1) Zero-temperature phase transitions in quantum systems with
symmetry-conserving impurities
Busiello,-G.; De-Cesare,-L.; Lukierska-Walasek,-K.; Walasek,-K.
Physica-A. 15 June 1989; 157(3): 1227-58.
Abstract:
The zero-temperature phase transitions in several quantum systems with quenched
symmetry-conserving impurities are investigated on the grounds of a diagrammatic
approach, starting from an appropriate random quantum field functional
Hamiltonian. The authors find that the peculiar random anisotropy induced along
the imaginary-time axis by the competition between random and quantum regime
fluctuations inhibits the occurrence of a sharp second-order transition, in
agreement with the renormalization group predictions. In particular, for
three-dimensional Ising-like systems a spin-glass-like behavior appears, while
for systems with continuous symmetry only smeared or first-order transitions
seem to be possible. An explicit calculation in the large-n limit suggests that
the second alternative is more realistic.
2) Exotic glassy phase in the site-diluted transverse Ising
model at zero temperature?
Busiello,-G.; de-Cesare,-L.; Lukierska-Walasek,-K.; Walasek,-K.
Physica-Status-Solidi-B. 1 June 1989; 153(2): 561-71.
Abstract:
A self-consistent diagrammatic approach is used for studying the
zero-temperature properties of the three-dimensional site-diluted transverse
Ising model via a path-integral representation. It is found, that the peculiar
competition between random and quantum fluctuations inhibits the occurrence of a
second-order phase transition and an exotic phase with some spin-glass-like
characteristics appears. This should provide a plausible explanation of recent
experiments on some mixed crystals.
3) New criticality in quantum models with pair interactions
decaying as a power law
De-Cesare,-L.; Rabuffo,-I.
Physical-Review-B-Condensed-Matter. 1 Oct. 1988; 38(10): 7109-12.
Abstract:
A renormalization-group study is made for a class of quantum models with pair
interactions decaying as a power law. An additional expansion parameter enters
the problem and allows exploration of the behavior of the models at realistic
dimensionalities. In the classical regime, a new criticality appears under an
appropriate constraint on the zero-wave-vector Fourier component of the
interaction potential. In the quantum regime, a runaway in the parameter space
below a borderline dimension signals a drastic change in the usual scenario of
quantum criticality.
4) Criticality and crossover phenomena in quantum systems
with random fields
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Edt.: Busiello,-G.; De-Cesare,-L.;
Mancini,-F.; Marinaro,-M.
International-Meeting-on-Advances-on-Phase-Transitions-and-Disorder-Phenomena.1987:
289-306. (World Scientific, Singapore)
Abstract:
The authors present a brief review of some progress in understanding random
field (RF) effects on criticality in quantum systems. Renormalization group (RG)
results to first order in epsilon =d/sub cu/-d are given both for classical and
quantum regimes and an estimate of the lower critical dimensionality is
obtained. Finally, unperturbative RF criticality and crossover phenomena in the
large-n limit are also sketched.
5) Some aspects of quenched disorder effects on quantum phase
transitions
De-Cesare,-L.
Edt.: Busiello,-G.; De-Cesare,-L.; Mancini,-F.; Marinaro,-M.
International-Meeting-on-Advances-on-Phase-Transitions-and-Disorder-Phenomena.
1987: 229-55. (World Scientific, Singapore)
Astract:
Some results connected with the effects of quenched impurities on quantum
criticality obtained via the standard renormalization group (RG) approach are
presented. These will be the starting point for considering the correspondence
between random quantum systems at zero temperature and classical systems with
extended defects. Then, the opportunity is taken to point out some open problems
and controversies which arise in the extended impurity problem from a novel RG
perturbative treatment.
6) A mechanical approach to classical critical phenomena in
the large-n limit
De-Cesare,-L.; Villasi,-G.
Edt.: Busiello,-G.; De-Cesare,-L.; Mancini,-F.;
Marinaro,-M.
International-Meeting-on-Advances-on-Phase-Transitions-and-Disorder-Phenomena.
1987: 99-110. PB: World Scientific, Singapore
Abstract:
A mechanical analogy based on a differential formulation of the dynamic
renormalization group (RG) in the large-n limit for a n-vector classical model
with purely relaxational dynamics is presented. By means of strictly mechanical
methods, a maximal set of the RG invariants is achieved and an equivalence
between the original statistical mechanics problem and an anisotropic hyperbolic
oscillator in a stabilizing potential is established. All these results may give
new information about the internal structure of the RG and an alternative
approach to the study of critical phenomena in the large-n limit.
