arXiv subjects

A. Debernardi

Publications and source records attributed to A. Debernardi.

8 recordsLinked to original sources

arXiv2020

Impurity induced topological phase transitions in Cd$_3$As$_2$ and Na$_3$Bi Dirac semimetals

Using first-principles density functional theory calculations, combined with a topological analysis, we have investigated the electronic properties of $Cd_3As_2$ and $Na_3Bi$ Dirac topological semimetals doped with non-magnetic and magnetic impurities. Our systematic analysis shows that the selective breaking of the inversion, rotational and time-reversal symmetry, controlled by specific choices of the impurity doping, induces phase transitions from the original Dirac semimetal to a variety of topological phases such as, topological insulator, trivial semimetal, non-magnetic and magnetic Weyl semimetal, and Chern insulator. The Dirac semimetal phase can exist only if the rotational symmetry $C_n$ with $n > 2$ is maintained. One particularly interesting phase emerging in doped $Cd_3As_2$ is a coexisting Dirac-Weyl phase, which occurs when only inversion symmetry is broken while time-reversal symmetry and rotational symmetry are both preserved. To further characterize the low-energy excitations of this phase, we have complemented our density functional results with a continuum four-band $k\cdot p$ model, which indeed displays nodal points of both Dirac and Weyl type. The coexisting phase appears as a transition point between two topologically distinct Dirac phases, but may also survive in a small region of parameter space controlled by external strain.

cond-mat.str-el↗

arXiv2020

Critical behavior of the insulator-to-metal transition in Te-hyperdoped Si

Hyperdoping Si with chalcogens is a topic of great interest due to the strong sub-bandgap absorption exhibited by the resulting material, which can be exploited to develop broadband room-temperature infrared photodetectors using fully Si-compatible technology. Here, we report on the critical behavior of the impurity-driven insulator-to-metal transition in Te-hyperdoped Si layers fabricated via ion implantation followed by nanosecond pulsed-laser melting. Electrical transport measurements reveal an insulator-to-metal transition, which is also confirmed and understood by density functional theory calculations. We demonstrate that the metallic phase is governed by a power law dependence of the conductivity at temperatures below 25 K, whereas the conductivity in the insulating phase is well described by a variable-range hopping mechanism with a Coulomb gap at temperatures in the range of 2-50 K. These results show that the electron wave-function in the vicinity of the transition is strongly affected by the disorder and the electron-electron interaction.

cond-mat.mtrl-sci↗

arXiv2018

Weighted norm inequalities for generalized Fourier-type transforms and applications

We obtain necessary and sufficient conditions on weights for the generalized Fourier-type transforms to be bounded between weighted $L^p-L^q$ spaces. As an important example, we investigate transforms with kernel of power type, as for instance the sine, Hankel or $\mathscr{H}_α$ transforms. The obtained necessary and sufficient conditions are given in terms of weights, but not in terms of their decreasing rearrangements, as in several previous investigations.

math.CA↗

arXiv2018

Uniform convergence of Hankel transforms

We investigate necessary and/or sufficient conditions for the pointwise and uniform convergence of the weighted Hankel transforms $$\mathcal{L}^α_{ν,μ}f(r) = r^μ\int_0^\infty (rt)^νf(t) j_α(rt)\, dt, \quad α\geq -1/2, \quad r\geq 0, $$ where $ν,μ\in \mathbb{R}$ are such that $0\leq μ+ν\leq α+3/2$. We subdivide these transforms into two classes in such a way that the uniform convergence criteria is remarkably different on each class. In more detail, we have the transforms satisfying $μ+ν=0$ (such as the classical Hankel transform), that generalize the cosine transform, and those satisfying $0<μ+ν\leq α+3/2$, generalizing the sine transform.

math.CA↗

arXiv2018

Breaking the doping limit in silicon by deep impurities

N-type doping in Si by shallow impurities, such as P, As and Sb, exhibits an intrinsic limit due to the Fermi-level pinning via defect complexes at high doping concentrations. Here we demonstrate that doping Si with the chalcogen Te by non-equilibrium processing, a deep double donor, can exceed this limit and yield higher electron concentrations. In contrast to shallow impurities, both the interstitial Te fraction decreases with increasing doping concentration and substitutional Te dimers become the dominant configuration as effective donors, leading to a non-saturating carrier concentration as well as to an insulator-to-metal transition. First-principle calculations reveal that the Te dimers possess the lowest formation energy and donate two electrons per dimer to the conduction band. These results provide novel insight into physics of deep impurities and lead to a possible solution for the ultra-high electron concentration needed in today's Si-based nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2002

Anharmonic Self-Energy of Phonons: Ab Initio Calculations and Neutron Spin Echo Measurements

We have calculated (ab initio) and measured (by spin-echo techniques) the anharmonic self-energy of phonons at the X-point of the Brillouin zone for isotopically pure germanium. The real part agrees with former, less accurate, high temperature data obtained by inelastic neutron scattering on natural germanium. For the imaginary part our results provide evidence that transverse acoustic phonons at the X-point are very long lived at low temperatures, i.e. their probability of decay approaches zero, as a consequence of an unusual decay mechanism allowed by energy conservation.

cond-mat.mtrl-sci↗

arXiv2002

Magneto-Optical Kerr Effect of Iron Thin Films on Paramagnetic Substrates

First principles calculations of the magnetic properties and the magneto-optical Kerr effect (MOKE) of iron thin films epitaxially grown on the [001] surface of paramagnetic metals: copper, silver, gold, palladium, and platinum are presented. The role of hybridization with the substrate is investigated and it is shown how the relaxation effects influence the complex Kerr angle. The results are obtained by means of the relativistic full-potential linear muffin-tin method, and the film is modeled using a slab geometry within a supercell technique.

cond-mat↗

arXiv2002

Electronic Properties of Mn-Compounds Under Strain

We study the physical properties of MnAs under strain by using accurate first-principles pseudopotential calculations. Our results provide new insight on the physics of strained multilayer that are grown epitaxially on different lattice mismatched substrates and which are presently of interest for spintronic applications. We compute the strain dependence of the structural parameters, electronic bands, density of states and magnetization. In the region of strain/stress that is easily directly accessible to measurements, the effects on these physical quantities are linear. We also address the case of uniaxial stress inducing sizeable and strongly non linear effects on electronic and magnetic properties.

cond-mat.mtrl-sci↗