-
Exciton radiative lifetimes in hexagonal diamond Ge and Si$_x$Ge$_{1-x}$ alloys
Authors:
Michele Re Fiorentin,
Michele Amato,
Maurizia Palummo
Abstract:
Recent reports of strong room-temperature photoluminescence in hexagonal diamond (2H) germanium stand in marked contrast to theoretical predictions of very weak band-edge optical transitions. Here we address radiative emission in 2H-Ge and related materials through a comprehensive investigation of their excitonic properties and radiative lifetimes, performing Bethe-Salpeter calculations on pristin…
▽ More
Recent reports of strong room-temperature photoluminescence in hexagonal diamond (2H) germanium stand in marked contrast to theoretical predictions of very weak band-edge optical transitions. Here we address radiative emission in 2H-Ge and related materials through a comprehensive investigation of their excitonic properties and radiative lifetimes, performing Bethe-Salpeter calculations on pristine and uniaxially strained 2H-Ge, 2H-Si$_x$Ge$_{1-x}$ alloys with $x=\frac{1}{6},\,\frac{1}{4},\,\frac{1}{2}$, and wurtzite GaN as a reference. Pristine 2H-Ge features sizable exciton binding energies ($\sim\!30$ meV) but extremely small dipole moments, yielding radiative lifetimes above $10^{-4}$ s. Alloying with Si reduces the lifetime by nearly two orders of magnitude, whereas a 2% uniaxial strain along the $c$ axis induces a band crossover that strongly enhances the in-plane dipole moment of the lowest-energy exciton and drives the lifetime down to the nanosecond scale. Although strained 2H-Ge approaches the radiative efficiency of GaN, its much lower exciton energy prevents a full match. These results provide the missing excitonic description of 2H-Ge and 2H-Si$_x$Ge$_{1-x}$, demonstrating that, even when excitonic effects are fully accounted for, the strong photoluminescence reported experimentally cannot originate from the ideal crystal.
△ Less
Submitted 14 April, 2026; v1 submitted 17 December, 2025;
originally announced December 2025.
-
Evidence for atomic-scale vibron-mediated electron bunching
Authors:
A. Maiti,
M. Amato,
V. S. Stolyarov,
H. Aubin,
J. Estève,
F. Pistolesi,
M. Aprili,
F. Massee
Abstract:
Due to the Coulomb blockade effect, electrons rarely bunch during transport, a phenomenon observed only in a few specially engineered mesoscopic configurations. In this work, we introduce an atomically resolved shot-noise study to demonstrate the possibility of electron bunching through vibrational coupling which takes place in an atomically sized nano-electro-mechanical system. Using tunnelling s…
▽ More
Due to the Coulomb blockade effect, electrons rarely bunch during transport, a phenomenon observed only in a few specially engineered mesoscopic configurations. In this work, we introduce an atomically resolved shot-noise study to demonstrate the possibility of electron bunching through vibrational coupling which takes place in an atomically sized nano-electro-mechanical system. Using tunnelling spectroscopy, we observe signatures of vibron-assisted tunnelling on an Fe impurity in Bi$_2$Se$_3$. Notably, simultaneous shot-noise measurements at the centre of the vibrating impurity reveal super-Poissonian noise. In the absence of alternative sources of super-Poissonian noise, this implies vibronic-coupling-induced bunching of electrons during the tunnelling process through the impurity, as theoretically predicted decades ago. As a future outlook, if coherence between electrons can be implemented, vibron-mediated electron bunching at single atomic sites may be exploited as a local injection source of $N$-paired electrons.
△ Less
Submitted 14 November, 2025;
originally announced November 2025.
-
Structural and thermodynamic stability of hexagonal-diamond $\text{Si}_{1 - x - y}\,\text{Ge}_{x}\,\text{B}_{y}$ alloys
Authors:
Marc Túnica,
Francesca Chiodi,
Michele Amato
Abstract:
Pushing dopant concentrations beyond the solubility limit in semiconductors -- a process known as hyperdoping -- has been demonstrated as an effective strategy for inducing superconductivity in cubic-diamond Si and SiGe materials. Additionally, previous studies have reported that several polytypes of Si may exhibit a type-I superconducting state under high pressure. In this work, we employ ground-…
▽ More
Pushing dopant concentrations beyond the solubility limit in semiconductors -- a process known as hyperdoping -- has been demonstrated as an effective strategy for inducing superconductivity in cubic-diamond Si and SiGe materials. Additionally, previous studies have reported that several polytypes of Si may exhibit a type-I superconducting state under high pressure. In this work, we employ ground-state Density Functional Theory simulations to investigate the effects of both low and high B doping concentrations on the structural and thermodynamic properties of hexagonal-diamond SiGe alloys, with a systematic comparison to their cubic-diamond counterparts. Our results highlight three key findings: (i) structural analysis confirms that the lattice parameters of SiGeB alloys adhere to a ternary Vegard's law, consistent with observations in cubic-diamond SiGe alloys. However, at high doping concentrations, B incorporation can locally disrupt the hexagonal symmetry, particularly in the presence of B clustering; (ii) dopant formation energy calculations reveal that B is thermodynamically more stable in the hexagonal phase than in the cubic phase across all Ge concentrations, regardless of the doping level; (iii) mixing enthalpy calculations demonstrate that hyperdoped hexagonal-diamond SiGe alloys are thermodynamically stable across the full range of Ge compositions and that their tendency for hyperdoping is more favorable than that of cubic-diamond SiGe alloys. Taken together, these findings indicate that hyperdoping is experimentally viable in hexagonal-diamond SiGe alloys and, in light of previous evidence, position these materials as a promising platform for the exploration of superconductivity in group IV semiconductors.
