arXiv subjects

Z. Bukowski

Publications and source records attributed to Z. Bukowski.

At least 19 recordsLinked to original sources

arXiv2021

57Fe and 151Eu Mössbauer studies of 3d-4f spin interplay in EuFe(2-x)Ni(x)As2

The EuFe(2-x)Ni(x)As2 compounds exhibiting 3d and/or 4f magnetic order were investigated by means of 57Fe and 151Eu Mössbauer spectroscopy. Additionally, results for the end members of this system, i.e. EuFe2As2 and EuNi2As2, are reported for comparison. It was found that spin-density-wave order of the Fe itinerant moments is monotonically suppressed by Ni-substitution. However, the 3d magnetic order survives at the lowest temperature up to at least x = 0.12 and it is certainly completely suppressed for x = 0.20. The Eu localized moments order regardless of the Ni concentration, but undergo a spin reorientation with increasing x from the alignment parallel to the a-axis in the parent compound, toward c-axis alignment for x > 0.07. The change of the 4f spins ordering from antiferromagnetic to ferromagnetic takes place simultaneously with a disappearance of the 3d spins order what is the evidence of a strong coupling between magnetism of Eu2+ ions and the conduction electrons of [Fe(2-x)Ni(x)As2]2- layers. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu2+ ordering for Ni-substituted samples with x > 0.04, while the transferred field is undetectable in EuFe2As2 and for compound with a low Ni-substitution level. It seems that the 4f ferromagnetic component arising from a tilt of the Eu2+ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms.

cond-mat.supr-con↗

arXiv2019

Spin reorientation of the Fe moments in Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$: Evidence for a strong interplay of Eu and Fe magnetism

Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$. Upon 50 \% dilution of the Eu sites with isovalent Ca$^{2+}$, the Eu sublattice is found to be still long-range ordered below $\mathit{T_{Eu}}$ = 10 K, in the A-typed antiferromagnetic (AFM) structure. The moment size of Eu$^{2+}$ spins is estimated to be as large as 6.74(4) $μ_{B}$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at $\mathit{T_{SDW}}$ = 192(2) K and displays an in-plane AFM structure above $\mathit{T_{Eu}}$. However, at 2.5 K, the Fe$^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of 14(4)° out of the $\mathit{ab}$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides a direct evidence of a strong interplay between the two magnetic sublattices in Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$.

cond-mat.str-el↗

arXiv2019

Charge and nematic orders in AFe$_2$As$_2$ (A= Rb,Cs) superconductors

We discuss the results of $^{75}$As Nuclear Quadrupole Resonance (NQR) and muon spin relaxation measurements in AFe$_2$As$_2$ (A= Cs, Rb) iron-based superconductors. We demonstrate that the crossover detected in the nuclear spin-lattice relaxation rate $1/T_1$ (around 150 K in RbFe$_2$As$_2$ and around 75 K in CsFe$_2$As$_2$), from a high temperature nearly localized to a low temperature delocalized behaviour, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is associated with a charge disproportionation at the Fe sites induced by competing Hund's and Coulomb couplings. In RbFe$_2$As$_2$ around 35 K, far below that crossover temperature, we observe a peak in the NQR $1/T_1$ which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order.

cond-mat.str-el↗

arXiv2018

Spiral magnetic ordering of the Eu moments in EuNi$_{2}$As$_{2}$

The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $\mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $\mathit{ab}$ plane with the moment size of 6.75(6) $μ_{B}$, but rotate spirally by 165.6(1)° around the $\mathit{c}$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $\mathit{T_{N}}$ = 15 K, shows critical behavior with a critical exponent of $β_{Eu}$ = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered.

cond-mat.str-el↗

arXiv2018

Magnetic phase diagram of Ca-substituted EuFe$_2$As$_2$

The simultaneous presence of a Fe-related spin-density wave and antiferromagnetic order of Eu$^{2+}$ moments ranks EuFe$_2$As$_2$ among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of Eu$^{2+}$ magnetic lattice through on-site Ca substitution. By employing macro- and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration $x(\mathrm{Ca}) = 0.12$ and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the Eu$^{2+}$ moments, $T_\mathrm{N}$, and the decrease of magnetic volume fraction with increasing $x(\mathrm{Ca})$. We establish that, similarly to the EuFe$_2$As$_2$ parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field.

