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Sep 17, 2013
09/13

by
F. Mancini

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A well-established method to deal with highly correlated systems is based on the expansion of the Green's function in terms of spectral moments. In the context of the Composite Operator Method one approximation is proposed: a set of n composite fields is assumed as fundamental basis and the dynamics is considered up to the order s. The resulting Green's function has a sn-pole structure. The truncation of the hierarchy of the equations of motion is made at the s-th order and the first s-1...

Source: http://arxiv.org/abs/cond-mat/0007341v1

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58

Sep 23, 2013
09/13

by
F. Mancini

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In this paper, we study the Hubbard model with intersite Coulomb interaction in the ionic limit (i.e. no kinetic energy). It is shown that this model is isomorphic to the spin-1 Ising model in presence of a crystal field and an external magnetic field. We show that for such models it is possible to find, for any dimension, a finite complete set of eigenoperators and eigenvalues of the Hamiltonian. Then, the hierarchy of the equations of motion closes and analytical expressions for the relevant...

Source: http://arxiv.org/abs/cond-mat/0510658v1

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59

Sep 19, 2013
09/13

by
F. Mancini

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The Mott-Hubbard transition is studied in the context of the two-dimensional Hubbard model. Analytical calculations show the existence of a critical value Uc of the potential strength which separates a paramagnetic metallic phase from a paramagnetic insulating phase. Calculations of the density of states and double occupancy show that the ground state in the insulating phase contains always a small fraction of empty and doubly occupied sites. The structure of the ground state is studied by...

Source: http://arxiv.org/abs/cond-mat/9812310v2

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Sep 18, 2013
09/13

by
F. Mancini

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A formulation of the Green's function method is presented in the n-pole approximation. Without referring to a specific model we give a general scheme of calculations that easily permits the computation of the "single-particle" Green's function in terms of the energy matrix. A theorem is proved which states that the moments of the spectral density function are conserved up to the order 2(n-l+1), where l is the order of the composite field. A comparison with the spectral density...

Source: http://arxiv.org/abs/cond-mat/9803276v1

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48

Sep 18, 2013
09/13

by
A. Avella; F. Mancini

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In this paper, we study the Ising model with general spin $S$ in presence of an external magnetic field by means of the equations of motion method and of the Green's function formalism. First, the model is shown to be isomorphic to a fermionic one constituted of $2S$ species of localized particles interacting via an intersite Coulomb interaction. Then, an exact solution is found, for any dimension, in terms of a finite, complete set of eigenoperators of the latter Hamiltonian and of the...

Source: http://arxiv.org/abs/cond-mat/0606274v1

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49

Sep 19, 2013
09/13

by
A. Avella; F. Mancini

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The half-filled two-dimensional Hubbard model in presence of a uniform and static parallel magnetic field has been studied by means of the Composite Operator Method. A fully self-consistent solution, fulfilling all the constrains coming from the Pauli principle, has been found. The relevant features of a metal-insulator transition in presence of a magnetic field have been analyzed. The results qualitatively agree with the ones recently obtained by means of experimental investigations.

Source: http://arxiv.org/abs/cond-mat/0209587v1

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Sep 22, 2013
09/13

by
F. Mancini; A. Avella

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Strongly correlated electron systems require the development of new theoretical schemes in order to describe their unusual and unexpected properties. The usual perturbation schemes are inadequate and new concepts must be introduced. In our scheme of calculations, the Composite Operator Method, is possible to recover, through a self-consistent calculation, a series of fundamental symmetries by choosing a suitable Hilbert space.

Source: http://arxiv.org/abs/cond-mat/9807019v1

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Sep 19, 2013
09/13

by
A. Avella; F. Mancini

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We have compared the results of our approximation scheme, the composite operator method, for the double occupancy and the internal energy of the two-dimensional Hubbard model with numerical data obtained by means of the Lanczos and quantum Monte Carlo schemes. The agreement is very good at both half-filling and away from it showing how reliable is the approximation scheme.