△ Less
Submitted 31 July, 2025;
originally announced July 2025.
-
Optical absorption in hexagonal-diamond Si and Ge nanowires: insights from STEM-EELS experiments and ab initio theory
Authors:
Luiz H. G. Tizei,
Michele Re Fiorentin,
Thomas Dursap,
Theodorus M. van den Berg,
Marc Túnica,
Maurizia Palummo,
Mathieu Kociak,
Laetitia Vincent,
Michele Amato
Abstract:
Hexagonal-diamond (2H) group IV nanowires are key for advancing group IV-based lasers, quantum electronics, and photonics. Understanding their dielectric response is crucial for performance optimization, but their optical absorption properties remain unexplored. We present the first comprehensive study of optical absorption in 2H-Si and 2H-Ge nanowires, combining high-resolution STEM, monochromate…
▽ More
Hexagonal-diamond (2H) group IV nanowires are key for advancing group IV-based lasers, quantum electronics, and photonics. Understanding their dielectric response is crucial for performance optimization, but their optical absorption properties remain unexplored. We present the first comprehensive study of optical absorption in 2H-Si and 2H-Ge nanowires, combining high-resolution STEM, monochromated EELS, and ab initio simulations. The nanowires, grown in situ in a TEM as nanobranches on GaAs stems, show excellent structural quality: single crystalline, strain-free, minimal defects, no substrate contamination, enabling access to intrinsic dielectric response. 2H-Si exhibits enhanced absorption in the visible range compared to cubic Si, with a marked onset above 2.5 eV. 2H-Ge shows absorption near 1 eV but no clear features at the direct bandgap, as predicted by ab initio simulations. A peak around 2 eV in aloof-beam spectra is attributed to a thin 3C-Ge shell. These findings clarify structure-optical response relationships in 2H materials.
△ Less
Submitted 12 June, 2025;
originally announced June 2025.
-
Interaction of dopants with the I$_3$-type basal stacking fault in hexagonal-diamond Si
Authors:
Marc Túnica,
Perpetua Wanjiru Muchiri,
Alberto Zobelli,
Anna Marzegalli,
Emilio Scalise,
Michele Amato
Abstract:
Recently synthesized hexagonal-diamond silicon, germanium, and silicon-germanium nanowires exhibit remarkable optical and electronic properties when compared to cubic-diamond polytypes. Because of the metastability of the hexagonal-diamond phase, I$_3$-type basal stacking faults are frequently observed in these materials. Understanding and modulating the interaction between these extended defects…
▽ More
Recently synthesized hexagonal-diamond silicon, germanium, and silicon-germanium nanowires exhibit remarkable optical and electronic properties when compared to cubic-diamond polytypes. Because of the metastability of the hexagonal-diamond phase, I$_3$-type basal stacking faults are frequently observed in these materials. Understanding and modulating the interaction between these extended defects and dopants are essential for advancing the design and performance of these novel semiconductors. In the present study, we employ density functional theory calculations to investigate the interaction of extrinsic dopants (group III, IV, and V elements) with the I$_3$-type basal stacking fault in hexagonal-diamond silicon. Contrary to the behavior observed in cubic-diamond silicon with intrinsic stacking faults, we demonstrate that neutral and negatively charged $p$-type impurities exhibit a marked tendency to occupy lattice sites far from the I$_3$-type basal stacking fault. The interaction of acceptors with the planar defect reduces their energetic stability. However, this effect is much less pronounced for neutral or positively charged $n$-type dopants and isovalent impurities. The thermodynamic energy barrier to segregation for these dopants is small and may even become negative, indicating a tendency to segregate into the fault. Through a detailed analysis of structural modifications, ionization effects, and impurity-level charge density distribution, we show that the origin of this behavior can be attributed to variations in the impurity's steric effects and its wave function character. Finally, all these results are validated by considering the extreme case of an abrupt hexagonal/cubic silicon interface, where acceptor segregation from the cubic to the hexagonal region is demonstrated, confirming the behavior observed for $p$-type dopants near the I$_3$-type defect.
△ Less
Submitted 12 June, 2025; v1 submitted 21 February, 2025;
originally announced February 2025.
-
Acceptor and donor impurity levels in hexagonal-diamond silicon
Authors:
Marc Túnica,
Alberto Zobelli,
Michele Amato
Abstract:
Recent advances in the characterization of hexagonal-diamond silicon (2H-Si) have shown that this material possesses remarkably different structural, electronic, and optical properties as compared to the common cubic-diamond (3C) polytype. Interestingly, despite the wide range of physical properties analyzed, to date no study has investigated impurity energy levels in 2H-Si. Here, we present resul…
▽ More
Recent advances in the characterization of hexagonal-diamond silicon (2H-Si) have shown that this material possesses remarkably different structural, electronic, and optical properties as compared to the common cubic-diamond (3C) polytype. Interestingly, despite the wide range of physical properties analyzed, to date no study has investigated impurity energy levels in 2H-Si. Here, we present results of ab initio DFT simulations to describe the effect of p- and n-type substitutional doping on the structural and electronic properties of hexagonal-diamond Si (2H-Si). We first provide a detailed analysis of how a given impurity can assume a different local symmetry depending on the host crystal phase. Then, by studying neutral and charged dopants, we carefully estimate donors and acceptors transition energy levels in 2H-Si and compare them with the cubic-diamond (3C) case. In the case of acceptors, the formation energy is always lower in 2H-Si and is associated with a shallower charge transition level with respect to 3C-Si. On the other hand, donors prefer the cubic phase and have transition energies smaller with respect to 2H-Si. Finally, by employing a simple model based on the 2H/3C band offset diagram, we prove the physical validity of our findings and we show how holes can be used to stabilize the 2H-Si phase. Overall, the described doping properties represent a robust starting point for further theoretical and experimental investigations.