cond-mat.mtrl-sci↗

arXiv2018

Mössbauer study of Eu0.57Ca0.43Fe2As2 and Eu0.73Ca0.27(Fe0.87Co0.13)2As2: A comparison to 122 iron-based superconductors parent compounds EuFe2As2 and CaFe2As2

57Fe and 151Eu Moessbauer spectra were obtained versus temperature for Eu0.57Ca0.43Fe2As2 compound with 3d and 4f magnetic order and Eu0.73Ca0.27(Fe0.87Co0.13)2As2 re-entrant superconductor, where the finite resistivity reappears while approaching the ground state. They were compared with previously obtained spectra for parent compounds EuFe2As2 and CaFe2As2. It was found that substitution beyond the Fe-As layers does not lead to the rotation (canting) of the Eu2+ magnetic moments and does not generate Eu3+ states. On the other hand, re-entrant superconductor exhibits rotation (canting) of the Eu2+ moments on the c-axis of the unit cell leading to the transferred hyperfine magnetic field on iron nuclei. Divalent europium orders magnetically within the bulk of the re-entrant superconducting phase. The re-entrant superconductor remains in the inhomogeneous state close to the ground state with about 27 % of the volume being free of 3d magnetism, while the remainder exhibits weak spin density wave. Those two regions slightly differ by the electric field gradient and electron density on iron nuclei.

cond-mat.supr-con↗

arXiv2017

Electron-Phonon Interaction in Ternary Rare-Earth Copper Antimonides LaCuSb2 and La(Cu0.8Ag0.2)Sb2 probed by Yanson Point-Contact Spectroscopy

Investigation of the electron-phonon interaction (EPI) in LaCuSb2 and La(Cu0.8Ag0.2)Sb2 compounds by Yanson point-contact spectroscopy (PCS) has been carried out. Point-contact spectra display a pronounced broad maximum in the range of 10÷20 mV caused by EPI. Variation of the position of this maximum is likely connected with anisotropic phonon spectrum in these layered compounds. The absence of phonon features after the main maximum allows the assessment of the Debye energy of about 40 meV. The EPI constant for the LaCuSb2 compound was estimated to be λ=0.2+/-0.03. A zero-bias minimum in differential resistance for the latter compound is observed for some point contacts, which vanishes at about 6 K, pointing to the formation of superconducting phase under point contact, while superconducting critical temperature of the bulk sample is only 1K.

cond-mat.supr-con↗

arXiv2017

Hydrostatic pressure effects on the static magnetism in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$

The effects of hydrostatic pressure on the static magnetism in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ are investigated by complementary electrical resistivity, ac magnetic susceptibility and single-crystal neutron diffraction measurements. A specific pressure-temperature phase diagram of Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ is established. The structural phase transition, as well as the spin-density-wave order of Fe sublattice, is suppressed gradually with increasing pressure and disappears completely above 2.0 GPa. In contrast, the magnetic order of Eu sublattice persists over the whole investigated pressure range up to 14 GPa, yet displaying a non-monotonic variation with pressure. With the increase of the hydrostatic pressure, the magnetic state of Eu evolves from the canted antiferromagnetic structure in the ground state, via a pure ferromagnetic structure under the intermediate pressure, finally to a possible "novel" antiferromagnetic structure under the high pressure. The strong ferromagnetism of Eu coexists with the pressure-induced superconductivity around 2 GPa. The change of the magnetic state of Eu in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ upon the application of hydrostatic pressure probably arises from the modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu$^{2+}$ moments tuned by external pressure.

cond-mat.str-el↗

arXiv2016

Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals

The magnetic ground state of the Eu$^{2+}$ moments in a series of Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ is established. The ground-state magnetic structure of the Eu$^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}$ = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu$^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu$^{2+}$ and Fe$^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals with 0.10 $\leqslant$ $\mathit{x}$ $\leqslant$ 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.