7) Destruction of long-range order in quantum ferroelectrics
via random field fluctuations
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Physica-A. July 1987; 144A(1): 192-200.
Abstract:
A method for exploring the destruction of long-range order in pure classical
systems with continuous symmetry is extended to quantum ferroelectrics in the
presence of correlated random fields. This gives a reliable estimate of the
lower critical dimensionality in terms of the parameters characterizing the
range of the interactions and random field correlations.
8) Large-n-limit model for quantum structural phase
transitions with correlated random fields
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.; D'Auria,-A.-C.
Physical-Review- B-Condensed-Matter. 1 March 1987; 35(7): 3536-44.
Abstract:
The effects of uncorrelated and correlated random fields on the critical
properties in the quantum displacive limit in systems undergoing structural
phase transitions are investigated within the Hartree approximation. An unusual
critical behavior, absent dimensional reduction, and breaking of the scaling
exponent relations, occurs as a competition of thermal, quantum, and random
field fluctuations when the transition is driven by the temperature. On the
contrary, by approaching the quantum displacive limit in terms of the
interaction-strength parameter a picture emerges which is consistent with recent
renormalization-group predictions. In particular the dimensional reduction is
present also for long-range interactions and correlated random fields due to the
absence of perturbative effects as the Griffiths singularities which disappear
in the large-n limit.
9) Random fields and the weakly first-order phase transition
in superconductors
Busiello,-G.; de-Cesare,-L.; Uzunov,-D.-I.
Physical-Review-B-Condensed-Matter. 1 Oct. 1986; 34(7): 4932-5.
Abstract:
The authors study the influence of random fields with short-range and long-range
correlations on the weakly first-order phase transition in superconductors.
Renormalization-group (RG) analysis near the upper critical dimensionality d/sub
u/=6 (short-range correlations) reveals a new critical behavior which holds if
the number of the order-parameter components is n>10. In the long-range case,
the RG transformation is self-consistent only if the parameter theta of the
long-range correlations is assumed of order epsilon , epsilon =6-d.
10) On the mechanical analogy of the renormalisation group in
the large-n limit
Busiello,-G.; De-Cesare,-L.; De-Filippo,-S.; Vilasi,-G.
Journal-of-Physics-A-Mathematical-and-General. 21 Oct. 1986; 19(15): L881-6.
Abstract:
Some aspects of the mechanical analogy recently proposed for the static and
dynamic renormalisation group (RG) in the large-n limit are pointed out. In
particular, starting from purely mechanical methods, a maximal set of the static
RG invariants is obtained. Similar results also hold in the most complex
dynamical case. Finally, by means of a canonical transformation, the original
statistical mechanics problem is reduced to an equivalent anisotropic hyperbolic
oscillator under stabilising conditions.
11) Weak disorder effects on criticality in quantum systems
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Edt.: Mancini,-F.
Quantum-Field-Theory.-Proceedings-of-the-International-Symposium-in-Honour-of-Hiroomi-Umezawa.
1986: 533-48.
PB: North-Holland, Amsterdam, Netherlands
Abstract:
The authors present
some recent results about the effects of symmetry-breaking and
symmetry-conserving quenched impurities on criticality in quantum systems. The
starting point is an appropriate random quantum field representation of
microscopic models in terms of which both static and intrinsic dynamic
properties can be studied in the same formalism.
12) Wilson's renormalization-group approach to a
d-dimensional quantum sine-Gordon model
Busiello,-G.; de-Cesare,-L.; Rabuffo,-I.
Physical-Review-B-Condensed-Matter. 1 Nov. 1985; 32(9): 5918-24.
Abstract:
Wilson's renormalization-group method is used for investigating the critical
properties and the role of quantum fluctuations for a d-dimensional quantum
sine-Gordon model. In the classical regime a phase transition is found in d=2-
epsilon ( epsilon >or=0) dimensions and the static critical exponents are
evaluated to leading order in epsilon >0. Within the same formalism, the
dynamic critical exponent z is also obtained as a characteristic of the
intrinsic dynamics of the model. The quantum regime is briefly discussed, and no
phase transition is found for d>1. The authors argue that a quantum-classical
crossover occurs for 1<d<or=2 in the low-temperature limit as a
consequence of a dimensional crossover d to d+1 which shifts the upper critical
dimensionality from d/sub cu/=2 to d/sub cu/=1.
