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Papers by Giulio Filippis
Proceedings
We discuss the quantum annealing of the fully-connected ferromagnetic p-spin model in a dissipati... more We discuss the quantum annealing of the fully-connected ferromagnetic p-spin model in a dissipative environment at low temperature. This model, in the large p limit, encodes in its ground state the solution to the Grover’s problem of searching in unsorted databases. In the framework of the quantum circuit model, a quantum algorithm is known for this task, providing a quadratic speed-up with respect to its best classical counterpart. This improvement is not recovered in adiabatic quantum computation for an isolated quantum processor. We analyze the same problem in the presence of a low-temperature reservoir, using a Markovian quantum master equation in Lindblad form, and we show that a thermal enhancement is achieved in the presence of a zero temperature environment moderately coupled to the quantum annealer.
Physical Review B
The Kubo formula for the electrical conductivity is expressed in terms of a weighted sum of Drude... more The Kubo formula for the electrical conductivity is expressed in terms of a weighted sum of Drude-like contributions associated to the exact eigenstates of the interacting system, each characterized by its own frequencydependent relaxation time. This alternative formulation considerably simplifies the access to the static properties (dc conductivity) and resolves the long-standing difficulty to connect the Boltzmann transport theory and the Kubo formula. In particular, at the lowest order of the perturbation theory, the correct transport scattering lifetime depending on the momentum k, which appears in the Boltzmann theory, instead of the single-electron lifetime appearing in the Green function, can be recovered. This alternative formulation is applied to (i) the elastic scattering in metals, and (ii) the inelastic scattering in the Fröhlich polaron model to obtain the exact result of the mobility in the low-temperature weak-coupling limit.
Physical Review B
We study theoretically the transport properties of a three-dimensional spin texture made from thr... more We study theoretically the transport properties of a three-dimensional spin texture made from three orthogonal helices, which is essentially a lattice of monopole-antimonopole pairs connected by skyrmion strings. This spin structure is proposed for MnGe based on neutron scattering experiments as well as Lorentz transmission electron microscopy observations. Equipped with a sophisticated spectral analysis method, we adopt the finite temperature Green's function technique to calculate the longitudinal dc electric transport in such a system. We consider conduction electrons interacting with spin waves of the topologically nontrivial spin texture, wherein fluctuations of monopolar emergent magnetic fields enter. We study in detail the behavior of electric resistivity under the influence of temperature, external magnetic field, and a characteristic monopole motion, especially a novel magnetoresistivity effect describing the latest experimental observations in MnGe, wherein a topological phase transition signifying strong correlations is identified.
Nature communications, 2017
Topological insulators are fascinating states of matter exhibiting protected edge states and robu... more Topological insulators are fascinating states of matter exhibiting protected edge states and robust quantized features in their bulk. Here we propose and validate experimentally a method to detect topological properties in the bulk of one-dimensional chiral systems. We first introduce the mean chiral displacement, an observable that rapidly approaches a value proportional to the Zak phase during the free evolution of the system. Then we measure the Zak phase in a photonic quantum walk of twisted photons, by observing the mean chiral displacement in its bulk. Next, we measure the Zak phase in an alternative, inequivalent timeframe and combine the two windings to characterize the full phase diagram of this Floquet system. Finally, we prove the robustness of the measure by introducing dynamical disorder in the system. This detection method is extremely general and readily applicable to all present one-dimensional platforms simulating static or Floquet chiral systems.
Nature Communications, 2016
Physical review letters, Jan 27, 2015
The transport properties at finite temperature of crystalline organic semiconductors are investig... more The transport properties at finite temperature of crystalline organic semiconductors are investigated, within the Su-Schrieffer-Heeger model, by combining an exact diagonalization technique, Monte Carlo approaches, and a maximum entropy method. The temperature-dependent mobility data measured in single crystals of rubrene are successfully reproduced: a crossover from super- to subdiffusive motion occurs in the range 150≤T≤200 K, where the mean free path becomes of the order of the lattice parameter and strong memory effects start to appear. We provide an effective model, which can successfully explain features of the absorption spectra at low frequencies. The observed response to slowly varying electric field is interpreted by means of a simple model where the interaction between the charge carrier and lattice polarization modes is simulated by a harmonic interaction between a fictitious particle and an electron embedded in a viscous fluid.
Electron-screening effects on the self-trapping of polarons
Physical review. B, Condensed matter, Jan 15, 1996
In a polar semiconductor with a number of electrons or holes in the conduction or valence band, t... more In a polar semiconductor with a number of electrons or holes in the conduction or valence band, the interactions of an external charge with the longitudinal optical phonon are screened by the electron/hole density. Consequently the self-energy depends on the density. We show that, for any value of the Fröhlich electron-phonon coupling constant alpha, an electronic density n* can be
Physical Review B, 2001
The interplay of the electron-phonon interaction and of the double and super exchange magnetic ef... more The interplay of the electron-phonon interaction and of the double and super exchange magnetic effects is analyzed in the La 1−x Ca x M nO 3 perovskites with 0 < x < 0.5. By using an analytical variational scheme that allows to treat the electron-phonon interaction in a fully quantum manner it is shown that in the intermediate electron-phonon coupling regime a charge density instability occurs near the metal-insulator transition, induced by either temperature or hole doping. The system segregates in antiferromagnetic or paramagnetic small polarons and ferromagnetic large polarons domains characterized by different values of the lattice distortions that are satisfactory compared with those experimentally observed.
