role of a fake superpotential, turning the dynamics into a first order problem, that may be analytically solved in a suitable time coordinate. In this framework, we propose integrable inflationary models with similar properties to the ones analysed in the literature and compatible with observations. Finally, we study scalar and tensor cosmological perturbations in each model by integrating Mukhanov-Sasaki equations via numerical and (semi-)analytic techniques. This allow us to compute the power spectrum, the spectral indices and the tensor-to-scalar ratio beyond the slow-roll approximation and compare our general results {against} the currently available observations and the theoretical predictions based on the slow-roll approximation.

hep-th↗
arXiv2023

The CUFF, Clenching Upper-limb Force Feedback wearable device: design, characterization and validation

This paper presents the design, characterization and validation of a wearable haptic device able to convey skin stretch, force feedback, and a combination of both, to the user's arm. In this work, we carried out physical and perceptual characterization with eleven able-bodied participants as well as two experiments of discrimination and manipulation task hiring a total of 32 participants. In both the experiments the CUFF was used in conjunction with the Pisa/IIT SoftHand. The first experiment was a discrimination task where the subjects had to recognize the dimension and the softness between pair of cylinder. in the second experiment the subjects were asked to control the robotic hand for grasping objects. After the experiments the subjects underwent to a subjective evaluation of the device. Results of the experiments and questionnaire showed the effectiveness of the proposed device. Thank to its versatility and structure, the device could be a viable solution for teleoperation application, guidance and rehabilitation tasks, including prosthesis applications.

cs.HC↗
arXiv2022

Conjugacy classes of maximal cyclic subgroups of metacyclic $p$-groups

In this paper, we set $η(G)$ to be the number of conjugacy classes of maximal cyclic subgroups of a finite group $G$. We compute $η(G)$ for all metacyclic $p$-groups. We show that if $G$ is a metacyclic $p$-group of order $p^n$ that is not dihedral, generalized quaternion, or semi-dihedral, then $η(G) \ge n-2$, and we determine when equality holds.

math.GR↗
arXiv2022

Conjugacy classes of maximal cyclic subgroups

In this paper, we set $η(G)$ to be the number of conjugacy classes of maximal cyclic subgroups of $G$. We consider $η$ and direct and semi-direct products. We characterize the normal subgroups $N$ so that $η(G/N) = η(G)$. We set $G^- = \{ g \in G \mid \langle g \rangle {\rm ~is~not ~maximal~cyclic} \}$. We show if $\langle G^- \rangle < G$, then $G/\langle G^- \rangle$ is either (1) an elementary abelian $p$-group for some prime $p$, (2) a Frobenius group whose Frobenius kernel is a $p$-group of exponent $p$ and a Frobenius complement has order $q$ for distinct primes $p$ and $q$, or (3) isomorphic to $A_5$.

math.GR↗
arXiv2021

Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors

The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS$_2$ and WS$_2$ on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9-2.3 eV between our systems. The transient conduction band signals decay on a sub-100 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe two timescales on the order of 200 fs and 50 ps, respectively, for the conduction band decay in MoS$_2$. These multiple timescales are explained by Auger recombination involving MoS$_2$ and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.

cond-mat.mtrl-sci↗
arXiv2019

The occupied electronic structure of ultrathin boron doped diamond

Using angle-resolved photoelectron spectroscopy, we compare the electronic band structure of an ultrathin (1.8 nm) δ-layer of boron-doped diamond with a bulk-like boron doped diamond film (3 μm). Surprisingly, the measurements indicate that except for a small change in the effective mass, there is no significant difference between the electronic structure of these samples, irrespective of their physical dimensionality. While this suggests that, at the current time, it is not possible to fabricate boron-doped diamond structures with quantum properties, it also means that nanoscale doped diamond structures can be fabricated which retain the classical electronic properties of bulk-doped diamond, without a need to consider the influence of quantum confinement.

cond-mat.mtrl-sci↗
arXiv2018

Influence of an anomalous temperature-dependence of the phase coherence length on the conductivity of magnetic topological insulators

Magnetotransport constitutes a useful probe to understand the interplay between electronic band topology and magnetism in spintronics devices based on topological materials. A recent theory of Lu and Shen [Phys. Rev. Lett. 112, 146601 (2014)] on magnetically doped topological insulators predicts that quantum corrections $Δκ$ to the temperature-dependence of the conductivity can change sign during the Curie transition. This phenomenon has been attributed to a suppression of the Berry phase of the topological surface states at the Fermi level, caused by a magnetic energy gap. Here, we demonstrate experimentally that $Δκ$ can reverse its sign even when the Berry phase at the Fermi level remains unchanged, provided that the inelastic scattering length decreases with temperature below the Curie transition.

cond-mat.mtrl-sci↗
arXiv2018

Probing Fuzzballs with Particles, Waves and Strings

We probe D1D5 micro-state geometries with massless particles, waves and strings. To this end, we study geodetic motion, Klein-Gordon equation and string scattering in the resulting gravitational background. Due to the reduced rotational symmetry, even in the simple case of a circular fuzzball, the system cannot be integrated elementarily. Yet, for motion in the plane of the string profile or in the orthogonal plane to it, one can compute the deflection angle or the phase shift and identify the critical impact parameter, at which even a massless probe is captured by the fuzzball if its internal momentum is properly tuned. We find agreement among the three approaches, thus giving further support to the fuzzball proposal at the dynamical level.

