German Samolyuk - Academia.edu (original) (raw)

Papers by German Samolyuk

Physical Review Materials

We have performed first-principles calculations to investigate the electronic structure, configur... more We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in p-type material while the aluminum vacancy has the lowest formation energy in n-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

Cornell University - arXiv, Jun 23, 2006

The RFe 2 Zn 20 series manifests strongly correlated electron behavior for the non-magnetic R = Y... more The RFe 2 Zn 20 series manifests strongly correlated electron behavior for the non-magnetic R = Y member and remarkably high temperature, ferromagnetic ordering (T C = 86 K) for the local moment bearing R = Gd member (a compound that is less than 5% atomic Gd). In contrast, the isostructural RCo 2 Zn 20 series manifests a more typical ordering temperature (T N = 5.7 K for GdCo 2 Zn 20) and YCo 2 Zn 20 does not show signs of correlated electron behavior. Studies of R(Fe x Co 1−x) 2 Zn 20 (R = Gd, Y), combined with bandstructure calculations for the end members, reveal that YFe 2 Zn 20 is a nearly ferromagnetic Fermi liquid and that the remarkably high T C associated with GdFe 2 Zn 20 is the result of submerging a large local moment into such a highly polarizable matrix. These results indicate that the RFe 2 Zn 20 series, and more broadly the RT 2 Zn 20 (T = Fe, Co, Ni, Mn, Ru, Rh, Os, Ir, Pt) isostructural family of compounds, offer an exceptionally promising phase space for the study of the interaction between local moment and correlated electron effects near the dilute R limit.

Physical Review B, 2020

Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry... more Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry measurements are performed on a set of medium (NiFeCo and NiFeCoCr) and high (NiFeCoCrPd and NiFeCoCrMn) entropy Cantor-Wu alloys. The bulk spin momentum densities determined by magnetic Compton scattering are remarkably isotropic, and this is a consequence of the smearing of the electronic structure by disorder scattering of the electron quasiparticles. Nonzero x-ray magnetic circular dichroism signals are observed for every element in every alloy indicating differences in the populations of the majority and minority spin states implying finite magnetic moments. When Cr is included in the solid solution, the Cr spin moment is unambiguously antiparallel to the total magnetic moment, while a vanishingly small magnetic moment is observed for Mn, despite calculations indicating a large moment. Some significant discrepancies are observed between the experimental bulk and surface magnetic moments. Despite the lack of quantitative agreement, the element specific surface magnetic moments seem to be qualitatively reasonable.

Materials Research Letters, 2019

The role of the electronic system in high energy displacement cascades is explored. The energy ex... more The role of the electronic system in high energy displacement cascades is explored. The energy exchange between the electronic and the atomic subsystem is described by the electron-phonon coupling. The electronic effects on the damage accumulation due to 100 keV Ni ion cascades in nickel, a prototype system to a large group of nickel-based high entropy alloys, are investigated for overlapping cascades. It is shown that the energy exchange between the two subsystems affects microstructure evolution, resulting in the formation of smaller clusters and more isolated defects. This effect is more significant for the vacancy cluster formation and size distribution. IMPACT STATEMENT The effect of electron-phonon coupling on the damage accumulation due to ion irradiation is investigated for the first time, with results showing significant effects on the vacancy cluster formation.

AIP Advances, 2018

Ion irradiation and modification: The role of coupled electronic and nuclear energy dissipation a... more Ion irradiation and modification: The role of coupled electronic and nuclear energy dissipation and subsequent nonequilibrium processes in materials Applied Physics Reviews 7, 041307 (2020);

Acta Materialia, 2018

Concentrated solid solution alloys have attracted rapidly increasing attention due to their poten... more Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present a computational study on swift heavy ion induced effects in Ni and equiatomic Ni-based alloys (Ni 50 Fe 50 , Ni 50 Co 50) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni 50 Co 50 the damage consists of a dislocation loop structure (d=2nm) and isolated point defects, while in Ni 50 Fe 50 , a defect cluster (d=4nm) along the ion path is, in addition, formed. The simulation results are supported by atomiclevel structural and defect characterizations in bismuth-irradiated Ni and Ni 50 Fe 50. The significance of the 2T-MD model is demonstrated by comparing the results to those

