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1
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Spin Density in Real and Momentum Space in Multi-Atom Alloys by KKR-CPA Method

100%
Kaprzyk S.
Acta Physica Polonica A
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1997
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vol. 91
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issue 1
135-150
EN
The application of the spin-polarized version of multiple scattering theory for obtaining electron charge and spin densities in both real and momentum spaces of concentrated, multi-atom disordered alloys is presented. This method is based on the Korringa-Kohn-Rostoker (KKR) band structure approach and coherent potential approximation (CPA) method. The effective one-electron potential is constructed within local spin density approximation. The magnetic neutron form factors are in real space of our main interest. With the recent developments of new synchrotron photon sources, the Compton profile becomes the most interesting target in momentum space. In the most of examples, spin momentum density and its specific structure due to Fermi surface will be shown. To get accurate enough description in momentum space and quantity like Compton profile, the determination of the Fermi surface must be done with high precision. In this context we show how to apply generalized Lloyd formula for accurate determination of the Fermi level. Also we show how to use efficiently complex energy integration method for the computation of matrix elements, G(r,r) or G(p,p), of the KKR-CPA Green function. Results for the iron-silicon ferromagnetic binary alloys and half-metallic ferromagnetic Heusler alloys are presented.
2
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Electronic Structure and Fermiology of High-T_{c} Superconductors

63%
Bansil A., Kaprzyk S.
Acta Physica Polonica A
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1993
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vol. 84
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issue 1
7-20
EN
We briefly discuss some of our recent results in three areas, all related to issues concerning the electronic structure and fermiology of the high-T_{c}'s. (i) First-principles band theory based computations of the electron-positron momentum density relevant for the interpretation of the positron annihilation (2D-ACAR) experiments; results for YBa_{2}Cu_{3}O_{7} are discussed. (ii) Another k-resolved spectroscopy where such first-principles comparisons between the computed and measured spectral intensities have been crucially important is the angle-resolved photoemission experiment; we discuss results for the (001)-surface of YBa_{2}Cu_{3}O_{7} where computations yield considerable insight into the nature and origin of the spectral signal associated with the CuO_{2} plane bands. (iii) The study of effects of substitutions and of oxygen-vacancy disorder in the high-T_{c}'s; here results for the LaSrCuO_{4}, and Ba-K-Bi-O and Ba-Pb-Bi-O systems, with focus on the nature of the van Hove singularity in the vicinity of the Fermi energy are presented.
3
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Electronic Structure of Zinc-Blende and Hexagonal Semiconductors: Comparative Theoretical Study

63%
Konior J., Kaprzyk S.
Acta Physica Polonica A
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1995
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vol. 87
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issue 1
269-274
EN
Using a self-consistent Korringa-Kohn-Rostoker Green function method and the local density approximation for the exchange and correlation poten­tials, we calculated the electronic structure and the corresponding densi­ties of states for two semiconducting compounds that exist in both cubic (zinc-blende) and hexagonal (wurtzite and NiAs-type) crystal structures, i.e., for CdSe and MnTe. Due to the use of the same method of calculation for both structures a comparison between the obtained results and finding the factors that determine the difference between the properties of these two phases are possible. The presented local density approximation-Korringa­-Kohn-Rostoker results might be of importance for the understanding of the factors that determine the relative stability of different phases of semicon­ducting compounds.
4
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Formation Energy and Electronic Structure of II-VI/IV Semiconductor Superlattices

51%
Jędrzejek C., Konior J., Kaprzyk S.
Acta Physica Polonica A
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1995
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vol. 87
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issue 2
349-352
EN
We calculate formation energy and electronic structure of ultrathin (001)II-VI/IV semiconductor superlattices using the Korringa-Kohn-Rosto­ker all-electron method. Formation energies (∆H) are 2.18 eV for (Ge_{2})_{1}(ZnSe)_{1} and 1.50 eV for (ZnS)_{1}(Si_{2})_{1}. The results of this work are significantly different from these by Ferraz and Srivastava who obtained ∆H = 0.88 eV for (001)(Ge_{2})_{1}(ZnSe)_{1} and moreover the one-layer super-lattices are metallic, which confirms the results by Ohno and Ito. The large formation energies surely lead to interfacial instability.
5
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Effect of Alloying with Ca in LaMnO_{3} System Studied by KKR-CPA Method and Giant Magnetoresistance

