Full-text resources of PSJD and other databases are now available in the new Library of Science.
Visit https://bibliotekanauki.pl
Preferences help
Polish version English version Language
enabled [disable] Abstract
Number of results

Results found: 11

Number of results on page
first rewind previous Page / 1 next fast forward last

Search results

help Sort By:

help Limit search:
first rewind previous Page / 1 next fast forward last
1
Content available remote

Indirect Exchange in Magnetic Semiconductors. III. Effect of Alloying

100%
Matlak M.
Acta Physica Polonica A
|
1991
|
vol. 79
|
issue 4
525-530
EN
We apply the extended s-f model to magnetic pseudobinary alloys. We calculate the Curie temperature and magnetic moment in the paramagnetic state as functions of concentration x. The calculated quantities are in good agreement with the quantities experimentally measured for Eu_{1-x}Gd_{x}S, Eu_{1-x}Gd_{x}Se, Re_{x}Gd_{1-x}Al_{2} (Re = Dy, Ho, Er, La, Lu, Y), Gd(Al_{1-x}Me_{x})_{2} (Me = Cu, Ag, Pd, In, Sn),...
2
Publication available in full text mode
Content available

Extended Hubbard Model in the Dimer Representation. II. Lattice Hamiltonian in the Large U Limit

64%
Grabiec B., Matlak M.
Acta Physica Polonica A
|
2002
|
vol. 101
|
issue 4
549-557
EN
Using the exact decomposition of the sc lattice into a set of interacting dimers (each dimer is described by the extended Hubbard Hamiltonian) and exact solution of the dimer problem (preceding paper) we exactly find the form of the extended Hubbard model in the case of a crystal in the large U limit. We apply a new, nonperturbative approach based on the exact projection procedure onto a dimer subspace occupied by electrons in this limit (it is the only assumption). The resulting Hamiltonian is very complicated and contains a variety of multiple magnetic and nonmagnetic interactions deeply hidden in its original form (site representation). We also present a simplified version of the model to better visualize a mixture of different interactions resulting from this approach.
3
Publication available in full text mode
Content available

Extended Hubbard Model in the Dimer Representation. I. Dimer Hamiltonian in the Large U Limit

64%
Grabiec B., Matlak M.
Acta Physica Polonica A
|
2002
|
vol. 101
|
issue 4
537-548
EN
We consider the extended Hubbard model for the single cubic lattice and rewrite it in the form of interacting dimers, using the exact solution of the dimer problem. We analytically derive the second quantization form of the dimer Hamiltonian eliminating from the considerations unoccupied dimer energy levels in the large U limit (it is the only assumption). The resulting dimer Hamiltonian written with the use of the Hubbard operators and spin operators contains three terms, visualizing explicitly competing magnetic interactions (ferromagnetic, antiferromagnetic) as a generalization of the t-J model. The presented, nonperturbative method, can in principle be applied to the cluster of any size (e.g. one central atom and z its nearest neighbours). The use of the projection technique can further be applied in the case of a crystal to obtain the second quantization form of the extended Hubbard model for the sc lattice in the large U limit.
4
Content available remote

Theoretical Study of Phase Diagrams for Reentrant Superconducting Er_{1-x}Ho_{x}X, Er_{1-x}Gd_{x}X, Er_{1-x}Tm_{x}X, Ho_{x}Lu_{1-x}X, and Sm_{1-x}Er_{x}X (X = Rh_{4}B_{4}) Alloys

64%
Gosławska E., Matlak M.
Acta Physica Polonica A
|
2000
|
vol. 97
|
issue 3
415-418
EN
We consider a simplified version of the s-f model, supplemented by the intersite Cooper pairs interaction between conduction electrons with opposite spins to investigate ferromagnetic and superconducting properties of an alloy Re_{1-x}^{(1)}Re_{x}^{(2)}X (Re^{(1),(2)} - rare earth elements, X = Rh_{4}B_{4}) using the virtual crystal approximation. For a suitable choice of the model parameters we can reproduce the phase boundaries of the experimentally measured phase diagrams for Er_{1-x}Ho_{x}X, Er_{1-x}Gd_{x} X, Er_{1-x}Tm_{x}X, Ho_{x}Lu_{1-x}X, and Sm_{1-x}Er_{x}X alloys.
5
Content available remote

Chemical Potential Derivative as Hallmark for Phase Transitions

64%
Pietruszka M., Matlak M.
|
|
vol. 96
|
issue 6
725-732
EN
We study antiferromagnetic properties of the two-band extended s-f model with fluctuating valence in the context of two mutually bound new effects of chemical potential critical behaviour, as well as of critical electron redistribution. In order to exemplify both phenomena we build phase diagrams of the system displaying the dependence of the critical Néel temperatures (T_{N}) of the system versus 4f (5f) level positions. The phase diagram consists of two different areas corresponding to antiferromagnetic and paramagnetic phases. We plot the magnetizations and the correlation functions of the system as functions of temperature. Next, we investigate the temperature dependence of the relative average occupation numbers Δn^{f(d)} and the chemical potential Δμ for a given 4f (5f) level position E_{f}. Plotting this quantities along the E_{f} cross-section lines we observe small (of the order of 10^{-4}-10^{-3}) but well localized kinks exactly at the Néel temperature T_{N}. Last but not least, we plot the first derivative of the chemical potential dμ/dT which, as it shows clearly visible jumps at T_{N}, may turn out to be very accurate and sensitive (auxiliary) tool to find critical temperatures of the considered system. Moreover, we plot the difference μ_{AF}-μ_{PARA} where we subtract a chemical potential value of a reference paramagnetic sample from the actual value of the antiferromagnetic system. Also in this case we report the observation of discontinuous change in slope at T_{N}. Our observations can be extended to point out to a new practical possibility of how to find experimentally the critical temperatures of the antiferromagnetic systems exclusively from the chemical potential measurements. We expect that the same type of measurement, according to our recent and present results, would also apply to all types of critical phenomena in real solids.
6
Content available remote