Journal of Alloys and Compounds, 2017

In high-energy irradiation events, energy from the fast moving ion is transferred to the system v... more In high-energy irradiation events, energy from the fast moving ion is transferred to the system via nuclear and electronic energy loss mechanisms. The nuclear energy loss results in the creation of point defects and clusters, while the energy transferred to the electrons results in the creation of high electronic temperatures, which can affect the damage evolution. We perform molecular dynamics simulations of 30 keV and 50 keV Ni ion cascades in nickel-based alloys without and with the electronic effects taken into account. We compare the results of classical molecular dynamics (MD) simulations, where the electronic effects are ignored, with results from simulations that include the electronic stopping only, as well as simulations where both the electronic stopping and the electronphonon coupling are incorporated, as described by the two temperature model (2T-MD). Our results indicate that the 2T-MD leads to a smaller amount of damage, more isolated defects and smaller defect clusters.

Scientific reports, May 18, 2016

The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentrati... more The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, Tc, goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess -TlnT (or power law) contribution to the low temperature heat capacity, and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x ≈ 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in this simple fcc material thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.

Physical Review B, 2015

Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next... more Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-field-induced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single crystal during the application of magnetic fields. Additionally, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni 2 Mn 1.14 Ga 0.86. The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based on the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be ∼150 kJ/m 3 .

Gd_5(Si_2Ge_2) compound demonstrates a wide class of peculiar phenomena related to its giant magn... more Gd_5(Si_2Ge_2) compound demonstrates a wide class of peculiar phenomena related to its giant magnetocaloric effect, magnetoresistance and magnetostriction. The microscopic origin of these observed properties so far is not clear. In this work we apply the LMTO method for the calculation of the total energy differences between related structural and magnetic phases, as well as of the magnetic stability parameters.

ABSTRACT Modern ab initio theories of the magnetic phase transition (Curie Temperature, TC) of Fe... more ABSTRACT Modern ab initio theories of the magnetic phase transition (Curie Temperature, TC) of Fe and Ni based on the Disordered Local Moment (DLM) type models generally rely on (constrained) density functional theory calculations performed at 0K and assume that the atoms occupy their equilibrium lattice sites. Here we point out that finite temperature lattice vibrations can result in large fluctuations in the local moments associated with individual site beyond those already accounted for in these approaches. These conclusions are based on large cell (˜10^4 -- atoms) ab initio calculations of the magnetic state of Fe and Ni based on the O[N] Locally Self-consistent Multiple Scattering (LSMS) method. Atom positions are obtained from freezes of individual time steps of molecular dynamics simulations based on classical interaction potentials. Calculations are performed for a range of temperatures up and beyond TC that illustrate the extent of the moment fluctuations. We discuss the consequences of these findings for the adequacy of existing theories TC.

Physical Review B, 2014

The magnetic phase diagrams of models for quasi one-dimensional compounds belonging to the iron-b... more The magnetic phase diagrams of models for quasi one-dimensional compounds belonging to the iron-based superconductors family are presented. The five-orbital Hubbard model and the realspace Hartree-Fock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed varying the Hubbard U and Hund J couplings and at zero temperature. The study is carried out at electronic density (electrons per iron) n = 5.0, which is of relevance for the already known material TlFeSe2, and also at n = 6.0, where representative compounds still need to be synthesized. At n = 5.0 there is a clear dominance of staggered spin order along the chain direction. At n = 6.0 and the realistic Hund coupling J/U = 0.25, the phase diagram is far richer including a variety of "block" states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent Density Matrix Renormalization Group calculations for the three-orbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange, and prevailing at large J/U) and antiferromagnetic order (dominating at small J/U). The density of states and orbital compositions of the many phases are also provided.