13) Thermodynamic properties of the low-dimensional perfect
Bose gas
Busiello,-G.; de-Cesare,-L.; Uzunov,-D.-I.
Physica-A. Aug. 1985; 132A(1): 199-206.
Abstract:
The chemical potential, the specific heat and the correlation function are
obtained for the perfect Bose gas at a constant volume at low spatial
dimensionalities. The Bose-Einstein condensation occurs only at zero
temperature. Some nonuniversal features of this condensation are discussed.
14) Self-consistent treatment of impurity influence in
quantum systems at T=0
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Journal-of-Physics-A-Mathematical-and-General. 11 Sept. 1985; 18(13):
L749-54.
Abstract:
The zero-temperature properties of quantum systems in the presence of
quenched impurities which coupled quadratically to the order parameter are
investigated by means of a self-consistent treatment. A physical interpretation
of the renormalisation group fixed point instability recently discovered is
suggested and a comparison with some experiments on doped quantum incipient
ferroelectrics is also made.
15) On the critical behaviour of bosonised models in a
long-range correlated random field
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Journal-of-Physics-A-Mathematical-and-General. 11 July 1985; 18(10):
1805-12.
Abstract:
The renormalisation group approach is used for exploring the critical
behaviour of bosonised models in the presence of a long-range correlated random
field. The zero-temperature regime is studied in some detail for different types
of random field correlation functions. As a result, the random critical
exponents are not derivable from the pure ones with appropriate dimensional
shifts.
16) Spectral-density method for classical systems: Heisenberg
ferromagnet
Campana,-L.-S.; Caramico-D'Auria,-A.; D'Ambrosio,-M.; Esposito,-U.; De-Cesare,-L.;
Kamieniarz,-G.
Physical-Review-B-Condensed-Matter. 1 Sept. 1984; 30(5): 2769-75.
Abstract: The authors formulate the spectral-density method in classical statistical
mechanics in strict analogy with the known quantum version, and they apply it to
the classical Heisenberg ferromagnetic model in an external field. A new formula
for the magnetization in the classical formulation of spin-vector Green
functions is derived for arbitrary spatial dimensionality. Furthermore, the
static properties of the one- and three-dimensional cases are considered in
detail. They obtain accurate results over a wide range of temperatures with the
external field for the one-dimensional model and without field for the
three-dimensional model. In the first case very good agreement with the exact
numerical transfer-matrix data is also found in the region of higher temperature
where the interacting-boson approach fails. In particular the zero-field
susceptibility results are found to be consistent with the exact results
obtained by Fisher (1964).
17) Random field effects in a Bose system: critical behaviour
and crossover phenomena in the Hartree limit
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Physica-A. June 1984; 126A(1-2): 195-236.
Abstract: The low temperature grand canonical critical properties of a d-dimensional
n-vector Bose system in the presence of a random field, which behaves like
[h/sub k/*h/sub k/]/sub av/~k/sup theta / ( theta >or=0), are investigated
with the use of replica trick and the Hartree approximation. With a boson free
particle spectrum, which behaves like k/sup sigma / (0< sigma <or=2),
several situations occur, both at zero and non-zero temperature regimes, which
are studied in detail for different values of d, sigma , theta . In particular,
some questions concerning the effective lowering of the space dimensionality and
the violation of the usual scaling laws when the random field is present are
also checked. Furthermore crossover phenomena appear where, alternatively, the
random field intensity and the temperature assume the role of crossover
parameter. It is shown that these crossover processes can be described in terms
of crossover scaling functions and effective critical exponents in conformity
with the standard crossover theory.
18) Quenched impurities and tricriticality in classical and
quantum systems
Busiello,-G.; De-Cesare,-L.; Uzunov,-D.-I.
Journal-of-Physics-A-Mathematical-and-General. 1 June 1984; 17(8): L441-5.
Abstract: A generalised model, intended to describe a number of quantum systems in the
presence of short-range correlated quenched impurities, is studied by the
renormalisation group approach. To first order in epsilon , it is shown that the
tricritical behaviour is unstable towards impurity perturbations for both finite
and zero temperatures. A brief discussion of the zero-temperature tricriticality
in pure systems is also presented.
19) Dynamical critical exponent for quantum systems with
long-range correlated impurities
Busiello,-G.; de-Cesare,-L.; Rabuffo,-I.
Physics-Letters-A. 30 April 1984; 102A(1-2): 41-4.