Source: http://arxiv.org/abs/cond-mat/0209673v1

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Jul 20, 2013
07/13

by
F. Mancini; D. Villani

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A new theoretical analysis of the current-charge and charge-charge propagators is presented for the Hubbard model, using the static approximation for the Composite Operator Method. This approximation manifestly preserves a sum rule which governs the single-site dynamics. We compare our results with those obtained by numerical analysis.

Source: http://arxiv.org/abs/cond-mat/9910463v1

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57

Sep 21, 2013
09/13

by
F. Mancini; F. P. Mancini

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Within the Green's function and equations of motion formalism it is possible to exactly solve a large class of models useful for the study of strongly correlated systems. Here, we present the exact solution of the one-dimensional extended Hubbard model with on-site $U$ and first nearest neighbor repulsive V interactions in the presence of an external magnetic field h, in the narrow band limit. At zero temperature our results establish the existence of four phases in the three-dimensional space...

Source: http://arxiv.org/abs/0901.0780v1

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Jul 20, 2013
07/13

by
F. Mancini; F. P. Mancini

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We present the exact solution of the one-dimensional extended Hubbard model in the atomic limit within the Green's function and equation of motion formalism. We provide a comprehensive and systematic analysis of the model by considering all the relevant response and correlation functions as well as thermodynamic quantities in the whole parameter space. At zero temperature we identify four phases in the plane (U,n) [U is the onsite potential and n is the filling] and relative phase transitions...

Source: http://arxiv.org/abs/0804.4419v2

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83

Jul 24, 2013
07/13

by
F. Mancini; F. P. Mancini

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We study the one-dimensional S=1 Blume-Emery-Griffiths model. Upon transforming the spin model into an equivalent fermionic model, we provide the exact solution within the Green's function and equations of motion formalism. We show that the relevant response functions as well as thermodynamic quantities can be determined, in the whole parameters space, in terms of a finite set of local correlators. Furthermore, considering the case of an antiferromagnetic chain with single-ion anisotropy in the...

Source: http://arxiv.org/abs/0807.2177v1

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39

Sep 17, 2013
09/13

by
F. Mancini; F. P. Mancini

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We present the exact solution of a system of Fermi particles living on the sites of a Bethe lattice with coordination number z and interacting through on-site U and nearest-neighbor V interactions. This is a physical realization of the extended Hubbard model in the atomic limit. Within the Green's function and equations of motion formalism, we provide a comprehensive analysis of the model and we study the phase diagram at finite temperature in the whole model's parameter space, allowing for the...

Source: http://arxiv.org/abs/0912.4137v1

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Sep 19, 2013
09/13

by
F. Mancini; D. Villani; H. Matsumoto

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In the low doping region an incommensurate magnetic phase is observed in LSCO. By means of the composite operator method we show that the single-band 2D Hubbard model describes the experimental situation. In the higher doping region, where experiments are not available, the incommensurability is depressed owing to the van Hove singularity near the Fermi level. A proportionality between the incommensurability amplitude and the critical temperature is predicted, suggesting a close relation...

Source: http://arxiv.org/abs/cond-mat/9703207v1

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Sep 23, 2013
09/13

by
A. Avella; F. Mancini; D. Villani

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Recent experimental data for the overdoped LSCO have firmly established the properties which characterize the transition between the superconducting and the Fermi liquid metallic phases. The thermodynamic response functions show a pronounced feature at this point, while the Fermi surface undergoes a dramatic change. By use of the Composite Operator Method for the two-dimensional Hubbard model, it is found that the presence of a van Hove singularity in the lower Hubbard band can explain these...

Source: http://arxiv.org/abs/cond-mat/9806371v1

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65

Sep 18, 2013
09/13

by
E. Plekhanov; A. Avella; F. Mancini

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We present a study of entanglement in the case of the 1D extended anisotropic Heisenberg model. We investigate two quantum phase transitions (QPTs) within the previously found ergodicity phase diagram [E. Plekhanov, A. Avella, and F. Mancini Phys. Rev. B \textbf{74}, 115120 (2006)]. Our calculations are done by means of the numerically exact Lanczos method at T=0, followed by a finite-size scaling. As a measure of entanglement we use the concurrence for two spins out of the system. We conclude...