Multiband s-f Model for Pseudobinary Intermetalic Gd(Al_{1-x}Me_{x})_{2} Alloys. II. Me=Pb, Bi, Si, Sb

64%
Matlak M., Zieliński J.
Acta Physica Polonica A
|
1991
|
vol. 79
|
issue 5
717-719
EN
With the use of the formulae derived in the preceding paper we calculate the Curie temperature and the magnetic moment as functions of the concentration x for Gd(Al_{1-x}Me_{x})_{2} (Me = Pb, Bi, Si, Sb) alloys. The agreement with the recent experimental data is relatively good.
7
Content available remote

Magnetic Properties of R^{(1)}_{1-x}R^{(2)}_{x}Al_{2} Alloys

64%
Matlak M., Ślebarski A.
Acta Physica Polonica A
|
1991
|
vol. 79
|
issue 5
721-725
EN
Using the multiband s-f model we derive the formulae determining the Curie temperature and magnetic moment as functions of concentration x for R^{(1)}_{1-x}R^{(2)}_{x}Al_{2} intermetallic alloys, where R^{1,2} denote magnetic rare earth metals. These formulae, applied to R^{(1)}_{1-x}R^{(2)} alloys (R^{(1)} = La, Lu, Y, Zr and R^{(2)} = Gd) give the linear dependence of the Curie temperature and magnetic moment versus x in full agreement with experimental data.
8
Content available remote

Chemical Potential Evidence for Phase Transitions in Magnetic and Superconducting Compounds and Alloys

64%
Matlak M., Pietruszka M.
Acta Physica Polonica A
|
2000
|
vol. 97
|
issue 1
253-256
EN
We announce that all phase transitions (induced by temperature or concentration) including structural ones and transitions between metastable or "exotic" states can be detected by the chemical potential critical behaviour, as well as, from the average occupation numbers of the electronic system (critical electron redistribution).
9
Content available remote

Chemical Potential as a Detector of Phase Transitions in Solids

51%
Matlak M., Gosławska E., Grabiec B., Eid K.
|
EN
We show that the chemical potential exhibits small but distinct kinks at all critical temperatures as the evidence for phase transitions in the electronic system, structural phase transitions included. In the case of, at least, two kinds of interacting electrons average occupation numbers exhibit the same behavior.
10
Content available remote

Phase Transitions in Magnetic and Superconducting Systems with Fluctuating Valence. Chemical Potential Evidence

51%
Gosławska E., Matlak M., Eid K.
Acta Physica Polonica A
|
2000
|
vol. 97
|
issue 4
713-718
EN
We investigate the critical behaviour of the chemical potential and average occupation numbers using the extended s -f model with intersite Cooper pairing for systems with fluctuating valence. The model is able to describe phase transitions from normal ferromagnet to normal paramagnet at T=T_{C}, from superconducting paramagnet to normal paramagnet at T=T_{S}, as well as reentrant phase transitions with three critical temperatures T_{S_{1}}, T_{C} and T_{S_{2}} (T_{S_{1}} < T_{C} < T_{S_{2}}). Present investigation, as well as recent results obtained for another models suggest one-to-one correspondence between critical temperatures of the system and kinks appearing in the temperature dependence of the chemical potential and average occupation numbers. This, in turn, indicates a possibility to apply the measurement of the chemical potential vs. temperature as an experimental universal tool when looking for phase transitions in solids.
11
Content available remote

Electronic Correlations within Fermionic Lattice Models

51%
Matlak M., Grabiec B., Krawiec S.
Acta Physica Polonica A
|
2007
|
vol. 112
|
issue 3
537-547
EN
We investigate two-site electronic correlations within generalized Hubbard model, which incorporates the conventional Hubbard model (parameters: t (hopping between nearest neighbours), U (Coulomb repulsion (attraction))) supplemented by the intersite Coulomb interactions (parameters: J^{(1)} (parallel spins), J^{(2)} (antiparallel spins)) and the hopping of the intrasite Cooper pairs (parameter: V). As a first step we find the eigenvalues E_α and eigenvectors |E_α〉 of the dimer and we represent each partial Hamiltonian E_α|E _α〉〈 E_α| (α=1,2,...,16) in the second quantization with the use of the Hubbard and spin operators. Each dimer energy level possesses its own Hamiltonian describing different two-site interactions which can be active only in the case when the level will be occupied by the electrons. A typical feature is the appearance of two generalized t-J interactions ascribed to two different energy levels which do not vanish even for U=J^{(1)}=J{(2)}=V=0 and their coupling constants are equal to ±t in this case. In the large linebreak U-limit for J^{(1)}=J^{(2)}=V=0 there is only one t-J interaction with coupling constant equal to 4t^2/|U| as in the case of a real lattice. The competition between ferromagnetism, antiferromagnetism and superconductivity (intrasite and intersite pairings) is also a typical feature of the model because it persists in the case U=J^{(1)}=J^{(2)}=V=0 and t≢0. The same types of the electronic, competitive interactions are scattered between different energy levels and therefore their thermodynamical activities are dependent on the occupation of these levels. It qualitatively explains the origin of the phase diagram of the model. We consider also a real lattice as a set of interacting dimers to show that the competition between magnetism and superconductivity seems to be universal for fermionic lattice models.
first rewind previous Page / 1 next fast forward last
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.