Physical Review Letters, 2009

The London penetration depth, λ(T), has been measured in several single crystals of Ba(Fe0.93Co0.... more The London penetration depth, λ(T), has been measured in several single crystals of Ba(Fe0.93Co0.07)2As2. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of λ(T) follows a power-law, ∆λ(T) ∼ T n with n = 2.4 ± 0.1, indicating the existence of normal quasiparticles down to at least 0.02Tc. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO1−xFx and SmFeAsO1−xFx, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe0.93Co0.07)2As2 may lead to an anisotropic s−wave gap with point nodes that would explain the observed λ(T).

Physical Review B, 2007

Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagne... more Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic SmAgSb2 and ferromagnetic CeAgSb2 using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of ∆ρ ≃ 20 pΩ allows a detailed study of the SdH spectra as a function of temperature. The effects of the Sm long-range magnetic ordering as well as its electronic structure (4f-electrons) on the Fermi surface topology is discussed.

Physical Review Letters, 2009

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of Ca... more We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2-parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three-to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (Ts) Fermi surfaces around Γ and X points are cylindrical and quasi-2D. Below Ts the former becomes a 3D ellipsoid, while the latter remains quasi-2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

Physical Review Letters, 2004

The observed magnetoresistance of single crystalline Gd 5 Si 2 Ge 2 is negative and strongly anis... more The observed magnetoresistance of single crystalline Gd 5 Si 2 Ge 2 is negative and strongly anisotropic. The absolute values measured along the [100] and [010] directions exceed those parallel to the [001] direction by more than 60%. First principles calculations demonstrate that a structural modification is responsible for the anisotropy of the magnetoresistance, and that the latter is due to a significant reduction of electronic velocity in the [100] direction and the anisotropy of electrical conductivity.

Physical Review B, 2013

Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenid... more Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenide compound BaFe2Se3 have unveiled a dominant spin arrangement involving ferromagnetically ordered 2×2 iron-superblocks, that are antiferromagnetically coupled among them (the "block-AFM" state). Using the electronic five-orbital Hubbard model, first principles techniques to calculate the electronic hopping amplitudes between irons, and the real-space Hartree-Fock approximation to handle the many-body effects, here it is shown that the exotic block-AFM state is indeed stable at realistic electronic densities close to n ∼ 6.0. Another state (the "CX" state) with parallel spins along the rungs and antiparallel along the legs of the ladders is close in energy. This state becomes stable in other portions of the phase diagrams, such as with hole doping, as also found experimentally via neutron scattering applied to KFe2Se3. In addition, the present study unveils other competing magnetic phases that could be experimentally stabilized varying either n chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques. A comparison of the results obtained with the realistic selenides hoppings amplitudes for BaFe2Se3 against those found using the hopping amplitudes for pnictides reveals several qualitative similarities, particularly at intermediate and large Hubbard couplings.

Physical Review B, 2008

Magnetization, resistivity and specific heat measurements were performed on the solution-grown, s... more Magnetization, resistivity and specific heat measurements were performed on the solution-grown, single crystals of six GdT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh and Ir) compounds, as well as their Y analogues. For the Gd compounds, the Fe column members manifest a ferromagnetic (FM) ground state (with an enhanced Curie temperature, T C , for T = Fe and Ru), whereas the Co column members manifest an antiferromagnetic (AFM) ground state. Thermodynamic measurements on the YT 2 Zn 20 revealed that the enhanced T C for GdFe 2 Zn 20 and GdRu 2 Zn 20 can be understood within the framework of Heisenberg moments embedded in a nearly ferromagnetic Fermi liquid. Furthermore, electronic structure calculations indicate that this significant enhancement is due to large, close to the Stoner FM criterion, transition metal partial density of states at Fermi level, whereas the change of FM to AFM ordering is associated with filling of electronic states with two additional electrons per formula unit. The degree of this sensitivity is addressed by the studies of the pseudo-ternary compounds Gd(Fe x Co 1−x) 2 Zn 20 and Y(Fe x Co 1−x) 2 Zn 20 which clearly reveal the effect of 3d band filling on their magnetic properties.