Source: http://arxiv.org/abs/0705.4413v1

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49

Sep 19, 2013
09/13

by
F. Mancini; D. Villani; H. Matsumoto

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By means of the composite operator method we calculate the specific heat of the 2D Hubbard model. Our results show that the model gives a detailed description of the experimental situation in the normal state of cuprate high Tc superconductors. The observed behavior is consistent with a Fermi liquid picture where the unusual properties are described in terms of the van Hove scenario. There is experimental evidence, based on the data for the spin magnetic susceptibility and specific heat, that...

Source: http://arxiv.org/abs/cond-mat/9703192v1

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Sep 19, 2013
09/13

by
M. Bak; A. Avella; F. Mancini

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The relevance of zero-energy functions, coming from zero-energy modes and present in the structure of bosonic Green's functions, is often underestimated. Usually, their values are fixed by assuming the ergodicity of the dynamics, but it can be shown that this is not always correct. As the zero-energy functions are connected to fundamental response properties of the system under analysis (specific heat, compressibility, susceptibility, etc.), their correct determination is not an irrelevant...

Source: http://arxiv.org/abs/cond-mat/0209527v1

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57

Sep 19, 2013
09/13

by
F. Mancini; D. Villani; H. Matsumoto

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A theoretical analysis of the thermodynamic response functions of the 2D single-band Hubbard model is realized by means of the composite operator method. It is shown that all the features of these quantities can be explained by looking at the dependence of the thermodynamic variables on their conjugate ones. It is found that the numerical data from quantum Monte Carlo techniques for the internal energy and electronic specific heat are well reproduced. The anomalous normal state properties in...

Source: http://arxiv.org/abs/cond-mat/9709189v3

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117

Sep 18, 2013
09/13

by
A. Avella; F. Mancini; S. Odashima

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The electronic states of the Hubbard model are investigated by use of the Composite Operator Method. In addition to the Hubbard operators, two other operators related with two-site composite excitations are included in the basis. Within the present formulation, higher-order composite excitations are reduced to the chosen operatorial basis by means of a procedure preserving the particle-hole symmetry. The positive comparison with numerical simulations for the double occupancy indicates that such...

Source: http://arxiv.org/abs/cond-mat/0401083v1

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46

Sep 18, 2013
09/13

by
F. Mancini; E. Plekhanov; G. Sica

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In this paper we present for the first time the exact solution in the narrow-band limit of the 1D extended Hubbard model with nearest-neighbour spin-spin interactions described by an exchange constant J. An external magnetic field h is also taken into account. This result has been obtained in the framework of the Green's functions formalism, using the Composite Operator Method. By means of this theoretical background, we have studied some relevant features such as double occupancy,...

Source: http://arxiv.org/abs/1211.6551v1

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49

Sep 18, 2013
09/13

by
E. Plekhanov; A. Avella; F. Mancini

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By using Density Matrix Renormalization Group (DMRG) technique we study the 1D extended anisotropic Heisenberg model. We find that starting from the ferromagnetic phase, the system undergoes two quantum phase transitions (QPTs) induced by frustration. By increasing the next-nearest-neighbor (NNN) interaction, the ground state of the system changes smoothly from a completely polarized state to a NNN correlated one. On the contrary, letting the in-plane interaction to be greater than the...

Source: http://arxiv.org/abs/0705.4361v1

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56

Sep 22, 2013
09/13

by
A. Avella; F. Mancini; D. Villani

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The results of G. Su and M. Suzuki [Phys. Rev. B 54, 8291 (1996); ibidem 57, 13367 (1998)] for the spin and pseudo-spin symmetry properties of the Hubbard model are reexamined. We point out that the exact relations they have found are valid down to zero temperature and that their solutions for both spin and pseudo-spin correlation functions are incorrect.