△ Less
Submitted 21 October, 2024; v1 submitted 1 August, 2024;
originally announced August 2024.
-
Elastic solids under frictionless rigid contact and configurational force
Authors:
Francesco Dal Corso,
Marco Amato,
Davide Bigoni
Abstract:
A homogeneous elastic solid, bounded by a flat surface in its unstressed configuration, undergoes a finite strain when in frictionless contact against a rigid and rectilinear constraint, ending with a rounded or sharp corner, in a two-dimensional formulation. With a strong analogy to fracture mechanics, it is shown that (i.) a path-independent $J$--integral can be defined for frictionless contact…
▽ More
A homogeneous elastic solid, bounded by a flat surface in its unstressed configuration, undergoes a finite strain when in frictionless contact against a rigid and rectilinear constraint, ending with a rounded or sharp corner, in a two-dimensional formulation. With a strong analogy to fracture mechanics, it is shown that (i.) a path-independent $J$--integral can be defined for frictionless contact problems, (ii.) which is equal to the energy release rate $G$ associated with an infinitesimal growth in the size of the frictionless constraint, and thus gives the value of the configurational force component along the sliding direction. Furthermore, it is found that (iii.) such a configurational sliding force is the Newtonian force component exerted by the elastic solid on the constraint at the frictionless contact. Assuming the kinematics of an Euler-Bernoulli rod for an elastic body of rectangular shape, the results (i.)--(iii.) lead to a new interpretation from a nonlinear solid mechanics perspective of the configurational forces recently disclosed for one-dimensional structures of variable length. Finally, approximate but closed-form solutions (validated with finite element simulations) are exploited to provide further insight into the effect of configurational forces. In particular, two applications are presented which show that a transverse compression can lead to Eulerian buckling or to longitudinal dynamic motion, both realizing novel examples of soft actuation mechanisms. As an application to biology, our results may provide a mechanical explanation for the observed phenomenon of negative durotaxis, where cells migrate from stiffer to softer environments.
△ Less
Submitted 20 May, 2024;
originally announced May 2024.
-
Tuning superconductivity in nanosecond laser annealed boron doped $Si_{1-x}Ge_{x}$ epilayers
Authors:
S. Nath,
I. Turan,
L. Desvignes,
L. Largeau,
O. Mauguin,
M. Túnica,
M. Amato,
C. Renard,
G. Hallais,
D. Débarre,
F. Chiodi
Abstract:
Superconductivity in ultra-doped $Si_{1-x}Ge_{x}:B$ epilayers is demonstrated by nanosecond laser doping, which allows introducing substitutional B concentrations well above the solubility limit and up to $7\,at.\%$. A Ge fraction $x$ ranging from 0 to 0.21 is incorporated in $Si:B$ : 1) through a precursor gas by Gas Immersion Laser Doping; 2) by ion implantation, followed by nanosecond laser ann…
▽ More
Superconductivity in ultra-doped $Si_{1-x}Ge_{x}:B$ epilayers is demonstrated by nanosecond laser doping, which allows introducing substitutional B concentrations well above the solubility limit and up to $7\,at.\%$. A Ge fraction $x$ ranging from 0 to 0.21 is incorporated in $Si:B$ : 1) through a precursor gas by Gas Immersion Laser Doping; 2) by ion implantation, followed by nanosecond laser annealing; 3) by UHV-CVD growth of a thin Ge layer, followed by nanosecond laser annealing. The 30 nm and 80 nm thick $Si_{1-x}Ge_{x}:B$ epilayers display superconducting critical temperatures $T_c$ tuned by B and Ge between 0 and 0.6 K. Within BCS weak-coupling theory, $T_c$ evolves exponentially with both the density of states and the electron-phonon potential. While B doping affects both, through the increase of the carrier density and the tensile strain, Ge incorporation allows addressing independently the lattice deformation influence on superconductivity. To estimate the lattice parameter modulation with B and Ge, Vegard's law is validated for the ternary $SiGeB$ bulk alloy by Density Functional Theory calculations. Its validity is furthermore confirmed experimentally by X-Ray Diffraction. We highlight a global linear dependence of $T_c$ vs. lattice parameter, common for both $Si:B$ and $Si_{1-x}Ge_{x}:B$, with $δT_c/T_c \sim 50\,\%$ for $δa/a \sim 1\,\%$.
△ Less
Submitted 3 April, 2024;
originally announced April 2024.
-
Strain engineering of the electronic states of silicon-based quantum emitters
Authors:
A. Ristori,
N. Granchi,
F. Intonti,
M. Khoury,
D. Hannani,
C. M. Ruiz,
M. Salvalaglio,
A. Filippatos,
M. Amato,
T. Herzig,
J. Meijer,
S. Pezzagna,
M. Bollani,
C. Barri,
M. Abbarchi,
F. Biccari
Abstract:
Light-emitting complex defects in silicon have been considered a potential platform for quantum technologies based on spin and photon degrees of freedom working at telecom wavelengths. Their integration in complex devices is still in its infancy, and it was mostly focused on light extraction and guiding. Here we address the control of the electronic states of carbon-related impurities (G-centers)…
▽ More
Light-emitting complex defects in silicon have been considered a potential platform for quantum technologies based on spin and photon degrees of freedom working at telecom wavelengths. Their integration in complex devices is still in its infancy, and it was mostly focused on light extraction and guiding. Here we address the control of the electronic states of carbon-related impurities (G-centers) via strain engineering. By embedding them in patches of silicon on insulator and topping them with SiN, symmetry breaking along [001] and [110] directions is demonstrated, resulting in a controlled splitting of the zero phonon line (ZPL), as accounted for by the piezospectroscopic theoretical framework. The splitting can be as large as 18 meV and it is finely tuned by selecting patch size or by moving in different positions on the patch. Some of the split, strained ZPLs are almost fully polarized and their overall intensity is enhanced up to 7 times with respect to the flat areas, whereas their recombination dynamics is slightly affected. Our technique can be extended to other impurities and Si-based devices such as suspended bridges, photonic crystal microcavities, Mie resonators, and integrated photonic circuits.