Physical Review Materials

We have performed first-principles calculations to investigate the electronic structure, configur... more We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in p-type material while the aluminum vacancy has the lowest formation energy in n-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

Cornell University - arXiv, Jun 23, 2006

The RFe 2 Zn 20 series manifests strongly correlated electron behavior for the non-magnetic R = Y... more The RFe 2 Zn 20 series manifests strongly correlated electron behavior for the non-magnetic R = Y member and remarkably high temperature, ferromagnetic ordering (T C = 86 K) for the local moment bearing R = Gd member (a compound that is less than 5% atomic Gd). In contrast, the isostructural RCo 2 Zn 20 series manifests a more typical ordering temperature (T N = 5.7 K for GdCo 2 Zn 20) and YCo 2 Zn 20 does not show signs of correlated electron behavior. Studies of R(Fe x Co 1−x) 2 Zn 20 (R = Gd, Y), combined with bandstructure calculations for the end members, reveal that YFe 2 Zn 20 is a nearly ferromagnetic Fermi liquid and that the remarkably high T C associated with GdFe 2 Zn 20 is the result of submerging a large local moment into such a highly polarizable matrix. These results indicate that the RFe 2 Zn 20 series, and more broadly the RT 2 Zn 20 (T = Fe, Co, Ni, Mn, Ru, Rh, Os, Ir, Pt) isostructural family of compounds, offer an exceptionally promising phase space for the study of the interaction between local moment and correlated electron effects near the dilute R limit.

Physical Review B, 2020

Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry... more Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry measurements are performed on a set of medium (NiFeCo and NiFeCoCr) and high (NiFeCoCrPd and NiFeCoCrMn) entropy Cantor-Wu alloys. The bulk spin momentum densities determined by magnetic Compton scattering are remarkably isotropic, and this is a consequence of the smearing of the electronic structure by disorder scattering of the electron quasiparticles. Nonzero x-ray magnetic circular dichroism signals are observed for every element in every alloy indicating differences in the populations of the majority and minority spin states implying finite magnetic moments. When Cr is included in the solid solution, the Cr spin moment is unambiguously antiparallel to the total magnetic moment, while a vanishingly small magnetic moment is observed for Mn, despite calculations indicating a large moment. Some significant discrepancies are observed between the experimental bulk and surface magnetic moments. Despite the lack of quantitative agreement, the element specific surface magnetic moments seem to be qualitatively reasonable.

Materials Research Letters, 2019

The role of the electronic system in high energy displacement cascades is explored. The energy ex... more The role of the electronic system in high energy displacement cascades is explored. The energy exchange between the electronic and the atomic subsystem is described by the electron-phonon coupling. The electronic effects on the damage accumulation due to 100 keV Ni ion cascades in nickel, a prototype system to a large group of nickel-based high entropy alloys, are investigated for overlapping cascades. It is shown that the energy exchange between the two subsystems affects microstructure evolution, resulting in the formation of smaller clusters and more isolated defects. This effect is more significant for the vacancy cluster formation and size distribution. IMPACT STATEMENT The effect of electron-phonon coupling on the damage accumulation due to ion irradiation is investigated for the first time, with results showing significant effects on the vacancy cluster formation.

AIP Advances, 2018

Ion irradiation and modification: The role of coupled electronic and nuclear energy dissipation a... more Ion irradiation and modification: The role of coupled electronic and nuclear energy dissipation and subsequent nonequilibrium processes in materials Applied Physics Reviews 7, 041307 (2020);

Acta Materialia, 2018

Concentrated solid solution alloys have attracted rapidly increasing attention due to their poten... more Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present a computational study on swift heavy ion induced effects in Ni and equiatomic Ni-based alloys (Ni 50 Fe 50 , Ni 50 Co 50) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni 50 Co 50 the damage consists of a dislocation loop structure (d=2nm) and isolated point defects, while in Ni 50 Fe 50 , a defect cluster (d=4nm) along the ion path is, in addition, formed. The simulation results are supported by atomiclevel structural and defect characterizations in bismuth-irradiated Ni and Ni 50 Fe 50. The significance of the 2T-MD model is demonstrated by comparing the results to those

