Alex Schmidt - Academia.edu (original) (raw)

Papers by Alex Schmidt

Physica a-Statistical Mechanics and Its Applications, 2002

In the calculation of the quantum mechanical partition function special tools are needed to deal ... more In the calculation of the quantum mechanical partition function special tools are needed to deal with the non-commuting operators forming the Hamiltonian. The method more currently used in the study of short-range and infinite range spin glasses in a transverse field is the Trotter-Suzuki formula , that maps a system of quantum spins in d-dimensions to a classical system of spins in (d + 1)-dimensions, and it is suited to perform numerical studies. Another way of dealing with the non-commutativity of quantum mechanical operators is to use Feynman's path integral formulations and to introduce time-ordening by means of an imaginary time 0 ≤ τ ≤ β, where β is the inverse temperature. Both methods use unitarity and closure to express the quantum mechanical partition function as a trace over M intermediate time steps τ ≈ O(1 M) , when M → ∞ , and they must lead to the same exact solution. We may ask, however, if the same holds true as regards to approximations. In the present work we investigate this problem by representing the spin operators as bilinear combinations of fermions fields in the Hamiltoniam for a long range Ising spin glass in a transverse field. This path integral formulation has a natural application in problems in condensed matter theory, where the fermions operators represent electrons that also participate in other physical processes. A possible criticism, however, may be that the spin eigenstates at each site do not belong to one irreducible representation S Z = ± 1 2 , but they are labeled instead by the fermionic occupation numbers n σ = 0 or 1 , giving two more unwanted states with S z = 0. We call this the "four states" (4S) model, and despite the presence of these two unwanted states the 4S-Ising spin glass model describes a spin glass transition with the same characteristics as the Sherrington-Kirkpatrick (SK) model in a replica symmetric theory. A way to get rid of the unwanted states consists in fixing the occupation number n i↑ + n i↓ by means of an integral constraint at every site. We refer this as the "two states" 2S-Ising spin glass model. We analyse the 4S-Ising and 2S-Ising spin glass models in a transverse field, within the static approximation in a replica symmetric theory. The static ansatz neglects time fluctuations and may be considered an approximation similar to mean field theory. When Γ = 0 the static approximation reproduces the exact results obtained by other methods, in particular for the 2S-Ising spin glass model we recover SK equations. The results in both models are very similar; they both exhibit a critical spin glass temperature T c (Γ) that decreases when the strength Γ of the transverse field increases, until it reaches a quantum critical point(QCP) at Γ c , T c (Γ c) = 0. The value of Γ c is the same for both models and the 4S-Ising and 2S-Ising models are identical close to the QCP.

Journal of Magnetism and Magnetic Materials, 2004

Journal of Magnetism and Magnetic Materials, 2001

We have analyzed a fermionic infinite-ranged quantum Heisenberg spin glass with BCS coupling in r... more We have analyzed a fermionic infinite-ranged quantum Heisenberg spin glass with BCS coupling in real space in the presence of a magnetic field. It has been possible to locate the transition line between the normal paramagnetic phase (NP) and the phase where there is a long range order corresponding to pair formation in sites (PAIR). The nature of the transition line is also investigated. This transition ends at T f , the transition temperature between NP and spin glass phase (SG) where the static approximation and replica symmetry ansatz are reliable.

Journal of Magnetism and Magnetic Materials, 2013

The present work studies the effects of a random magnetic field on the competition among Kondo ef... more The present work studies the effects of a random magnetic field on the competition among Kondo effect, spin glass (SG) phase and ferromagnetic (FE) order in disordered cerium systems such as CeNi 1Àx Cu x. A Kondo lattice model is used with an intersite disordered interaction J ij between localized canonical ones [2-6]. Particularly, the competition between Kondo effect, SG-type state and FE has been investigated using distinct types of random exchange interactions as, for example, the Sherrington-Kirkpatrick (SK) model [2,3], the van Hemmen model [6] and the generalized Mattis model [4]. Nevertheless, the disorder in these studies has been introduced only in the random exchange interaction between the localized f-spins.