△ Less
Submitted 12 June, 2023;
originally announced June 2023.
-
Configurational Forces in Penetration Processes
Authors:
Davide Bigoni,
Marco Amato,
Francesco Dal Corso
Abstract:
With a loose reference to problems of penetration in biomechanics (for instance, a nanoparticle penetrating through a cell's membrane or a cell sucked with a pipette), the role of configurational forces is investigated during the process in which a compliant intruder is inserted into an elastic structure. For insertion into a rigid constraint, a configurational force proportional to the square of…
▽ More
With a loose reference to problems of penetration in biomechanics (for instance, a nanoparticle penetrating through a cell's membrane or a cell sucked with a pipette), the role of configurational forces is investigated during the process in which a compliant intruder is inserted into an elastic structure. For insertion into a rigid constraint, a configurational force proportional to the square of the strain needed to deform the body, which is penetrating, is found. This force has a more complex structure when the compliance of the constraint is kept into account, but in all cases, it tends to expel the penetrating body.
△ Less
Submitted 22 November, 2022;
originally announced December 2022.
-
Electronic structure of folded hexagonal boron nitride
Authors:
Anthony Impellizzeri,
Michele Amato,
Chris P. Ewels,
Alberto Zobelli
Abstract:
Folded regions are commonly encountered in a number of hexagonal boron nitride (h-BN) based bulk and nanostructured materials. Two types of structural modifications occur in folded h-BN layers: local curvature at the folded edges and interlayer shear of the layers which changes the stacking of the overlapping flat regions. In this work we discuss, via density functional theory simulations, the imp…
▽ More
Folded regions are commonly encountered in a number of hexagonal boron nitride (h-BN) based bulk and nanostructured materials. Two types of structural modifications occur in folded h-BN layers: local curvature at the folded edges and interlayer shear of the layers which changes the stacking of the overlapping flat regions. In this work we discuss, via density functional theory simulations, the impact of these structural modifications on the ground state electronic structure of the pristine monolayer. We show that, depending on the fold orientation, the overlapping region might present different stacking configurations with subsequent variations of the fundamental band gap; further gap changes occur at the folded regions. The overall electronic structure of a BN folded monolayer can finally be described as a type-II junction between two wide gap semiconductors located at the curved and flat overlapping zones.
△ Less
Submitted 18 November, 2022;
originally announced November 2022.
-
Interplay of quantum confinement and strain effects in type I to type II transition in Ge/Si core-shell nanocrystals
Authors:
Ivan Marri,
Stefano Ossicini,
Michele Amato,
Simone Grillo,
Olivia Pulci
Abstract:
The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied within Density Functional Theory. Effects induced by quantum confinement and strain on the near-band-edge states localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that…
▽ More
The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied within Density Functional Theory. Effects induced by quantum confinement and strain on the near-band-edge states localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that Si(core)/Ge(shell) nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) Ge(core)/Si(shell) nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms inducing a type I alignment. In larger Ge(core)/Si(shell) nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The conditions that favor the transition from a type I to a type II alignment for Ge(core)/Si(shell) nanocrystals are discussed in detail.
△ Less
Submitted 22 June, 2022;
originally announced June 2022.
-
Atomic scale visualization of the p-d hybridization in III-V semiconductors doped with transition metal impurities
Authors:
K. Badiane,
G. Rodary,
M. Amato,
A. Gloter,
C. David,
H. Aubin,
J. -C. Girard
Abstract:
p-d hybridization of transition metal impurities in a semiconductor host is the mechanism that couples valence-band electrons and localized spins. We use scanning tunneling microscopy and spectroscopy combined with density functional theory to probe at the atomic scale hybridization of Cr single impurities with GaAs host. Combining spatial density of states mapping and in-gap states spectroscopy o…
▽ More
p-d hybridization of transition metal impurities in a semiconductor host is the mechanism that couples valence-band electrons and localized spins. We use scanning tunneling microscopy and spectroscopy combined with density functional theory to probe at the atomic scale hybridization of Cr single impurities with GaAs host. Combining spatial density of states mapping and in-gap states spectroscopy of the Cr substituted at the surface of the semiconductor, we give a detailed picture of the spatial extension and the electronic structure of the strongly anisotropic wave function of Cr on GaAs(110). First principles calculations allow to identify electronic character and origin of each states and show that the main resonance peaks and the wave function with "drop-eyes" lobes experimentally observed for 3d metal impurities in III-V semiconductor are direct local evidences of the p-d hybridization.
△ Less
Submitted 17 May, 2022;
originally announced May 2022.