51%
Tobola J., Kaprzyk S., Pierre J.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
461-464
EN
La_{1-x} Ca_{x}MnO_{3} perovskites show semiconducting properties in the paramagnetic range. The gap is reduced near x=0.33, where ferromagnetic ordering is observed. The ferromagnetic ordering then induces a semiconductor-metal transition, and gives rise to a giant magnetoresistance effect. The ground state electronic structure calculations were done with KKR-CPA method for hypothetical cubic and ferromagnetic LaMnO_{3} and CaMnO_{3} compounds, as well as for disordered La_{1-x}Ca_{x}MnO_{3} (x=0.33, 0.4, 0.5) alloys with real crystal data. As a result, we get a magnetic moment per formula 4.00μ_{B} and 3.00μ_{B} and half-metallic behaviour for end-compounds, respectively. In the ferromagnetic region a linear decrease in the magnetic moment of La_{1-x}Ca_{x}MnO_{3} is observed, together with the decrease in the gap width for spin-down carriers, if doping Ca in La_{1-x}Ca_{x}MnO_{3}. A simple model is developed, which describes magnetic and transport properties as resulting from an exchange-induced band-crossing semiconductor-metal transition, as for instance in EuO.
6
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Interplay of Crystal Structure Preference and Magnetic Ordering in High Entropy CrCoFeNiAl Alloys

51%
Jasiewicz K., Kaprzyk S., Tobola J.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
511-513
EN
In this work, some relations between crystal structure and magnetic ordering in CrCoFeNiAl HEAs are discussed in view of the KKR-CPA calculations. Remarkably, it is noticed that the transition between fcc and bcc phases in the analysed CrCoFeNi_{x}Al and CrCoFeNiAl_{x} alloys is closely related to change in ordering of the local magnetic moments, namely the magnetic moment on Cr is either parallel (ferromagnetic) or antiparallel (ferrimagnetic) to the magnetic moments of other atoms (Co, Fe and Ni). On the whole, the theoretical diagram showing the fcc-bcc phase preference and their coexistence vs. alloy composition well corroborates with available experimental data.
7
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Superconductivity, Weak Itinerant Ferromagnetism and Electronic Band Structure of Y_9Co_7

45%
Kołodziejczyk A., Wiendlocha B., Zalecki R., Toboła J., Kaprzyk S.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
513-526
EN
The paper presents experimental results for the intermetallic compound Y_9Co_7 which is the first very weak itinerant ferromagnetic superconductor. The measurements of electrical resistivity, susceptibility, magnetization, specific heat, nuclear magnetic resonance, and other properties show the magnetic transition at T_C≈ 4.5 K and the onset of superconductivity at about T_S≈ 2.5 K, revealing the coexistence state within a temperature interval below T_S. A few of the results are shortly reviewed and discussed. We focus attention on our recent ultraviolet photoemission spectroscopy and their comparison with our band structure calculations. Interpretations are considered which take into account the characteristic structure of the compound and different possible types of magnetic ordering.
8
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Magnetic and Transport Properties of (Fe_{1-x}Ni_{x})_{2}P in View of KKR-CPA Results

39%
Kaprzyk S., Nizioł S., Toboła J., Zach R., Bacmann M., Fruchart D., Wolfers P.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
475-478
EN
We present results of the electronic structure study for the solid solution (Fe_{1-x}Ni_{x})_{2}P (space group P6̅2m). The samples were synthesized for x=0.0 up to 0.15. From the electrical conductivity measurements on these materials, it was verified that resistivity increases with increasing temperature up to the Curie point T_{C} and changes the slope at T_{C} decreasing when temperature is raised above T_{C}. To get insight into the possible origin of such observations, we carried out the Korringa-Kohn-Rostoker-coherent potential approximation electronic structure calculations for (Fe_{1-x}Ni_{x})_{2}P in the range of concentration x=0.0 up to 1.0. It was found that alloying with Ni in Fe_{2} P causes vanishing of spin band splitting and non-rigid band behaving of electronic valence states near the Fermi level. The polarization factor for electrons at E_{F} changes in (Fe_{1-x}Ni_{x})_{2}P with increasing x, and interestingly, it reaches maximum at x=0.1, closely to the concentration (x=0.08), when the largest value of T_{C} and a kink in temperature dependent resistivity were detected.
9
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Magnetoelastic Properties of MnRhAs_{1-x}p_{x} and Band Structure Results from KKR-CPA

39%
Zach R., Kaprzyk S., Nizioł S., Toboła J., Bacmann M., Fruchart D., Wolfers P.
Acta Physica Polonica A
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1997
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vol. 91
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issue 2
471-474
EN
The magnetoelastic phase transitions were observed in MnRhAs_{1-x}P_{x} system (space group P6̅2m) by X-ray measurements (80 K < T < 300 K). For x = 0.33 and x = 0.4 contents an abrupt change of both lattice parameters and of the elementary cell volume at the metamagnetic transition was detected. The Korringa-Kohn-Rostoker electronic structure calculations performed for MnRhP compound show significant variations of densities of states at the Fermi level, while changing lattice data. The Korringa-Kohn- Rostoker-coherent potential approximation studies of disordered MnRhAs_{1-x}P_{x} system (x=0.6, 0.8, 1.0) were done at the ferromagnetic region. From our calculations we observe moving of bands near E_{F}, while substituting with isoelectronic metalloid, likely as while changing the crystal data in MnRhP.
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