Journal of Superconductivity and Novel Magnetism, 2009

A minimal two-band model to describe superconducting Fe-pnictide materials is considered. It is a... more A minimal two-band model to describe superconducting Fe-pnictide materials is considered. It is assumed that the electronic structure of these materials may be modeled in terms of two tight-binding bands, whereas the pairing mechanism is driven by an attractive interaction with intraband and inter-band pairing. In the framework of a meanfield BCS-type approach, the two-gap equations at the critical temperature T c are solved, and T c and the chemical potential μ, as functions of the number of carriers n varying the pairing interaction V and cutoff energy ω D , are inferred. Furthermore, using a self-consistent approach, the isotope exponent α, as a function of the number carriers by considering an s-wave pairing symmetry, is calculated. The relevance of the present approach for the description of the recent experimental results of Fe-pnictide materials is finally discussed.

The European Physical Journal B - Condensed Matter, 2002

The Kondo lattice model has been analyzed in the presence of a random inter-site interaction amon... more The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean J0 and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J0, J and its relation with the Kondo interaction coupling JK. These results could be used to address part of the experimental data for the CeN i1−xCux compound, when x ≤ 0.8.

soil degassing CO 2 flux carbon dioxide carbon isotopes volcanic surveillance hydrothermal system... more soil degassing CO 2 flux carbon dioxide carbon isotopes volcanic surveillance hydrothermal system A new method combining measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux was developed in order to qualitatively and quantitatively characterise the CO 2 source feeding the soil CO 2 diffuse degassing. The method was tested in March 2007 at the Solfatara of Pozzuoli volcano degassing area (Naples, Italy) where more than 300 measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux were performed, surveying Solfatara crater and its surroundings. The wide range of CO 2 flux and CO 2 isotopic composition values (from 8.4 g m − 2 d − 1 to 28,834 g m − 2 d − 1 , and from 0.73‰ to −33.54‰, respectively), together with their statistical distributions suggests the occurrence of multiple CO 2 sources feeding soil degassing. The combined interpretation of flux and isotopic data allows us to identify and characterise two distinct gas sources: a hydrothermal and a biogenic source. The soil CO 2 from the hydrothermal source is characterised by a mean δ 13 C CO 2 of −2.3‰ ± 0.9‰, hence close to the isotopic composition of the fumarolic CO 2 (δ 13 C CO 2 = − 1.48‰ ± 0.22‰) and by a mean CO 2 flux of 2875 g m − 2 d − 1. The CO 2 from the biogenic source is characterised by a mean δ 13 C CO 2 of − 19.4‰ ± 2.1‰, and by a mean CO 2 flux of 26 g m − 2 d − 1 , which are both in the range of the typical values for biologic CO 2 soil degassing. This reliable characterisation of the biogenic CO 2 flux would not have been possible by solely applying a statistical analysis of the CO 2 flux values, which is commonly applied in volcanological studies for the partitioning between background fluxes and anomalous CO 2 fluxes. A map of the Solfatara diffuse degassing structure was derived from the estimated threshold for the biogenic CO 2 flux, highlighting that soil degassing of hydrothermal CO 2 mixed in different proportion with biogenic CO 2 occurs over a large area (~0.8 km 2), which extends over the inner part of the Solfatara crater as well as the eastern periphery, corresponding with a NW-SE fault system. The presented method and data analysis are important means of surveillance of the volcanic activity.

En la primera mitad del siglo XX, la obra del historiador correntino Hernán Félix Gómez (1888Góme... more En la primera mitad del siglo XX, la obra del historiador correntino Hernán Félix Gómez (1888Gómez ( -1945, -quien fuera actor central de la historiografía de la provincia-, constituyó el primer intento por brindar un marco teórico-metodológico a los estudios históricos correntinos, así como delinear una perspectiva correntina de la historia argentina. El análisis de sus presupuestos ideológicos y de sus opciones teóricas y metodológicas nos enfrenta con una clara propuesta de reversión de la mirada en torno a la relación centro/periferia, cuyo interés se ve aumentado por la amplia difusión que tuvieron sus obras, que aún hoy son de consulta obligada. Su contribución, enmarcada en la problemática de la inserción de Corrientes en el esquema político contemporáneo, se halla en estrecha vinculación con sus ideas y actividades políticas y educativas.