-
Fermi resonance in the Raman spectrum of graphene
Authors:
Dipankar Kalita,
Michele Amato,
Alexandre Artaud,
Laëtitia Marty,
Vincent Bouchiat,
Johann Coraux,
Christian Brouder,
Michele Lazzeri,
Nedjma Bendiab
Abstract:
We report the observation of an intense anomalous peak at 1608 cm$^{-1}$ in the Raman spectrum of graphene associated to the presence of chromium nanoparticles in contact with graphene. Bombardment with an electron beam demonstrates that this peak is distinct from the well studied D$'$ peak appearing as defects are created in graphene; the new peak is found non dispersive. We argue that the bondin…
▽ More
We report the observation of an intense anomalous peak at 1608 cm$^{-1}$ in the Raman spectrum of graphene associated to the presence of chromium nanoparticles in contact with graphene. Bombardment with an electron beam demonstrates that this peak is distinct from the well studied D$'$ peak appearing as defects are created in graphene; the new peak is found non dispersive. We argue that the bonding of chromium atoms with carbon atoms softens the out-of-plane optical (ZO) phonon mode, in such a way that the frequency of its overtone decreases to $2ω_{\rm ZO}\simω_{\rm G}$, where $ω_{\rm G}$=1585~cm$^{-1}$ is the frequency of the Raman-active E$_{\rm 2g}$ mode. Thus, the observed new peak is attributed to the 2ZO mode which becomes Raman-active following a mechanism known as Fermi resonance. First-principles calculations on vibrational and anharmonic properties of the graphene/Cr interface support this scenario.
△ Less
Submitted 18 November, 2020;
originally announced November 2020.
-
Extrinsic doping in group IV hexagonal-diamond type crystals
Authors:
Michele Amato,
Thanayut Kaewmaraya,
Alberto Zobelli
Abstract:
Over the last few years, group IV hexagonal-diamond type crystals have acquired great attention in semiconductor physics thanks to the appearance of novel and very effective growth methods. However, many questions remain unaddressed on their extrinsic doping capability and on how it compares to those of diamond-like structures. This point is here investigated through numerical simulations conducte…
▽ More
Over the last few years, group IV hexagonal-diamond type crystals have acquired great attention in semiconductor physics thanks to the appearance of novel and very effective growth methods. However, many questions remain unaddressed on their extrinsic doping capability and on how it compares to those of diamond-like structures. This point is here investigated through numerical simulations conducted in the framework of the Density Functional Theory (DFT). The comparative analysis for group III and V dopant atoms shows that: i) in diamond-type crystals the bulk sites symmetry ($T_d$) is preserved by doping while in hexagonal crystals the impurity site moves towards a higher ($T_d$) or lower ($C_{3v}$) symmetry configuration dependently on the valence of the dopant atoms; ii) for Si and Ge, group III impurities can be more easily introduced in the hexagonal-diamond phase, whose local $C_{3v}$ symmetry better accommodates the three-fold coordination of the impurity, while n-type impurities do not reveal any marked phase preference; iii) for C, both n and p dopants are more stable in the hexagonal-diamond structure than in the the cubic one, but this tendency is much more pronounced for n-type impurities.
△ Less
Submitted 8 July, 2020;
originally announced July 2020.
-
Contribution of individual degrees of freedom to Lyapunov vectors in many-body systems
Authors:
L Filho,
M Amato,
Y Elskens,
T Rocha Filho
Abstract:
We use the weight $δ$I, deduced from the estimation of Lyapunov vectors, in order to characterise regions in the kinetic (x, v) space with particles that most contribute to chaoticity. For the paradigmatic model, the cosine Hamiltonian mean field model, we show that this diagnostic highlights the vicinity of the separatrix, even when the latter hardly exists.
We use the weight $δ$I, deduced from the estimation of Lyapunov vectors, in order to characterise regions in the kinetic (x, v) space with particles that most contribute to chaoticity. For the paradigmatic model, the cosine Hamiltonian mean field model, we show that this diagnostic highlights the vicinity of the separatrix, even when the latter hardly exists.
△ Less
Submitted 22 February, 2019;
originally announced February 2019.
-
Tuning the Work Function of Si(100) Surface by Halogen Absorption: A DFT Study
Authors:
Matteo Bertocchi,
Michele Amato,
Ivan Marri,
Stefano Ossicini
Abstract:
First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well…
▽ More
First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well as to the charge redistribution at the interface. All these results are examined with respect to previous theoretical works and experimental data obtained for the (100) as well as other Si surface orientations. Based on this analysis, we argue that the changes in the electronic properties caused by variations of the interfacial chemistry strongly depend on the chemisorbed species and much less on the surface crystal orientation.
△ Less
Submitted 21 September, 2018;
originally announced September 2018.
-
Observation of a phonon avalanche in highly photoexcited hybrid perovskite single crystals
Authors:
G. M. Vanacore,
J. Hu,
E. Baldini,
C. A. Rozzi,
M. Amato,
H. Wei,
J. Huang,
S. Polishchuk,
M. Puppin,
A. Crepaldi,
M. Grioni,
M. Chergui,
F. Carbone,
A. H. Zewail
Abstract:
In hybrid lead halide perovskites, the coupling between photogenerated charges and the ionic degrees of freedom plays a crucial role in defining the intrinsic limit of carrier mobility and lifetime. However, direct investigation of this fundamental interaction remains challenging because its relevant dynamics occur on ultrashort spatial and ultrafast temporal scales. Here, we unveil the coupled el…
▽ More
In hybrid lead halide perovskites, the coupling between photogenerated charges and the ionic degrees of freedom plays a crucial role in defining the intrinsic limit of carrier mobility and lifetime. However, direct investigation of this fundamental interaction remains challenging because its relevant dynamics occur on ultrashort spatial and ultrafast temporal scales. Here, we unveil the coupled electron-lattice dynamics of a CH3NH3PbI3 single crystal upon intense photoexcitation through a unique combination of ultrafast electron diffraction, time-resolved photoelectron spectroscopy, and time-dependent ab initio calculations. We observe the structural signature of a hot-phonon bottleneck effect that prevents rapid carrier relaxation, and we uncover a phonon avalanche mechanism responsible for breaking the bottleneck. The avalanche involves a collective emission of low-energy phonons - mainly associated with the organic sub-lattice - that proceeds in a regenerative manner and correlates with the accumulation and confinement of photocarriers at the crystal surface. Our results indicate that in hybrid perovskites carrier transport and spatial confinement are key to controlling the electron-phonon interaction and their rational engineering is relevant for future applications in optoelectronic devices.