Abstract: The dynamical critical exponent of some quantum systems in the presence of
long-range-correlated disorder is calculated with the use of a double
renormalization group expansion. The impurity effects on the static critical
behaviour in the quantum regime are also outlined.
20) Quenched-impurity influence on quantum critical behavior
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Physical-Review-B-Condensed-Matter. 1 April 1984; 29(7): 4189-91.
Abstract: The effects of short-range-correlated impurities on the critical behavior of
some quantum models with a free-energy spectrum ~k/sup sigma / (0< sigma
<or=2) are investigated by using the renormalization-group approach via the
replica trick. At zero temperature, two nonsimultaneous expansion parameters
epsilon /sub q/=d/sub q/-d (d/sub q/= sigma or /sup 3///sub 2/ sigma ) and
epsilon =2 sigma -d are involved in terms of which an unusual phase transition
is predicted for d<2 sigma.
21)Spectral density approach for the damping of fermion
elementary excitations in an anisotropic spin-1/2 linear chain
Campana,-L.-S.; Caramico-D'Auria,-A.; D'Ambrosio,-M.; De-Cesare,-L.;
Esposito,-U.
Physica-A. Jan. 1984; 123A(1): 279-95.
Abstract: The paper is devoted to an investigation of the damping effects of the
Fermion elementary excitations in the Jordan-Wigner representation of a
one-dimensional anisotropic spin-1/2 model in an external field. This is
realized on the basis of the `Gaussian ansatz', recently proposed by Nolting and
Oles (1980), as substituting the standard `polar ansatz' in the Spectral Density
Method, for exploring damping effects of quasi-particles in many-body problems.
By using a first order approximation, the authors obtain reasonable results in
the low temperature limit.
22) Random-field effects on the critical behavior of an
interacting Bose model
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Physical-Review-B-Condensed-Matter. 1 Dec. 1983; 28(11): 6463-9.
Abstract: The critical properties of a d-dimensional Bose model in the presence of a
random quenched field are investigated by renormalization-group techniques via
the replica trick. To first order in epsilon =6-d it is found that both at
nonzero- and zero-temperature regimes the grand-canonical critical exponents are
the same as those of the corresponding classical model so that thermal and
quantum fluctuations appear to be irrelevant. Furthermore, at zero temperature,
the scaling laws already established for the pure model are retrieved with the
replacement of d with d-4, but the random critical exponents are not derivable
from the pure ones with the dimensional shift d to d-4.
23) A proposal to include damping effects in the spectral
density approach
Campana,-L.-S.; Caramico-D'Auria,-A.; D'Ambrosio,-M.; De-Cesare,-L.;
Esposito,-U.
Journal-of-Physics-C-Solid-State-Physics. 20 June 1983; 16(17): L549-53.
Abstract: Recently there has been proposed a modification of the spectral density
method (SDM) to take into account damping effects for Fermi systems. There are
in any case some difficulties concerning Bose and classical systems. The authors
suggest a simple way of avoiding such difficulties and preserving the earliest
idea for discussing damping effects. A classical ferromagnetic linear chain in
external magnetic field is studied.
24) Differential formulation of the dynamic renormalisation
group in the large-n limit
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Journal-of-Physics-A-Mathematical-and-General. 21 June 1983; 16(9): 1955-66.
Abstract: A differential formulation of the dynamic renormalisation group in the
large-n limit is presented for the time-dependent generalisation of the n-vector
classical model with purely relaxational dynamics for both conserved and
non-conserved order parameter. A discussion of the dynamical properties with the
same steps used in the traditional perturbative approaches is made in terms of
which known and new explicit results are obtained. Furthermore, a mechanical
analogy is introduced which may give new information about the structure of the
dynamic renormalisation group in the large-n limit. This relies on the
possibility of using the well known geometrical and mathematical techniques from
the Hamilton-Jacobi theory for classical mechanics.
25)Renormalisation group and quantum critical phenomena in
the large-n limit
Busiello,-G.; de-Cesare,-L.; Rabuffo,-I.
Physica-A. Feb. 1983; 117A(2-3): 445-81.
Abstract: The paper is devoted to a systematic study of the critical behaviour in the
classical and quantum regime of a wide class of quantum systems on the basis of
the renormalisation group (RG) approach in the large-n limit. It can be
considered as a complement to recent perturbative RG investigations for the same
systems and as a convenient basis for a treatment of the classical-quantum
crossover. The differences between `bosonised systems' and several quantum
systems for which a dimensional crossover occurs in the low temperature limit
are emphasized, consistently with the perturbative results and a calculation of
the quantum nonlinear scaling fields is also realized. Finally, a differential
formulation of the quantum RG in the large-n limit is presented and its
convenience in investigating some aspects of the problem is pointed out.