Physica a-Statistical Mechanics and Its Applications, 2002

In the calculation of the quantum mechanical partition function special tools are needed to deal ... more In the calculation of the quantum mechanical partition function special tools are needed to deal with the non-commuting operators forming the Hamiltonian. The method more currently used in the study of short-range and infinite range spin glasses in a transverse field is the Trotter-Suzuki formula , that maps a system of quantum spins in d-dimensions to a classical system of spins in (d + 1)-dimensions, and it is suited to perform numerical studies. Another way of dealing with the non-commutativity of quantum mechanical operators is to use Feynman's path integral formulations and to introduce time-ordening by means of an imaginary time 0 ≤ τ ≤ β, where β is the inverse temperature. Both methods use unitarity and closure to express the quantum mechanical partition function as a trace over M intermediate time steps τ ≈ O(1 M) , when M → ∞ , and they must lead to the same exact solution. We may ask, however, if the same holds true as regards to approximations. In the present work we investigate this problem by representing the spin operators as bilinear combinations of fermions fields in the Hamiltoniam for a long range Ising spin glass in a transverse field. This path integral formulation has a natural application in problems in condensed matter theory, where the fermions operators represent electrons that also participate in other physical processes. A possible criticism, however, may be that the spin eigenstates at each site do not belong to one irreducible representation S Z = ± 1 2 , but they are labeled instead by the fermionic occupation numbers n σ = 0 or 1 , giving two more unwanted states with S z = 0. We call this the "four states" (4S) model, and despite the presence of these two unwanted states the 4S-Ising spin glass model describes a spin glass transition with the same characteristics as the Sherrington-Kirkpatrick (SK) model in a replica symmetric theory. A way to get rid of the unwanted states consists in fixing the occupation number n i↑ + n i↓ by means of an integral constraint at every site. We refer this as the "two states" 2S-Ising spin glass model. We analyse the 4S-Ising and 2S-Ising spin glass models in a transverse field, within the static approximation in a replica symmetric theory. The static ansatz neglects time fluctuations and may be considered an approximation similar to mean field theory. When Γ = 0 the static approximation reproduces the exact results obtained by other methods, in particular for the 2S-Ising spin glass model we recover SK equations. The results in both models are very similar; they both exhibit a critical spin glass temperature T c (Γ) that decreases when the strength Γ of the transverse field increases, until it reaches a quantum critical point(QCP) at Γ c , T c (Γ c) = 0. The value of Γ c is the same for both models and the 4S-Ising and 2S-Ising models are identical close to the QCP.

Journal of Magnetism and Magnetic Materials, 2004

Journal of Magnetism and Magnetic Materials, 2001

We have analyzed a fermionic infinite-ranged quantum Heisenberg spin glass with BCS coupling in r... more We have analyzed a fermionic infinite-ranged quantum Heisenberg spin glass with BCS coupling in real space in the presence of a magnetic field. It has been possible to locate the transition line between the normal paramagnetic phase (NP) and the phase where there is a long range order corresponding to pair formation in sites (PAIR). The nature of the transition line is also investigated. This transition ends at T f , the transition temperature between NP and spin glass phase (SG) where the static approximation and replica symmetry ansatz are reliable.

Journal of Magnetism and Magnetic Materials, 2013

The present work studies the effects of a random magnetic field on the competition among Kondo ef... more The present work studies the effects of a random magnetic field on the competition among Kondo effect, spin glass (SG) phase and ferromagnetic (FE) order in disordered cerium systems such as CeNi 1Àx Cu x. A Kondo lattice model is used with an intersite disordered interaction J ij between localized canonical ones [2-6]. Particularly, the competition between Kondo effect, SG-type state and FE has been investigated using distinct types of random exchange interactions as, for example, the Sherrington-Kirkpatrick (SK) model [2,3], the van Hemmen model [6] and the generalized Mattis model [4]. Nevertheless, the disorder in these studies has been introduced only in the random exchange interaction between the localized f-spins.