△ Less
Submitted 19 September, 2020; v1 submitted 11 January, 2018;
originally announced January 2018.
-
Ensemble Inequivalence and Maxwell Construction in the Self-Gravitating Ring Model
Authors:
T. M. Rocha Filho,
C. H. Silvestre,
M. A. Amato
Abstract:
Equilibrium Statistical Mechanics is undoubtedly a cornerstone for the description of many particle systems. The common interpretation is based on ensemble theory as put forward by Gibbs, alongside the basic assumptions that different ensembles are equivalent, i.~e.\ the properties of the system can equally be obtained in any ensemble with the same results. However, the simplicity of the argument…
▽ More
Equilibrium Statistical Mechanics is undoubtedly a cornerstone for the description of many particle systems. The common interpretation is based on ensemble theory as put forward by Gibbs, alongside the basic assumptions that different ensembles are equivalent, i.~e.\ the properties of the system can equally be obtained in any ensemble with the same results. However, the simplicity of the argument that provides such equivalence, mathematically grounded by the existence of Legendre transformation between the ensembles and the existence of its inverse, may break down for physical systems with long range interactions. In this paper we study the behavior of a simple toy model with a long range interaction and show from first principles, by solving numerically the mechanical equations of motion and Monte Carlo simulations, the inequivalence of ensembles, and discuss in what situations and how the Maxwell construction is applicable.
△ Less
Submitted 16 November, 2017; v1 submitted 11 April, 2017;
originally announced April 2017.
-
Lyapunov Exponent and Criticality in the Hamiltonian Mean Field Model
Authors:
L. H. Miranda Filho,
M. A. Amato,
T. M. Rocha Filho
Abstract:
We investigate the dependence of the largest Lyapunov exponent of a $N$-particle self-gravitating ring model at equilibrium with respect to the number of particles and its dependence on energy. This model has a continuous phase-transition from a ferromagnetic to homogeneous phase, and we numerically confirm with large scale simulations the existence of a critical exponent associated to the largest…
▽ More
We investigate the dependence of the largest Lyapunov exponent of a $N$-particle self-gravitating ring model at equilibrium with respect to the number of particles and its dependence on energy. This model has a continuous phase-transition from a ferromagnetic to homogeneous phase, and we numerically confirm with large scale simulations the existence of a critical exponent associated to the largest Lyapunov exponent, although at variance with the theoretical estimate. The existence of chaos in the magnetized state evidenced by a positive Lyapunov exponent, even in the thermodynamic limit, is explained by the resonant coupling of individual particle oscillations to the diffusive motion of the center of mass of the system due to the thermal excitation of a classical Goldstone mode. The transition from "weak" to "strong" chaos occurs at the onset of the diffusive motion of the center of mass of the non-homogeneous equilibrium state, as expected. We also discuss thoroughly for the model the validity and limits of a geometrical approach for their analytical estimate.
△ Less
Submitted 19 December, 2017; v1 submitted 9 April, 2017;
originally announced April 2017.
-
Brownian regime of finite-N corrections to particle motion in the XY hamiltonian mean field model
Authors:
Bruno V Ribeiro,
Marco A Amato,
Yves Elskens
Abstract:
We study the dynamics of the N-particle system evolving in the XY hamiltonian mean field (HMF) model for a repulsive potential, when no phase transition occurs. Starting from a homogeneous distribution, particles evolve in a mean field created by the interaction with all others. This interaction does not change the homogeneous state of the system, and particle motion is approximately ballistic wit…
▽ More
We study the dynamics of the N-particle system evolving in the XY hamiltonian mean field (HMF) model for a repulsive potential, when no phase transition occurs. Starting from a homogeneous distribution, particles evolve in a mean field created by the interaction with all others. This interaction does not change the homogeneous state of the system, and particle motion is approximately ballistic with small corrections. For initial particle data approaching a waterbag, it is explicitly proved that corrections to the ballistic velocities are in the form of independent brownian noises over a time scale diverging not slower than $N^{2/5}$ as $N \to \infty$, which proves the propagation of molecular chaos. Molecular dynamics simulations of the XY-HMF model confirm our analytical findings.
△ Less
Submitted 18 May, 2016;
originally announced May 2016.
-
Microcanonical Monte Carlo Study of One Dimensional Self-Gravitating Lattice Gas Models
Authors:
J. M. Maciel,
M. A. Amato,
T. M. Rocha Filho,
A. D. Figueiredo
Abstract:
In this study we present a Microcanonical Monte Carlo investigation of one dimensional self-gravitating toy models. We study the effect of hard-core potentials and compare to those results obtained with softening parameters and also the effect of the geometry of the models. In order to study the effect of the geometry and the borders in the system we introduce a model with the symmetry of motion i…
▽ More
In this study we present a Microcanonical Monte Carlo investigation of one dimensional self-gravitating toy models. We study the effect of hard-core potentials and compare to those results obtained with softening parameters and also the effect of the geometry of the models. In order to study the effect of the geometry and the borders in the system we introduce a model with the symmetry of motion in a line instead of a circle, which we denominate as $1/r$ model. The hard-core particle potential introduces the effect of the size of particles and, consequently, the effect of the density of the system that is redefined in terms of the packing fraction of the system. The latter plays a role similar to the softening parameter $ε$ in the softened particles' case. In the case of low packing fractions both models with hard-core particles show a behavior that keeps the intrinsic properties of the three dimensional gravitational systems such as negative heat capacity. For higher values of the packing fraction the ring the system behaves as the Hamiltonian Mean Field model and while for the $1/r$ it is similar to the one-dimensional systems.