Journal of Alloys and Compounds, 2017

In high-energy irradiation events, energy from the fast moving ion is transferred to the system v... more In high-energy irradiation events, energy from the fast moving ion is transferred to the system via nuclear and electronic energy loss mechanisms. The nuclear energy loss results in the creation of point defects and clusters, while the energy transferred to the electrons results in the creation of high electronic temperatures, which can affect the damage evolution. We perform molecular dynamics simulations of 30 keV and 50 keV Ni ion cascades in nickel-based alloys without and with the electronic effects taken into account. We compare the results of classical molecular dynamics (MD) simulations, where the electronic effects are ignored, with results from simulations that include the electronic stopping only, as well as simulations where both the electronic stopping and the electronphonon coupling are incorporated, as described by the two temperature model (2T-MD). Our results indicate that the 2T-MD leads to a smaller amount of damage, more isolated defects and smaller defect clusters.

Scientific reports, May 18, 2016

The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentrati... more The face centered cubic (fcc) alloy NiCoCrx with x ≈ 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, Tc, goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess -TlnT (or power law) contribution to the low temperature heat capacity, and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x ≈ 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in this simple fcc material thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.

Physical Review B, 2015

Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next... more Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-field-induced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single crystal during the application of magnetic fields. Additionally, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni 2 Mn 1.14 Ga 0.86. The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based on the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be ∼150 kJ/m 3 .

Gd_5(Si_2Ge_2) compound demonstrates a wide class of peculiar phenomena related to its giant magn... more Gd_5(Si_2Ge_2) compound demonstrates a wide class of peculiar phenomena related to its giant magnetocaloric effect, magnetoresistance and magnetostriction. The microscopic origin of these observed properties so far is not clear. In this work we apply the LMTO method for the calculation of the total energy differences between related structural and magnetic phases, as well as of the magnetic stability parameters.

ABSTRACT Modern ab initio theories of the magnetic phase transition (Curie Temperature, TC) of Fe... more ABSTRACT Modern ab initio theories of the magnetic phase transition (Curie Temperature, TC) of Fe and Ni based on the Disordered Local Moment (DLM) type models generally rely on (constrained) density functional theory calculations performed at 0K and assume that the atoms occupy their equilibrium lattice sites. Here we point out that finite temperature lattice vibrations can result in large fluctuations in the local moments associated with individual site beyond those already accounted for in these approaches. These conclusions are based on large cell (˜10^4 -- atoms) ab initio calculations of the magnetic state of Fe and Ni based on the O[N] Locally Self-consistent Multiple Scattering (LSMS) method. Atom positions are obtained from freezes of individual time steps of molecular dynamics simulations based on classical interaction potentials. Calculations are performed for a range of temperatures up and beyond TC that illustrate the extent of the moment fluctuations. We discuss the consequences of these findings for the adequacy of existing theories TC.

Physical Review B, 2014

The magnetic phase diagrams of models for quasi one-dimensional compounds belonging to the iron-b... more The magnetic phase diagrams of models for quasi one-dimensional compounds belonging to the iron-based superconductors family are presented. The five-orbital Hubbard model and the realspace Hartree-Fock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed varying the Hubbard U and Hund J couplings and at zero temperature. The study is carried out at electronic density (electrons per iron) n = 5.0, which is of relevance for the already known material TlFeSe2, and also at n = 6.0, where representative compounds still need to be synthesized. At n = 5.0 there is a clear dominance of staggered spin order along the chain direction. At n = 6.0 and the realistic Hund coupling J/U = 0.25, the phase diagram is far richer including a variety of "block" states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent Density Matrix Renormalization Group calculations for the three-orbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange, and prevailing at large J/U) and antiferromagnetic order (dominating at small J/U). The density of states and orbital compositions of the many phases are also provided.