Journal of Superconductivity and Novel Magnetism, 2009

A minimal two-band model to describe superconducting Fe-pnictide materials is considered. It is a... more A minimal two-band model to describe superconducting Fe-pnictide materials is considered. It is assumed that the electronic structure of these materials may be modeled in terms of two tight-binding bands, whereas the pairing mechanism is driven by an attractive interaction with intraband and inter-band pairing. In the framework of a meanfield BCS-type approach, the two-gap equations at the critical temperature T c are solved, and T c and the chemical potential μ, as functions of the number of carriers n varying the pairing interaction V and cutoff energy ω D , are inferred. Furthermore, using a self-consistent approach, the isotope exponent α, as a function of the number carriers by considering an s-wave pairing symmetry, is calculated. The relevance of the present approach for the description of the recent experimental results of Fe-pnictide materials is finally discussed.

The European Physical Journal B - Condensed Matter, 2002

The Kondo lattice model has been analyzed in the presence of a random inter-site interaction amon... more The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean J0 and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J0, J and its relation with the Kondo interaction coupling JK. These results could be used to address part of the experimental data for the CeN i1−xCux compound, when x ≤ 0.8.

soil degassing CO 2 flux carbon dioxide carbon isotopes volcanic surveillance hydrothermal system... more soil degassing CO 2 flux carbon dioxide carbon isotopes volcanic surveillance hydrothermal system A new method combining measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux was developed in order to qualitatively and quantitatively characterise the CO 2 source feeding the soil CO 2 diffuse degassing. The method was tested in March 2007 at the Solfatara of Pozzuoli volcano degassing area (Naples, Italy) where more than 300 measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux were performed, surveying Solfatara crater and its surroundings. The wide range of CO 2 flux and CO 2 isotopic composition values (from 8.4 g m − 2 d − 1 to 28,834 g m − 2 d − 1 , and from 0.73‰ to −33.54‰, respectively), together with their statistical distributions suggests the occurrence of multiple CO 2 sources feeding soil degassing. The combined interpretation of flux and isotopic data allows us to identify and characterise two distinct gas sources: a hydrothermal and a biogenic source. The soil CO 2 from the hydrothermal source is characterised by a mean δ 13 C CO 2 of −2.3‰ ± 0.9‰, hence close to the isotopic composition of the fumarolic CO 2 (δ 13 C CO 2 = − 1.48‰ ± 0.22‰) and by a mean CO 2 flux of 2875 g m − 2 d − 1. The CO 2 from the biogenic source is characterised by a mean δ 13 C CO 2 of − 19.4‰ ± 2.1‰, and by a mean CO 2 flux of 26 g m − 2 d − 1 , which are both in the range of the typical values for biologic CO 2 soil degassing. This reliable characterisation of the biogenic CO 2 flux would not have been possible by solely applying a statistical analysis of the CO 2 flux values, which is commonly applied in volcanological studies for the partitioning between background fluxes and anomalous CO 2 fluxes. A map of the Solfatara diffuse degassing structure was derived from the estimated threshold for the biogenic CO 2 flux, highlighting that soil degassing of hydrothermal CO 2 mixed in different proportion with biogenic CO 2 occurs over a large area (~0.8 km 2), which extends over the inner part of the Solfatara crater as well as the eastern periphery, corresponding with a NW-SE fault system. The presented method and data analysis are important means of surveillance of the volcanic activity.

En la primera mitad del siglo XX, la obra del historiador correntino Hernán Félix Gómez (1888Góme... more En la primera mitad del siglo XX, la obra del historiador correntino Hernán Félix Gómez (1888Gómez ( -1945, -quien fuera actor central de la historiografía de la provincia-, constituyó el primer intento por brindar un marco teórico-metodológico a los estudios históricos correntinos, así como delinear una perspectiva correntina de la historia argentina. El análisis de sus presupuestos ideológicos y de sus opciones teóricas y metodológicas nos enfrenta con una clara propuesta de reversión de la mirada en torno a la relación centro/periferia, cuyo interés se ve aumentado por la amplia difusión que tuvieron sus obras, que aún hoy son de consulta obligada. Su contribución, enmarcada en la problemática de la inserción de Corrientes en el esquema político contemporáneo, se halla en estrecha vinculación con sus ideas y actividades políticas y educativas.