△ Less
Submitted 4 December, 2015;
originally announced December 2015.
-
Excitons and stacking order in h-BN
Authors:
Romain Bourrellier,
Michele Amato,
Luiz Henrique Galvão Tizei,
Christine Giorgetti,
Alexandre Gloter,
Malcolm I. Heggie,
Katia March,
Odile Stéphan,
Lucia Reining,
Mathieu Kociak,
Alberto Zobelli
Abstract:
The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN) makes this material one of the most promising candidate for light emitting devices in the far ultraviolet (UV). However, single excitons occur only in perfect monocrystals that are extremely hard to synthesize, while regular h-BN samples present a complex emission spectrum with several additional peaks. The microscopic orig…
▽ More
The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN) makes this material one of the most promising candidate for light emitting devices in the far ultraviolet (UV). However, single excitons occur only in perfect monocrystals that are extremely hard to synthesize, while regular h-BN samples present a complex emission spectrum with several additional peaks. The microscopic origin of these additional emissions has not yet been understood. In this work we address this problem using an experimental and theoretical approach that combines nanometric resolved cathodoluminescence, high resolution transmission electron microscopy and state of the art theoretical spectroscopy methods. We demonstrate that emission spectra are strongly inhomogeneus within individual flakes and that additional excitons occur at structural deformations, such as faceted plane folds, that lead to local changes of the h-BN stacking order.
△ Less
Submitted 9 January, 2014;
originally announced January 2014.
-
Scaling of the dynamics of homogeneous states of one-dimensional long-range interacting systems
Authors:
A. Figueiredo,
T. M. Rocha Filho,
A. E. Santana,
M. A. Amato
Abstract:
Quasi-Stationary States of long-range interacting systems have been studied at length over the last fifteen years. It is known that the collisional terms of the Balescu-Lenard and Landau equations vanish for one-dimensional systems in homogeneous states, thus requiring a new kinetic equation with a proper dependence on the number of particles. Here we show that previous scalings described in the l…
▽ More
Quasi-Stationary States of long-range interacting systems have been studied at length over the last fifteen years. It is known that the collisional terms of the Balescu-Lenard and Landau equations vanish for one-dimensional systems in homogeneous states, thus requiring a new kinetic equation with a proper dependence on the number of particles. Here we show that previous scalings described in the literature are due either to small size effects or the use of improper variables to describe the dynamics. The correct scaling is proportional to the square of the number of particles and deduce the kinetic equation valid for the homogeneous regime and numerical evidence is given for the Hamiltonian Mean Field and ring models.
△ Less
Submitted 22 August, 2014; v1 submitted 19 May, 2013;
originally announced May 2013.
-
Dynamics and physical interpretation of quasi-stationary states in systems with long-range interactions
Authors:
T. M. Rocha Filho,
A. E. Santana,
J. R. S. Moura,
M. A. Amato,
A. Figueiredo
Abstract:
Although the Vlasov equation is used as a good approximation for a sufficiently large $N$, Braun and Hepp have showed that the time evolution of the one particle distribution function of a $N$ particle classical Hamiltonian system with long range interactions satisfies the Vlasov equation in the limit of infinite $N$. Here we rederive this result using a different approach allowing a discussion of…
▽ More
Although the Vlasov equation is used as a good approximation for a sufficiently large $N$, Braun and Hepp have showed that the time evolution of the one particle distribution function of a $N$ particle classical Hamiltonian system with long range interactions satisfies the Vlasov equation in the limit of infinite $N$. Here we rederive this result using a different approach allowing a discussion of the role of inter-particle correlations on the system dynamics. Otherwise for finite N collisional corrections must be introduced. This has allowed the a quite comprehensive study of the Quasi Stationary States (QSS) but many aspects of the physical interpretations of these states remain unclear. In this paper a proper definition of timescale for long time evolution is discussed and several numerical results are presented, for different values of $N$. Previous reports indicates that the lifetimes of the QSS scale as $N^{1.7}$ or even the system properties scales with $\exp(N)$. However, preliminary results presented here shows indicates that time scale goes as $N^2$ for a different type of initial condition. We also discuss how the form of the inter-particle potential determines the convergence of the $N$-particle dynamics to the Vlasov equation. The results are obtained in the context of following models: the Hamiltonian Mean Field, the Self Gravitating Ring Model, and a 2-D Systems of Gravitating Particles. We have also provided information of the validity of the Vlasov equation for finite $N$, i. e.\ how the dynamics converges to the mean-field (Vlasov) description as $N$ increases and how inter-particle correlations arise.
△ Less
Submitted 19 May, 2013; v1 submitted 13 May, 2013;
originally announced May 2013.
-
Truncated Lévy Flights and Weak Ergodicity Breaking in the Hamiltonian Mean Field Model
Authors:
A. Figueiredo,
Z. T. Oliveira Jr,
T. M. Rocha Filho,
R. Matsushita,
M. A. Amato
Abstract:
The dynamics of the Hamiltonian mean field model is studied in the context of continuous time random walks. We show that the sojourn times in cells in the momentum space are well described by a Lévy truncated distribution. Consequently the system in weakly non-ergodic for long times that diverge with the number of particles. For a finite number of particles ergodicity is only attained for very lon…
▽ More
The dynamics of the Hamiltonian mean field model is studied in the context of continuous time random walks. We show that the sojourn times in cells in the momentum space are well described by a Lévy truncated distribution. Consequently the system in weakly non-ergodic for long times that diverge with the number of particles. For a finite number of particles ergodicity is only attained for very long times both at thermodynamical equilibrium and at quasi-stationary out of equilibrium states.
△ Less
Submitted 3 January, 2014; v1 submitted 23 August, 2012;
originally announced August 2012.