Source: http://arxiv.org/abs/cond-mat/9807402v1

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54

Sep 22, 2013
09/13

by
F. Mancini; F. P. Mancini; A. Naddeo

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We consider the extended Hubbard model in the atomic limit on a Bethe lattice with coordination number z. By using the equations of motion formalism, the model is exactly solved for both attractive and repulsive intersite potential V. By focusing on the case of negative V, i.e., attractive intersite interaction, we study the phase diagram at finite temperature and find, for various values of the filling and of the on-site coupling U, a phase transition towards a state with phase separation. We...

Source: http://arxiv.org/abs/0811.3310v1

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49

Sep 17, 2013
09/13

by
F. Mancini; F. P. Mancini; A. Naddeo

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We study the phase diagram at finite temperature of a system of Fermi particles on the sites of the Bethe lattice with coordination number z and interacting through onsite U and nearest-neighbor V interactions. This is a physical realization of the extended Hubbard model in the atomic limit. By using the equations of motion method, we exactly solve the model. For an attractive intersite potential, we find, at half filling, a phase transition towards a broken particle-hole symmetry state. The...

Source: http://arxiv.org/abs/0711.0318v1

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55

Sep 22, 2013
09/13

by
M. M. Sanchez; A. Avella; F. Mancini

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The one-dimensional Hubbard model at half-filling is studied in the framework of the Composite Operator Method using a static approximation. A solution characterized by strong antiferromagnetic correlations and a gap for any nonzero on-site interaction U is found. The corresponding ground-state energy, double occupancy and specific heat are in excellent agreement with those obtained within the Bethe Ansatz. These results show that the Composite Operator Method is an appropriate framework for...

Source: http://arxiv.org/abs/cond-mat/9807368v1

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41

Sep 22, 2013
09/13

by
F. Mancini; N. B. Perkins; N. M. Plakida

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Spin dynamics is calculated in the ferromagnetic (FM) state of the generalized Kondo lattice model taking into account strong on-site correlations between e_g electrons and antiferromagnetic (AFM) exchange among t_{2g} spins. Our study suggests that competing FM double-exchange and AFM super-exchange interaction lead to a rather nontrivial spin-wave spectrum. While spin excitations have a conventional Dq^2 spectrum in the long-wavelength limit, there is a strong deviation from the spin-wave...

Source: http://arxiv.org/abs/cond-mat/0011464v1

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49

Sep 22, 2013
09/13

by
A. Avella; F. Mancini; G. Scelza; S. Chaturvedi

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We study the two-orbital Hubbard model in the limit of vanishing kinetic energy. The phase diagram in the $V-J$ plane, with $V$ and $J$ denoting the interorbital hybridization and exchange coupling respectively, at half filling is obtained. A singlet(dimer)-triplet transition is found for a critical value of the ratio $V/J.$ The entropy of formation, both in the mode and in the particle picture, presents a jump as the same critical line in conformity with the suggested relation between...

Source: http://arxiv.org/abs/0906.2370v1

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54

Sep 17, 2013
09/13

by
V. Fiorentino; F. Mancini; E. Zasinas; A. F. Barabanov

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The p-d model which well describes the CuO_2 planes of the high-Tc superconductors is studied by means of the Composite Operator Method (COM). The relevant quasi-particle excitations are represented by composite operators. As a result of taking into account spin excitations we find a p- like band near the Fermi level. The dispersion of this band gives a Fermi surface which is in good agreement with the experimental measurements. Due to the strong mixing of the relevant excitations, the spectral...

Source: http://arxiv.org/abs/cond-mat/0007069v1

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49

Sep 22, 2013
09/13

by
A. Avella; F. Mancini; M. M. Sanchez; R. Sridhar

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We propose a theoretical framework to describe the ladder systems. The N-chain Hubbard model has been studied within the Composite Operator Method. In this scheme of calculations the single-particle Green's function for any number of coupled chains is obtained by solving self-consistently a system of integral equations.