26)Classical quantum crossover for a Bose system in the large
n-limit
de-Cesare,-L..
Nuovo-Cimento-D. May-June 1982; 1D ser. 1(3): 289-312.
Abstract: On the basis of the Hartree approximation, the low-temperature properties of
a d-dimensional n-vector Bose model are discussed in detail, for both normal and
ordered phase. The change of critical behaviour which occurs for 2<d<4
when the temperature goes to zero is described in terms of the usual crossover
theory. In particular, several crossover scaling functions and the corresponding
effective critical exponents are calculated. The results constitute a useful
guide for future renormalization group investigations of the classical quantum
crossover in a Bose system.
27) Differential formulation of the renormalisation group in
the large-n limit
Busiello,-G.; De-Cesare,-L.; Rabuffo,-I.
Journal-of-Physics-A-Mathematical-and-General. Dec. 1981; 14(12): L513-17.
Abstract: A differential formulation of the renormalisation group in the large-n limit
is proposed, in terms of which all the standard results can be reproduced in a
more natural way. Some advantages of the formulation with respect to the
traditional approach are pointed out.
28)On a self-consistent approach to a Bose model near the
criticality
Campana,-L.-S.; D'Ambrosio,-M.; De-Cesare,-L.
Lettere-al-Nuovo-Cimento. 12 Sept. 1981; 32 ser. 2(2): 39-44.
Abstract: The authors use a critical effective Hamiltonian, which is simpler than the
microscopic Hamiltonian, to explore the potentiality of the spectral density
method second-order approximation. This idea is used to investigate the critical
behaviour of a Bose model.
29)Spectral density method in classical statistical mechanics
d'Auria,-A.-C.; de-Cesare,-L.; Esposito,-U.
Physics-Letters-A. 28 Sept. 1981; 85A(4): 197-200.
Abstract: The spectral density method in classical statistical mechanics is formulated
and an application to a linear classical spin system is made in the simplest
approximation.
30)Renormalization group approach to quantum bosonized
systems
Busiello,-G.; De-Cesare,-L.
Nuovo-Cimento-B. 11 Oct. 1980; 59B ser. 2(2): 327-49.
Abstract: To leading order in a perturbative renormalization group analysis the
authors study the critical behaviour of Bose and related quantum systems at
temperature T not=0 and T=0. In both cases they specify the meaning of the grand
canonical variable which corresponds to the usual critical deviation T-T/sub c/
in the canonical Wilson scheme. In particular, for T not=0, when the grand
canonical critical exponents are the same as the classical one, the canonical
critical exponents are calculated with the use of Fisher's renormalization
relations. The question of dimensional crossover which appears for T to 0 is
briefly discussed. For the more interesting case T=0, the critical surface and
the exponents associated with the stable nontrivial fixed point are calculated.
The authors discover an unusual behaviour which, nevertheless, can be still
included in a universality class in Wilson's sense.
31)The renormalisation group in the large-n limit for the
vectorial paramagnon problem at T=O
Busiello,-G.; De-Cesare,-L.
Journal-of-Physics-A-Mathematical-and-General. Dec. 1980; 13(12): 3779-84.
Abstract: Uses the extension of the Ma RG approach (1973) to quantum functionals in
the limit n to infinity to obtain non-perturbative information for quantum
systems at T=O, restricting oneself to the n-vector paramagnon problem.
32) Bogoliubov approximation as a second-order approximation
in the spectral-density method. II
Caramico-D'Auria,-A.; De-Cesare,-L.; Esposito,-U.; Esposito,-F.
Nuovo-Cimento-A. 21 Sept. 1980; 59A ser. 2(2): 351-71.
Abstract: For pt.I see ibid., vol.52A, ser.2, no.2, p.141-150 (1979). The authors
discuss a method, based on the second-order approximation. In the
spectral-density approach, which avoids the difficulties which appear in the
Bogoliubov approximation for an interacting Bose system. They indicate the
prescriptions under which, starting from the original number-conserving
Hamiltonian, the known Bogoliubov results and the possibility to obtain
corrections to those results emerge in a natural way. Explicit calculations are
made at zero temperature for a high-density Bose model with a repulsive soft
core and a strong attraction between the particles. The self-consistency of the
above prescriptions is explicitly proved.