-
Nonequilibrium phase transitions and violent relaxation in the Hamiltonian Mean Field model
Authors:
M. Rocha Filho,
M. A. Amato,
A. Figueiredo
Abstract:
We discuss the nature of nonequilibrium phase transitions in the Hamiltonian Mean Field model using detailed numerical simulation of the Vlasov equation and molecular dynamics. Starting from fixed magnetization waterbag initial distributions and varying the energy, the states obtained after a violent relaxation undergoes a phase transition from magnetized to non-magnetized states when going from l…
▽ More
We discuss the nature of nonequilibrium phase transitions in the Hamiltonian Mean Field model using detailed numerical simulation of the Vlasov equation and molecular dynamics. Starting from fixed magnetization waterbag initial distributions and varying the energy, the states obtained after a violent relaxation undergoes a phase transition from magnetized to non-magnetized states when going from lower to higher energies. The phase transitions are either first order or composed by a cascade of phase reentrances. This result is at variance with most previous results in the literature mainly based in Lynden-Bell theory of violent relaxation. The latter is a rough approximation and consequently not suited for an accurate description of nonequilibrium phase transition in long range interacting systems.
△ Less
Submitted 29 February, 2012;
originally announced March 2012.
-
Phase transitions in simplified models with long-range interactions
Authors:
T. M. Rocha Filho,
M. A. Amato,
B. A. Mello,
A. Figueiredo
Abstract:
We study the origin of phase transitions in some simplified models with long range interactions. For the ring model, we show that a possible new phase transition predicted in a recent paper by Nardini and Casetti from an energy landscape analysis does not occur. Instead of such phase transitions we observe a sharp, although without any non-analiticity, change from a core-halo to an only core confi…
▽ More
We study the origin of phase transitions in some simplified models with long range interactions. For the ring model, we show that a possible new phase transition predicted in a recent paper by Nardini and Casetti from an energy landscape analysis does not occur. Instead of such phase transitions we observe a sharp, although without any non-analiticity, change from a core-halo to an only core configuration in the spatial distribution functions for low energies. By introducing a new class of solvable simplified models without any critical points in the potential energy, we show that a similar behaviour to the ring model is obtained, with a first order phase transition from an almost homogeneous high energy phase to a clustered phase, and the same core-halo to core configuration transition at lower energies. We discuss the origin of these features of the simplified models, and show that the first order phase transition comes from the maximization of the entropy of the system as a function of energy an an order parameter, as previously discussed by Kastner, which seems to be the main mechanism causing phase transitions in long-range interacting systems.
△ Less
Submitted 26 August, 2011; v1 submitted 13 April, 2011;
originally announced April 2011.
-
Ergodicity and Central Limit Theorem in Systems with Long-Range Interactions
Authors:
Annibal Figueiredo,
Tarcisio Marciano da Rocha Filho,
Marco Antonio Amato
Abstract:
In this letter we discuss the validity of the ergodicity hypothesis in theories of violent relaxation in long-range interacting systems. We base our reasoning on the Hamiltonian Mean Field model and show that the life-time of quasi-stationary states resulting from the violent relaxation does not allow the system to reach a complete mixed state. We also discuss the applicability of a generalizati…
▽ More
In this letter we discuss the validity of the ergodicity hypothesis in theories of violent relaxation in long-range interacting systems. We base our reasoning on the Hamiltonian Mean Field model and show that the life-time of quasi-stationary states resulting from the violent relaxation does not allow the system to reach a complete mixed state. We also discuss the applicability of a generalization of the central limit theorem. In this context, we show that no attractor exists in distribution space for the sum of velocities of a particle other than the Gaussian distribution. The long-range nature of the interaction leads in fact to a new instance of sluggish convergence to a Gaussian distribution.
△ Less
Submitted 11 May, 2008;
originally announced May 2008.
-
Reply to ``Comment on `Entropy of classical systems with long-range interactions' ''
Authors:
Tarcisio Marciano Rocha Filho,
Annibal Figueiredo,
Marco Antonio Amato
Abstract:
We offer o reply for the comments of Rapisarda et al. cond-mat/0601409 on our letter "Entropy of Classical Systems with Long Range Interactios", published in Phys. Rev. Lett. Vol 95 (2005) 190601.
We offer o reply for the comments of Rapisarda et al. cond-mat/0601409 on our letter "Entropy of Classical Systems with Long Range Interactios", published in Phys. Rev. Lett. Vol 95 (2005) 190601.
△ Less
Submitted 16 March, 2006;
originally announced March 2006.
-
On the entropy of classical systems with long-range interaction
Authors:
T. M. Rocha Filho,
A. Figueiredo,
M. A. Amato
Abstract:
We discuss the form of the entropy for classical hamiltonian systems with long-range interaction using the Vlasov equation which describes the dynamics of a $N$-particle in the limit $N\to\infty$. The stationary states of the hamiltonian system are subject to infinite conserved quantities due to the Vlasov dynamics. We show that the stationary states correspond to an extremum of the Boltzmann-Gi…
▽ More
We discuss the form of the entropy for classical hamiltonian systems with long-range interaction using the Vlasov equation which describes the dynamics of a $N$-particle in the limit $N\to\infty$. The stationary states of the hamiltonian system are subject to infinite conserved quantities due to the Vlasov dynamics. We show that the stationary states correspond to an extremum of the Boltzmann-Gibbs entropy, and their stability is obtained from the condition that this extremum is a maximum. As a consequence the entropy is a function of an infinite set of Lagrange multipliers that depend on the initial condition. We also discuss in this context the meaning of ensemble inequivalence and the temperature.
△ Less
Submitted 3 October, 2005;
originally announced October 2005.