cond-mat.str-el↗

arXiv2016

Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFe$_2$As$_2$

$^{75}$As, $^{87}$Rb and $^{85}$Rb nuclear quadrupole resonance (NQR) and $^{87}$Rb nuclear magnetic resonance (NMR) measurements in RbFe$_2$As$_2$ iron-based superconductor are presented. We observe a marked broadening of $^{75}$As NQR spectrum below $T_0\simeq 140$ K which is associated with the onset of a charge order in the FeAs planes. Below $T_0$ we observe a power-law decrease in $^{75}$As nuclear spin-lattice relaxation rate down to $T^*\simeq 20$ K. Below that temperature the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in the light of an orbital selective behaviour expected for the electronic correlations.

cond-mat.supr-con↗

arXiv2016

Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFe$_2$As$_2$ under pressure

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe$_2$As$_2$, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe$_2$As$_2$ crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the hole-like Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

cond-mat.supr-con↗

arXiv2016

Raman scattering study of spin-density-wave-induced anisotropic electronic properties in AFe2As2 (A=Ca,Eu)

We present a polarization-resolved and temperature-dependent Raman scattering study of AFe2As2 (A = Ca, Eu). In the spin-density-wave (SDW) phase, spectral weight redistribution is observed in the fully symmetric and non-symmetric scattering channels at different energies. An anisotropic Raman response is observed in the fully symmetric channel in spontaneously detwinned CaFe2As2 samples. We calculate the orbital-resolved electronic structures using a combination of density functional theory and dynamical mean field theory (DFT+DMFT). We identify the electronic transitions corresponding to these two spectral features and find that the anisotropic Raman response originates from the lifted degeneracy of the dxz/yz orbitals in the broken symmetry phase.

cond-mat.str-el↗

arXiv2016

Probing the pairing symmetry in the over-doped Fe-based superconductor Ba_0.35Rb_0.65Fe_2As_2 as a function of hydrostatic pressure

We report muon spin rotation experiments on the magnetic penetration depth lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find that in this system lambda^{-2}(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x = 0.35 system which is known to be a nodeless s^{+-}-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s^{+-} to d-wave in Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order parameter is robust against pressure, suggesting that d is the common and dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %, while at optimal doping x = 0.35 a significant decrease of lambda(0) was reported. The superconducting transition temperature T_c as well as the gap to T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system x = 0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.

cond-mat.supr-con↗

arXiv2014

Electric field gradient wave (EFGW) in iron-based superconductor Ba(0.6)K(0.4)Fe2As2 studied by Mössbauer spectroscopy

The optimally doped 122 iron-based superconductor Ba(0.6)K(0.4)Fe2As2 has been studied by 57Fe Moessbauer spectroscopy versus temperature ranging from 4.2 K till 300 K with particular attention paid to the superconducting transition around 38 K. The spectra do not contain magnetic components and they exhibit quasi-continuous distribution of quadrupole split doublets. A distribution follows the electric field gradient (EFG) spatial modulation (wave) - EFGW. The EFGW is accompanied by some charge density wave (CDW) having about an order of magnitude lesser influence on the spectrum. The EFGW could be modeled as widely separated narrow sheets with the EFG increasing from small till maximum value almost linearly and subsequently dropping back to the original value in a similar fashion - across the sheet. One encounters very small and almost constant EFG between sheets. The EFGW shape and amplitude as well as the amplitude of CDW are strongly affected by a superconducting transition. All modulations are damped significantly at transition (38 K) and recover at a temperature being about 14 K lower. The maximum quadrupole splitting at 4.2 K amounts to about 2.1 mm/s, while the dispersion of CDW seen on the iron nuclei could be estimated far away from the superconducting gap opening and at low temperature as 0.5 el./a.u.^3. It drops to about 0.3 el./a.u.^3 just below transition to the superconducting state.

cond-mat.supr-con↗

arXiv2013

Magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 as revealed by single-crystal neutron diffraction

The magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu2+ moments along the c-direction is revealed below the magnetic phase transition temperature Tc = 17 K. In addition, the antiferromagnetism of the Fe2+ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe-spin density wave (SDW) order and the structural phase transition are significantly suppressed to Tn = 70 K and Ts = 90 K, respectively, compared to the parent compound EuFe2As2.We present the microscopic evidences for the coexistence of the Eu-ferromagnetism (FM) and the Fe-SDW in the superconducting crystal. The superconductivity (SC) competes with the Fe-SDW in Eu(Fe0.82Co0.18)2As2.Moreover, the comparison between Eu(Fe1-xCox)2As2 and Ba(Fe1-xCox)2As2 indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.

cond-mat.supr-con↗

arXiv2013

Mössbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors

A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Moessbauer spectroscopy of 57Fe and 151Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the 122 compounds a longitudinal spin density wave (SDW) develops. In the case of the EuFe2As2 a divalent europium orders antiferromagnetically at much lower temperature as compared to the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe(1+x)Te parent of the 11 family one has a transversal SDW and magnetic order of the interstitial iron with relatively high and localized magnetic moments. These two systems are strongly coupled one to another. For the grand parent of the iron-based superconductors FeAs one observes two mutually orthogonal phase-related transversal SDW on the iron sites. There are two sets of such spin arrangements due to two crystallographic iron sites. The FeAs exhibits the highest covalency among compounds studied, but it has still a metallic character.

cond-mat.supr-con↗

arXiv2013

The electronic specific heat of Ba1-xKxFe2As2 (x=0 to 1.0) from 2K to 380K

Using a high-resolution differential technique we have determined the electronic specific heat coefficient gamma(T) of Ba1-xKxFe2As2 with x=0 to 1.0, at temperatures (T) from 2K to 380K and in magnetic fields H=0 to 13T. In the normal state gamma_n(x,T) increases strongly with x at low temperature, compatible with a mass renormalisation ~12 at x=1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x=0.2 to 1, with transition temperatures and condensation energies peaking sharply at x=0.4. Superconducting coherence lengths xi_{ab}~20Å and xi_c~3Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T-dependences of gamma_s(T,H) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad "knee" and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at T_{c,s} and a T-dependence gamma_s(T)\propto\sqrt{1-T/T_{c,s}}. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T-dependence Delta_s(T)\propto(1-T/T_{c,s})^{0.75} near T_{c,s}. We estimate band fractions and T=0 gaps from 3-band alpha-model fits to our data and compare the x-dependences of the band fractions with spectroscopic studies of the Fermi surface.

cond-mat.supr-con↗

arXiv2013

Magnetic anisotropy and lattice dynamics in FeAs studied by Mössbauer spectroscopy

Iron mono-arsenide in the powder form has been investigated by transmission 57Fe Moessbauer spectroscopy in the temperature range 4.2 - 1000 K. Additional spectra have been obtained at 20 K and 100 K applying external magnetic field of 7 T. It was found that the spin spiral propagating along the c-axis leads to the complex variation of the hyperfine magnetic field amplitude with the spin orientation varying in the a-b plane. The magnitude of the hyperfine field pointing in the direction of the local magnetic moment depends on the orientation of this moment in the a-b plane. Patterns are vastly different for iron located in the [0 k 0] positions and for iron in the [0 k+1/2 0] positions within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon transition to the paramagnetic state at 69.2 K primarily in the a-c plane as seen by iron atoms. This effect is quite large considering lack of the structural transition. Two previously mentioned iron sites are discernible in the paramagnetic region till 300 K by different electron densities on the iron nuclei. The anisotropy of the iron vibrations developed at the transition to the paramagnetic state increases with the temperature in accordance with the harmonic approximation, albeit tends to saturation at high temperatures indicating gradual onset of the quasi-harmonic conditions. It seems that neither hyperfine fields nor magnetic moments are correct order parameters in light of the determined static critical exponents. Sample starts to loose arsenic at about 1000 K and under vacuum.

cond-mat.mtrl-sci↗