33) The critical exponents eta and z to second order in
epsilon =2-d for a Bose system at T=0
Busiello,-G.; De-Cesare,-L.
Physics-Letters-A. 12 May 1980; 77A(2-3): 177-80.
Abstract: The critical exponents eta and z for a d-dimensional Bose system at T=0 are
calculated to second order in epsilon =2-d via Wilson's renormalization group
approach. The result seems to confirm that a cross-over (d, n=2) to (d+2, n=-2)
occurs for T to 0.
34) Bogolubov approximation as a second-order approximation
in the spectral density method
Campana,-L.-S.; Caramico-D'Auria,-A.; De-Cesare,-L.
Lettere-al-Nuovo-Cimento. 3 Nov. 1979; 26 ser. 2(10): 301-6.
Abstract: Recently (1979) the authors have stated an extension to finite temperature
of the Bogolubov approximation (BA) for a high-density Bose model by using the
formalism of temperature two-time Green functions. Incidentally they conjectured
that the BA can be obtained in the framework of a second-order approximation of
the spectral-density method (SDM) for the complete Hamiltonian. They establish
more clearly the connection between the BA and the SDM and to obtain in general
the prescriptions in order to reproduce it to the second order in the SDM.
Furthermore the possibility to obtain corrections to BA is pointed out.
35) Low-temperature behaviour of a high-density Bose model
Campana,-L.-S.; D'Auria,-A.-C.; De-Cesare,-L.; Esposito,-U.
Nuovo-Cimento-A. 21 July 1979; 52A ser. 2(2): 141-50.
Abstract: The low-temperature thermodynamic properties of a high-density Bose gas with
strong attraction between the particles are obtained within the Bogolyubov
approximation. An approach is used which also gives an interesting connection
between the Bogolyubov approximation and the spectral-density method.
36) Spectral density method for an anisotropic linear chain
Campana,-L.-S.; D'Auria,-A.-C.; de-Cesare,-L.; Esposito,-U.
Physica-A. March 1979; 95A(3): 417-48.
Abstract: The spectral density method is used to study the thermodynamic properties of
an anisotropic antiferromagnetic Heisenberg linear chain in an external magnetic
field. In second order approximation without the decoupling procedure for the
two particle correlation function the authors are able to obtain many results in
terms of both the temperature and the magnetic field for different values of the
anisotropy parameter. The ground state of the model is investigated and the
singular behaviour of some thermodynamic quantities near the critical field is
determined. In particular, explicit analytical solutions are given for small
anisotropy parameters. The results are compared with the ones obtained by means
of other methods and some exact results are reproduced.
37)On the spectral-density approach to the correlation
functions of a linear magnetic model
Campana,-L.-S.; d'Auria,-A.-C.; de-Cesare,-L.; Esposito,-U.
Nuovo-Cimento-B. 11 Dec. 1978; 48B ser. 2(2): 179-88.
Abstract: It is shown that, when one uses the spectra-density method (SDM) to study
the spin-spin correlation functions for a Heisenberg linear chain, a difficulty
appears in a simple procedure usually utilized to avoid the decoupling in the
higher spectral density. Such a difficulty does not exist at the first and
second order in the SDM with the decoupling procedure. In these approximations
the correlation functions are obtained for arbitrary values of the temperature,
the magnetic field and the anisotropy parameter.
38) Renormalization group approach to a functional
formulation of a Bose system
De-Cesare,-L..
Lettere-al-Nuovo-Cimento. 24 June 1978; 22 ser. 2(8): 325-31.
Abstract: The conventional renormalisation group approach is applied to a functional
formulation of the grand canonical partition function of a Bose system obtained
by a Wiegel and Hijmans (1974) by means of path integral methods. It leads to a
classical effective Hamiltonian also containing the Matsubara frequencies.
39) Thermodynamic stability and critical point in simple
fluids and ferromagnets
De-Cesare,-L..
Physica-Scripta. May-June 1977; 15(5-6): 355-65.
Abstract: By using a rigorous characterisation of a critical point, based on Tiszo's
thermodynamic theory of phase transitions, the usual and new inequalities among
critical indices for simple fluids and ferromagnets have been obtained in a
unified way without making additional hypotheses. It has been established when
the index relations are valid as strict equalities. The derivation of critical
indices in terms of critical amplitudes gives the possibility of finite cusps
and discontinuities for the specific heats at the critical point. The precise
connection between the static scaling hypotheses and the thermodynamic stability
is established. The analogy between fluids and ferromagnets is stressed as are
differences that follow from the slope of the saturation curve being zero for
ferromagnets, unlike that for fluids.