New formulation of the Kubo Optical Conductivity: a Shortcut to Transport Properties
Mobility of Holstein polaron: an unbiased approach
Nature Communications, 2014
Physical Review B, 2009
The temperature dependence of the transport properties of ͑SrMnO 3 ͒ n / ͑LaMnO 3 ͒ 2n superlatti... more The temperature dependence of the transport properties of ͑SrMnO 3 ͒ n / ͑LaMnO 3 ͒ 2n superlattices has been systematically investigated by changing the number of interfaces while keeping the total thickness of the system fixed. The superlattices undergo a rich variety of transitions among metal, Mott variable range hopping insulator, interaction-induced Efros-Shklovskii insulator, and polaronic insulator. Interfaces play a fundamental role in tuning the metal-insulator transitions controlling the effective doping of the different layers. In the case of a bilayer, as clarified by a theoretical model, the single interface can extend its effects well far from it in the bulk layer.
Physical Review B - Condensed Matter and Materials Physics, 2010
Double-exchange ͑DE͒ mechanisms in RE 1−x AE x MnO 3 manganites ͑where RE is a trivalent rare-ear... more Double-exchange ͑DE͒ mechanisms in RE 1−x AE x MnO 3 manganites ͑where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion͒ relies on the strong exchange interaction between two Mn 3+ and Mn 4+ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the RE-AE elements by Mn 2+ ions, in La-deficient La x MnO 3−␦ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn 2+ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. Mn 2+ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling Mn 2+ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of Mn 2+ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.
Physical Review B, 2003
Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in... more Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the M n − O − M n bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out important in influencing the balance of coexisting phases in the film.
Springer Series in Materials Science, 2007
Physical Review B, 2001
The influence of next-nearest neighbor electron hopping, t ′ , on the polaron and bipolaron forma... more The influence of next-nearest neighbor electron hopping, t ′ , on the polaron and bipolaron formation in a square Hubbard-Holstein model is investigated within a variational approach. The results for electron-phonon and electronelectron correlation functions show that a negative value of t ′ induces a strong anisotropy in the lattice distortions favoring the formation of nearest neighbor intersite bipolaron. The role of t ′ , electron-phonon and electron-electron interactions is briefly discussed in view of the formation of charged striped domains.
Proceedings
We discuss the quantum annealing of the fully-connected ferromagnetic p-spin model in a dissipati... more We discuss the quantum annealing of the fully-connected ferromagnetic p-spin model in a dissipative environment at low temperature. This model, in the large p limit, encodes in its ground state the solution to the Grover’s problem of searching in unsorted databases. In the framework of the quantum circuit model, a quantum algorithm is known for this task, providing a quadratic speed-up with respect to its best classical counterpart. This improvement is not recovered in adiabatic quantum computation for an isolated quantum processor. We analyze the same problem in the presence of a low-temperature reservoir, using a Markovian quantum master equation in Lindblad form, and we show that a thermal enhancement is achieved in the presence of a zero temperature environment moderately coupled to the quantum annealer.
Physical Review B
The Kubo formula for the electrical conductivity is expressed in terms of a weighted sum of Drude... more The Kubo formula for the electrical conductivity is expressed in terms of a weighted sum of Drude-like contributions associated to the exact eigenstates of the interacting system, each characterized by its own frequencydependent relaxation time. This alternative formulation considerably simplifies the access to the static properties (dc conductivity) and resolves the long-standing difficulty to connect the Boltzmann transport theory and the Kubo formula. In particular, at the lowest order of the perturbation theory, the correct transport scattering lifetime depending on the momentum k, which appears in the Boltzmann theory, instead of the single-electron lifetime appearing in the Green function, can be recovered. This alternative formulation is applied to (i) the elastic scattering in metals, and (ii) the inelastic scattering in the Fröhlich polaron model to obtain the exact result of the mobility in the low-temperature weak-coupling limit.
Physical Review B
We study theoretically the transport properties of a three-dimensional spin texture made from thr... more We study theoretically the transport properties of a three-dimensional spin texture made from three orthogonal helices, which is essentially a lattice of monopole-antimonopole pairs connected by skyrmion strings. This spin structure is proposed for MnGe based on neutron scattering experiments as well as Lorentz transmission electron microscopy observations. Equipped with a sophisticated spectral analysis method, we adopt the finite temperature Green's function technique to calculate the longitudinal dc electric transport in such a system. We consider conduction electrons interacting with spin waves of the topologically nontrivial spin texture, wherein fluctuations of monopolar emergent magnetic fields enter. We study in detail the behavior of electric resistivity under the influence of temperature, external magnetic field, and a characteristic monopole motion, especially a novel magnetoresistivity effect describing the latest experimental observations in MnGe, wherein a topological phase transition signifying strong correlations is identified.