Source: http://arxiv.org/abs/cond-mat/9807355v1

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50

Sep 19, 2013
09/13

by
E Plekhanov; A Avella; F Mancini; F P Mancini

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The orbital degrees of freedom are of vital importance in explanation of various phenomena. Among them is the orbital-selective Mott transition (OSMT), which is thought to occur in several materials as Ca$_{2-x}$Sr$_x$RuO$_4$ and La$_{n+1}$Ni${_n}$O$_{3n+1}$. OSMT is usually studied in the infinite-dimension limit, and for the time being, it is not clear if it could survive in one-dimensional (1D) case. There exist two scenarios for the OSMT: upon increasing the interaction in a two-band system...

Source: http://arxiv.org/abs/1009.4448v1

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48

Sep 23, 2013
09/13

by
A. Avella; F. Mancini; D. Villani; L. Siurakshina; V. Yu. Yushankhai

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The two-dimensional Hubbard model is analyzed in the framework of the two-pole expansion. It is demonstrated that several theoretical approaches, when considered at their lowest level, are all equivalent and share the property of satisfying the conservation of the first four spectral momenta. It emerges that the various methods differ only in the way of fixing the internal parameters and that it exists a unique way to preserve simultaneously the Pauli principle and the particle-hole symmetry. A...

Source: http://arxiv.org/abs/cond-mat/9708009v2

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52

Sep 18, 2013
09/13

by
Yu. A. Izyumov; N. I. Chaschin; D. S. Alexeev; F. Mancini

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The method of generating functional is generalized to the case of strongly correlated systems, and applied to the Hubbard model. For the electronic Green's function constructed for Hubbard operators, an equation using variational derivatives with respect to the fluctuating fields has been derived and its multiplicative form has been determined. Corrections for the electronic self-energy are calculated up to the second order with respect to the parameter W/U (W width of the band), and a mean...

Source: http://arxiv.org/abs/cond-mat/0504723v1

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25

Jun 28, 2018
06/18

by
J. Rajeswari; H. Ping; G. F. Mancini; Y. Murooka; T. Latychevskaia; D. McGrouther; M. Cantoni; E. Baldini; J. S. White; A. Magrez; T. Giamarchi; H. M. Rønnow; F. Carbone

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Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects or external stimuli, on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3x7.3{\mu}m$^{2}$) single crystal nano-slice of Cu$_{2}$OSeO$_{3}$, we image up to...

Topics: Mesoscale and Nanoscale Physics, Condensed Matter

Source: http://arxiv.org/abs/1506.07694

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3.0

Jun 29, 2018
06/18

by
Elisabeth R. Shanblatt; Christina L. Porter; Dennis F. Gardner; Giulia F. Mancini; Robert M. Karl; Michael D. Tanksalvala; Charles S. Bevis; Victor H. Vartanian; Henry C. Kapteyn; Daniel E. Adams; Margaret M. Murnane

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Characterizing buried layers and interfaces is critical for a host of applications in nanoscience and nano-manufacturing. Here we demonstrate non-invasive, non-destructive imaging of buried interfaces using a tabletop, extreme ultraviolet (EUV), coherent diffractive imaging (CDI) nanoscope. Copper nanostructures inlaid in SiO2 are coated with 100 nm of aluminum, which is opaque to visible light and thick enough that neither optical microscopy nor atomic force microscopy can image the buried...

Topics: Optics, Physics

Source: http://arxiv.org/abs/1603.01301

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Jun 30, 2018
06/18

by
B. Mansart; M. J. G. Cottet; G. F. Mancini; T. Jarlborg; S. B. Dugdale; S. L. Johnson; S. O. Mariager; C. J. Milne; P. Beaud; S. Grübel; J. A. Johnson; T. Kubacka; G. Ingold; K. Prsa; H. M. Rønnow; K. Conder; E. Pomjakushina; M. Chergui; F. Carbone

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The strength of the electron-phonon coupling parameter and its evolution throughout a solid's phase diagram often determines phenomena such as superconductivity, charge- and spin-density waves. Its experimental determination relies on the ability to distinguish thermally activated phonons from those emitted by conduction band electrons, which can be achieved in an elegant way by ultrafast techniques. Separating the electronic from the out-of-equilibrium lattice subsystems, we probed their...

Topics: Superconductivity, Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1401.4072