40) Ground state of an antiferromagnetic isotropic linear
chain
Caremico-D'Auria,-A.; De-Cesare,-L.; Esposito,-U.
Lettere-al-Nuovo-Cimento. 20 Aug. 1977; 19(16): 607-10.
Abstract: In recent work (see Nuovo Cimento vol.38B, p.116 (1977)) the authors applied
the spectral density method to an isotropic antiferromagnetic linear chain in an
external magnetic field. The Hamiltonian is expressed in terms of Fermi
operators. The analysis is completed by showing the existence of a well defined
value of the adimensional external field which marks the breakdown of the
antiferromagnetic coupling when the magnetic field is increased.
41) Decoupling effect in the spectral density approach to an
Heisenberg linear chain
Campana,-L.-S.; Caramico-D'Auria,-A.; De-Cesare,-L.; Esposito,-U.
Lettere-al-Nuovo-Cimento. 13 Aug. 1977; 19(15): 577-80.
42) The spectral-density approach to an antiferromagnetic
Heisenberg linear chain
Caramico-D'Auria,-A.; De-Cesare,-L.; Esposito,-U.
Nuovo-Cimento-B. 11 March 1977; 38B ser. 2(1): 116-32.
Abstract: An antiferromagnetic Heisenberg linear chain is studied by using the
spectral-density method. The values of the thermodynamic quantities are obtained
as functions of the temperature; the results are compared with the ones obtained
by other authors at T=0.
43) Superfluid neutron star cores
Angelis,-U.-De.; De-Cesare,-L.
Lettere-al-Nuovo-Cimento. 10 May 1975; 13 ser. 2(2): 79-82.
Abstract: It is suggested that there exists a point just before equilibrium density is
reached when neutron pairs can be treated as individual particles i.e. bosons.
44) Type II/1 and type II/2 superconductors
De-Cesare,-L.; Mancini,-F.
Physica-Status-Solidi-B. 1 Sept. 1974; 65(1): 419-27.
Abstract: The problem of a first-order transition at the lower critical field in type
II superconductors is investigated by means of the boson formulation of
superconductivity. The critical value kappa /sub c/ of the Ginzburg-Landau
parameter which separates type II/1 from type II/2 superconductors is
theoretically determined and is in good agreement with the experimental value.
The distribution of the magnetic field of an isolated flux line and the
interaction potential between two flux lines are also computed in the entire
region including the core.
45) Temperature dependence of the lattice parameter d/sub 0/
in type-II/1 superconductors
De-Cesare,-L.; Mancini,-F.
Solid-State-Communications. 1 Sept. 1974; 15(5): 815-18.
Abstract: The distance r/sub m/ where the magnetic field of an isolated flux line has
a minimum, is computed as function of the temperature. Good agreement with the
experimental data for the lattice parameter is found in the region
0<or=T/T/sub c/<or=0.5.
46) Evolution of central stars of planetary nebulae towards
the crystallizing white dwarf stage
de-Angelis,-U.; de-Cesare,-L.; Forlani,-A.; Platania,-G.
Astrophysics-and-Space-Science. March 1974; 27(1): 217-25.
Abstract: The evolution of the central stars of planetary nebulae, interpreted as hot
white dwarfs with liquefying cores, towards the cold white dwarf stage is
discussed and theoretical (non-computational) evolutionary tracks are built for
such central stars as they cool towards the crystallizing region. The
conclusions seem to hint a picture in which crystalline white dwarfs can be
looked at as final stages of the central stars of planetary nebulae.
47) The cooling of liquefying hot white dwarfs
de-Angelis,-U.; de-Cesare,-L.; Forlani,-A.; Platania,-G.
Astrophysics-and-Space-Science. March 1974; 27(1): 117-35.
Abstract: The consequences of gas-liquid phase transitions in the core of hot white
dwarf stars are discussed. Expressions for the latent heat and the liquefaction
curve T/sub l/=T/sub l/( rho ) are obtained. Then a model for a hot white dwarf
is introduced and the corresponding liquefaction sequences are built on the H-R
diagram; relations luminosity-central temperature and effective
temperature-central temperature are also given for liquefying white dwarfs.