Nature communications, 2017
Topological insulators are fascinating states of matter exhibiting protected edge states and robu... more Topological insulators are fascinating states of matter exhibiting protected edge states and robust quantized features in their bulk. Here we propose and validate experimentally a method to detect topological properties in the bulk of one-dimensional chiral systems. We first introduce the mean chiral displacement, an observable that rapidly approaches a value proportional to the Zak phase during the free evolution of the system. Then we measure the Zak phase in a photonic quantum walk of twisted photons, by observing the mean chiral displacement in its bulk. Next, we measure the Zak phase in an alternative, inequivalent timeframe and combine the two windings to characterize the full phase diagram of this Floquet system. Finally, we prove the robustness of the measure by introducing dynamical disorder in the system. This detection method is extremely general and readily applicable to all present one-dimensional platforms simulating static or Floquet chiral systems.
Nature Communications, 2016
Physical review letters, Jan 27, 2015
The transport properties at finite temperature of crystalline organic semiconductors are investig... more The transport properties at finite temperature of crystalline organic semiconductors are investigated, within the Su-Schrieffer-Heeger model, by combining an exact diagonalization technique, Monte Carlo approaches, and a maximum entropy method. The temperature-dependent mobility data measured in single crystals of rubrene are successfully reproduced: a crossover from super- to subdiffusive motion occurs in the range 150≤T≤200 K, where the mean free path becomes of the order of the lattice parameter and strong memory effects start to appear. We provide an effective model, which can successfully explain features of the absorption spectra at low frequencies. The observed response to slowly varying electric field is interpreted by means of a simple model where the interaction between the charge carrier and lattice polarization modes is simulated by a harmonic interaction between a fictitious particle and an electron embedded in a viscous fluid.
Electron-screening effects on the self-trapping of polarons
Physical review. B, Condensed matter, Jan 15, 1996
In a polar semiconductor with a number of electrons or holes in the conduction or valence band, t... more In a polar semiconductor with a number of electrons or holes in the conduction or valence band, the interactions of an external charge with the longitudinal optical phonon are screened by the electron/hole density. Consequently the self-energy depends on the density. We show that, for any value of the Fröhlich electron-phonon coupling constant alpha, an electronic density n* can be
Physical Review B, 2001
The interplay of the electron-phonon interaction and of the double and super exchange magnetic ef... more The interplay of the electron-phonon interaction and of the double and super exchange magnetic effects is analyzed in the La 1−x Ca x M nO 3 perovskites with 0 < x < 0.5. By using an analytical variational scheme that allows to treat the electron-phonon interaction in a fully quantum manner it is shown that in the intermediate electron-phonon coupling regime a charge density instability occurs near the metal-insulator transition, induced by either temperature or hole doping. The system segregates in antiferromagnetic or paramagnetic small polarons and ferromagnetic large polarons domains characterized by different values of the lattice distortions that are satisfactory compared with those experimentally observed.
New formulation of the Kubo Optical Conductivity: a Shortcut to Transport Properties
Mobility of Holstein polaron: an unbiased approach
Nature Communications, 2014
Physical Review B, 2009
The temperature dependence of the transport properties of ͑SrMnO 3 ͒ n / ͑LaMnO 3 ͒ 2n superlatti... more The temperature dependence of the transport properties of ͑SrMnO 3 ͒ n / ͑LaMnO 3 ͒ 2n superlattices has been systematically investigated by changing the number of interfaces while keeping the total thickness of the system fixed. The superlattices undergo a rich variety of transitions among metal, Mott variable range hopping insulator, interaction-induced Efros-Shklovskii insulator, and polaronic insulator. Interfaces play a fundamental role in tuning the metal-insulator transitions controlling the effective doping of the different layers. In the case of a bilayer, as clarified by a theoretical model, the single interface can extend its effects well far from it in the bulk layer.
Physical Review B - Condensed Matter and Materials Physics, 2010
Double-exchange ͑DE͒ mechanisms in RE 1−x AE x MnO 3 manganites ͑where RE is a trivalent rare-ear... more Double-exchange ͑DE͒ mechanisms in RE 1−x AE x MnO 3 manganites ͑where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion͒ relies on the strong exchange interaction between two Mn 3+ and Mn 4+ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the RE-AE elements by Mn 2+ ions, in La-deficient La x MnO 3−␦ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn 2+ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. Mn 2+ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling Mn 2+ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of Mn 2+ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.
Physical Review B, 2003
Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in... more Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the M n − O − M n bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out important in influencing the balance of coexisting phases in the film.
Springer Series in Materials Science, 2007
Physical Review B, 2001
The influence of next-nearest neighbor electron hopping, t ′ , on the polaron and bipolaron forma... more The influence of next-nearest neighbor electron hopping, t ′ , on the polaron and bipolaron formation in a square Hubbard-Holstein model is investigated within a variational approach. The results for electron-phonon and electronelectron correlation functions show that a negative value of t ′ induces a strong anisotropy in the lattice distortions favoring the formation of nearest neighbor intersite bipolaron. The role of t ′ , electron-phonon and electron-electron interactions is briefly discussed in view of the formation of charged striped domains.