hep-th↗
arXiv2016

Non-magnetic ground state of Ni adatoms on Te-terminated bismuth chalcogenide topological insulators

We report on the quenching of single Ni adatom moments on Te-terminated Bi2Te2Se and Bi2Te3 topological insulator surfaces. The effect becomes manifested as a missing X-ray magnetic circular dichroism for resonant L3,2 transitions into partially filled Ni 3d states of occupancy n_d = 9.2. On the basis of a comparative study of Ni and Fe using scanning tunneling microscopy and ab initio calculations we are able to relate the element specific moment formation to a local Stoner criterion. While Fe adatoms form large spin moments of m_s = 2.54 mu_B with out-of-plane anisotropy due to a sufficiently large density of states at the Fermi energy, Ni remains well below an effective Stoner threshold for local moment formation. With the Fermi level remaining in the bulk band gap after adatom deposition, non-magnetic Ni and preferentially out-of-plane oriented magnetic Fe with similar structural properties on Bi2Te2Se surfaces constitute a perfect platform to study off-on effects of time-reversal symmetry breaking on topological surface states.

cond-mat.mtrl-sci↗
arXiv2016

Topological insulator homojunctions including magnetic layers: the example of n-p type ($n$-QLs Bi$_2$Se$_3$/Mn-Bi$_2$Se$_3$) heterostructures

Homojunctions between Bi$_2$Se$_3$ and its Mn-doped phase are investigated as a sample geometry to study the influence of spin degrees of freedom on topological insulator properties. $n$ quintuple layers (QLs) of Bi$_2$Se$_3$ are grown ontop of Mn-doped Bi$_2$Se$_3$ by molecular beam epitaxy for $0\le n \le 30\,$QLs, allowing to unhamperedly monitor the development of electronic and topological properties by surface sensitive techniques like angle resolved photoemission spectroscopy. With increasing $n$, a Mn-induced gap at the Dirac point is gradually filled in an "hourglass" fashion to reestablish a topological surface state at $n \sim 9\,$QLs. Our results suggest a competition of upwards and downwards band bending effects due to the presence of an n-p type interface, which can be used to tailor topological and quantum well states independently.

cond-mat.mtrl-sci↗
arXiv2015

One-dimensional spin texture of Bi(441); Quantum Spin Hall properties without a topological insulator

The high index (441) surface of bismuth has been studied using Scanning Tunnelling Microscopy (STM), Angle Resolved Photoemission Spectroscopy (APRES) and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localised states are observed, both of which are linearly dispersing in one in-plane direction ($k_x$), and dispersionless in the orthogonal in-plane direction ($k_y$), and both of which have a Dirac-like crossing at $k_x$=0. Spin ARPES reveals a strong in-plane polarisation, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its {possible} origin as an edge-state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one dimensional Quantum Spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.

cond-mat.mes-hall↗
arXiv2013

Evidence for a direct band gap in the topological insulator Bi2Se3 from theory and experiment

Using angle-resolved photoelectron spectroscopy and ab-initio GW calculations, we unambiguously show that the widely investigated three-dimensional topological insulator Bi2Se3 has a direct band gap at the Gamma point. Experimentally, this is shown by a three-dimensional band mapping in large fractions of the Brillouin zone. Theoretically, we demonstrate that the valence band maximum is located at the Brillouin center only if many-body effects are included in the calculation. Otherwise, it is found in a high-symmetry mirror plane away from the zone center.

cond-mat.mtrl-sci↗
arXiv2012

Controllable magnetic doping of the surface state of a topological insulator

A combined experimental and theoretical study of doping individual Fe atoms into Bi2Se3 is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally-activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab-initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.

cond-mat.mtrl-sci↗
arXiv2012

Unconventional spin texture of a topologically nontrivial semimetal Sb(110)

The surfaces of antimony are characterized by the presence of spin-split states within the projected bulk band gap and the Fermi contour is thus expected to exhibit a spin texture. Using spin-resolved density functional theory calculations, we determine the spin polarization of the surface bands of Sb(110). The existence of the unconventional spin texture is corroborated by the investigations of the electron scattering on this surface. The charge interference patterns formed around single scattering impurities, imaged by scanning tunneling microscopy, reveal the absence of direct backscattering signal. We identify the allowed scattering vectors and analyze their bias evolution in relation to the surface-state dispersion.

cond-mat.mes-hall↗
arXiv2011

Large tuneable Rashba spin splitting of a two-dimensional electron gas in Bi2Se3

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi$_2$Se$_3$ from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

cond-mat.mtrl-sci↗
arXiv2009

Electron-phonon coupling in potassium-doped graphene: Angle-resolved photoemission spectroscopy

The electron-phonon coupling in potassium-doped graphene on Ir(111) is studied via the renormalization of the pi* band near the Fermi level, using angle-resolved photoemission spectroscopy. The renormalization is found to be fairly weak and almost isotropic, with a mass enhancement parameter of lambda= 0.28(6) for both the K-M and the K-G direction. These results are found to agree well with recent first principles calculations.

cond-mat.mes-hall↗