Physical Review Letters, 2009

The London penetration depth, λ(T), has been measured in several single crystals of Ba(Fe0.93Co0.... more The London penetration depth, λ(T), has been measured in several single crystals of Ba(Fe0.93Co0.07)2As2. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of λ(T) follows a power-law, ∆λ(T) ∼ T n with n = 2.4 ± 0.1, indicating the existence of normal quasiparticles down to at least 0.02Tc. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO1−xFx and SmFeAsO1−xFx, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe0.93Co0.07)2As2 may lead to an anisotropic s−wave gap with point nodes that would explain the observed λ(T).

Physical Review B, 2007

Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagne... more Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic SmAgSb2 and ferromagnetic CeAgSb2 using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of ∆ρ ≃ 20 pΩ allows a detailed study of the SdH spectra as a function of temperature. The effects of the Sm long-range magnetic ordering as well as its electronic structure (4f-electrons) on the Fermi surface topology is discussed.

Physical Review Letters, 2009

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of Ca... more We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2-parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three-to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (Ts) Fermi surfaces around Γ and X points are cylindrical and quasi-2D. Below Ts the former becomes a 3D ellipsoid, while the latter remains quasi-2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

Physical Review Letters, 2004

The observed magnetoresistance of single crystalline Gd 5 Si 2 Ge 2 is negative and strongly anis... more The observed magnetoresistance of single crystalline Gd 5 Si 2 Ge 2 is negative and strongly anisotropic. The absolute values measured along the [100] and [010] directions exceed those parallel to the [001] direction by more than 60%. First principles calculations demonstrate that a structural modification is responsible for the anisotropy of the magnetoresistance, and that the latter is due to a significant reduction of electronic velocity in the [100] direction and the anisotropy of electrical conductivity.

Physical Review B, 2013

Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenid... more Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenide compound BaFe2Se3 have unveiled a dominant spin arrangement involving ferromagnetically ordered 2×2 iron-superblocks, that are antiferromagnetically coupled among them (the "block-AFM" state). Using the electronic five-orbital Hubbard model, first principles techniques to calculate the electronic hopping amplitudes between irons, and the real-space Hartree-Fock approximation to handle the many-body effects, here it is shown that the exotic block-AFM state is indeed stable at realistic electronic densities close to n ∼ 6.0. Another state (the "CX" state) with parallel spins along the rungs and antiparallel along the legs of the ladders is close in energy. This state becomes stable in other portions of the phase diagrams, such as with hole doping, as also found experimentally via neutron scattering applied to KFe2Se3. In addition, the present study unveils other competing magnetic phases that could be experimentally stabilized varying either n chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques. A comparison of the results obtained with the realistic selenides hoppings amplitudes for BaFe2Se3 against those found using the hopping amplitudes for pnictides reveals several qualitative similarities, particularly at intermediate and large Hubbard couplings.

Physical Review B, 2008

Magnetization, resistivity and specific heat measurements were performed on the solution-grown, s... more Magnetization, resistivity and specific heat measurements were performed on the solution-grown, single crystals of six GdT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh and Ir) compounds, as well as their Y analogues. For the Gd compounds, the Fe column members manifest a ferromagnetic (FM) ground state (with an enhanced Curie temperature, T C , for T = Fe and Ru), whereas the Co column members manifest an antiferromagnetic (AFM) ground state. Thermodynamic measurements on the YT 2 Zn 20 revealed that the enhanced T C for GdFe 2 Zn 20 and GdRu 2 Zn 20 can be understood within the framework of Heisenberg moments embedded in a nearly ferromagnetic Fermi liquid. Furthermore, electronic structure calculations indicate that this significant enhancement is due to large, close to the Stoner FM criterion, transition metal partial density of states at Fermi level, whereas the change of FM to AFM ordering is associated with filling of electronic states with two additional electrons per formula unit. The degree of this sensitivity is addressed by the studies of the pseudo-ternary compounds Gd(Fe x Co 1−x) 2 Zn 20 and Y(Fe x Co 1−x) 2 Zn 20 which clearly reveal the effect of 3d band filling on their magnetic properties.