Finally the cooling curves are obtained for such stars taking into account the
effect of latent heat emission. The results seem to suggest a possible
identification of the central stars of planetary nebulae as hot liquefying white
dwarfs.
48) Superconducting critical field in cold magnetic white
dwarfs and in neutron stars
de-Cesare,-L..
Astrophysics-and-Space-Science. Nov. 1973; 25(1): 133-48.
Abstract: For an evaluation of the superconducting critical field H/sub c/ as a
function of the mass-density in the external layers of cold magnetic white
dwarfs and in the superconducting proton fluid in neutron stars, the solution of
a differential equation involving H/sub c/ as a function of the pressure is
used. The differential equation and its solution are obtained by pure
thermodynamic way. Finally other thermodynamic quantities are calculated for the
above superconducting astrophysical systems.
49) Thermodynamics of white dwarf matter in crystalline phase
de-Cesare,-L.; Forlani,-A.; Platania,-G.
Memorie-della-Societa-Astronomica-Italiana. 1973; 44(1): 13-22.
Abstract: The authors construct these thermodynamic functions at temperatures and
densities characteristic of the interiors of white dwarfs and of neutron star
crusts. The model is a plasma of heavy ions, forming a body-centered cubic
lattice (bcc), immersed in a uniform neutralizing background of non-relativistic
or relativistic strongly degenerate electrons.
50) Nuclei of planetary nebulae as hot liquefying white dwarfs
De-Angelis,-Y.; De-Cesare,-L.; Forlani,-A.; Platania,-G.
Nature-Physical-Science. 27 Aug. 1973; 244(139): 133-5.
Abstract: Suggests the following picture. (i) The central stars of planetary nebulae
in the diagonal branch of the H-R diagram are hot white dwarfs whose cores are
undergoing liquefaction. (ii) The cores of the stars are chemically pure (/sup
12/C or higher Z), and the envelopes are thin and composed of elements of mean
charge Z<10. (iii) The masses of the stars are in the range 0.45 to 1.3 M(.),
which means that the nebula ejection (for the higher masses of the original
stars) has to occur before the star reaches the diagonal branch (that is, in the
horizontal branch). (iv) No neutrino luminosity is present and the cooling times
are those obtained for a gaseous core liquefying with emission of latent heat.
51) Gas-liquid phase transitions and critical point in hot
white dwarf matter
de-Cesare,-L.; Forlani,-A.; Platania,-G.
Astrophysics-and-Space-Science. April 1973; 21(2): 461-74.
Abstract: Considers the gas-liquid first-order phase transitions and proves the
existence of the critical point in white dwarf matter. The latent heat released
in the liquefaction processes can be used in the interpretation of the spreading
of the white dwarf sequence in H-R diagram. Some thermodynamic quantities, e.g.,
the saturation pressure, the latent heat, etc., are calculated along the
gas-liquid coexistence curve, and their behaviour near critical point is
studied.
52)A possibility to obtain equalities for the critical
indices by thermodynamics way. II
de-Cesare,-L.
Lettere-al-Nuovo-Cimento. 4 Nov. 1972; 5 ser. 2(10): 718-22.
Abstract: For Pt. I see abstr. A49033 of 1972. Results of the previous paper for
ferromagnets are extended to show that they contain the equivalent Coopersmith
and Fisher inequalities. The conditions under which the inequalities for the
critical isotherm become strict equalities are established and several critical
indices are determined.
53)A possibility of obtaining equalities for the critical
indices by thermodynamic method
de-Cesare,-L.
Lettere-al-Nuovo-Cimento. 17 June 1972; 4 ser. 2(7): 251-6.
Abstract: It is shown how, by making definite the rigorous mathematical condition that
characterises a critical point, it is possible to establish criteria on the
basis of which the expectation of inequalities for critical indices can be
predicted.
54)Phase-transitions in white-dwarf matter
De-Angelis,-U.; De-Cesare,-L.; Platania,-G.
Abstracts-of-the-meetings-on-critical-phenomena. 1971: 60.
PB: Ruhr Univ. at Bochum, Bochum, West Germany
Abstract: Tisza's theory of phase transitions has been applied to white dwarf matter
to establish the order of transition, the density-temperature regions where
crystallization may actually occur, and to what extent the use of `traditional'
thermodynamics in dealing with such problems is correct. In fact, according to
modern statistical mechanics, the actual thermodynamic functions are obtained
from the functions provided by statistical mechanics only after a suitable
`thermodynamic limit' has been taken.
