Publications (304)693.27 Total impact
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ABSTRACT: In the temperatureconcentration phase diagram of most ironbased superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In Ba{1x}KxFe2As2 and Ba{1x}NaxFe2As2, it has recently become clear that the usual stripelike magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we report measurements of the electrical resistivity of Ba{1x}KxFe2As2 under applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We track the onset of the tetragonal magnetic phase using the sharp anomaly it produces in the resistivity. In the temperatureconcentration phase diagram of Ba{1x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic phase, while the stripelike phase shrinks. This raises the interesting possibility that the fluctuations of the former phase might be involved in the pairing mechanism responsible for the superconductivity.  [Show abstract] [Hide abstract]
ABSTRACT: The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of single crystalline FeSe was studied using measurements of the interplane resistivity, $\rho_{\textrm{c}}$ in magnetic fields parallel to tetragonal $c$axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_\textrm{c}$, of FeSe exhibits a nonmonotonic dependence with local maximum at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of the upper critical field at $T_\textrm{c}$, $\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also exhibits a nonmonotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p<p_1$ the slope can be described within multiband orbital model. For both $p_1<p <p_2$ and $p>p_2$ the slope is in good quantitative agreement with a single band, orbital HelfandWerthamer theory with Fermi velocities determined from Shubnikovde Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.  [Show abstract] [Hide abstract]
ABSTRACT: The inplane resistivity anisotropy is studied in straindetwinned single crystals of FeSe. In contrast to other ironbased superconductors, FeSe does not develop longrange magnetic order below the nematic/structural transition at $T_{s}\approx$90~K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strainfree samples. The anisotropy peaks slightly below $T_{s}$ and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the inplane resistivity anisotropy in FeSe is dominated by inelastic scattering by anisotropic spin fluctuations.  [Show abstract] [Hide abstract]
ABSTRACT: We report $^{75}$As nuclear magnetic resonance (NMR) measurements of singlecrystalline Ca(Fe$_{1x}$Co$_x$)$_2$As$_2$ ($x$ = 0.023, 0.028, 0.033, and 0.059) annealed at 350~$^{\circ}$C for 7 days. From the observation of a characteristic shape of $^{75}$As NMR spectra in the stripetype antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\rm N}$ = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x$ = 0.023 ($T_{\rm N}$ = 106 K) and $x$ = 0.028 ($T_{\rm N}$ = 53 K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (1/$T_1$), although stripetype AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature $T^*$ which was nearly independent of Cosubstitution concentration, and is attributed to a pseudogaplike behavior in the spin excitation spectra of these systems. The $T^*$ feature finds correlation with features in the temperaturedependent interplane resistivity, $\rho_c(T)$, but not with the inplane resistivity $\rho _a (T)$. The temperature evolution of anisotropic stripetype AFM spin fluctuations are tracked in the paramagnetic and pseudogap phases by the 1/$T_1$ data measured under magnetic fields parallel and perpendicular to the $c$ axis. Based on our NMR data, we have added a pseudogaplike phase to the magnetic and electronic phase diagram of Ca(Fe$_{1x}$Co$_x$)$_2$As$_2$.  [Show abstract] [Hide abstract]
ABSTRACT: We present quantum oscillations observed in the heavy fermion compound YbPtBi in magnetic fields far beyond its fieldtuned, quantum critical point. Quantum oscillations are observed in magnetic fields as low as 60 kOe at 60 mK and up to temperatures as high as 3 K, which confirms the very high quality of the samples as well as the small effective mass of conduction carriers far from the quantum critical point. Although the electronic specific heat coefficient of YbPtBi reaches ~ 7.4 J/mol K2 in zero field, which is one of the highest effective mass value among heavy fermion systems, it is suppressed quickly by applied magnetic field. The quantum oscillations were used to extract the quasiparticle effective masses of the order of the bare electron mass, which is consistent with the behavior observed in specific heat measurements. Such a small effective masses at high fields can be understood by considering the suppression of Kondo screening. 
Article: Naturally structurally detwinned Fe pnictide superconducting family with metallic spacer layers
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ABSTRACT: We report a comprehensive study of single crystals of the newly discovered 112 iron pnictide superconductors (FBS). In Ca$_{0.73}$La$_{0.27}$FeAs$_2$, we unraveled a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which a distinct magnetic structure appears with the spins ordering 45$^\circ$ away from the stripe direction. Both phase transitions can be suppressed upon Co substitution on Fe sites and bulk superconductivity is stabilized up to 20 K. Furthermore, we demonstrate, as the structural and chemical consequences of the As chains in the spacer layers, this magnetic FBS is naturally structurally detwinned at ambient pressure with the formation of spin rotation walls (Swalls). Finally, in addition to the centralhole and cornerelectron Fermi pockets usually appearing in FBS, angleresolved photoemission (ARPES) measurements resolve a novel Fermiology where an extra electron pocket exists at the Brillouin zone edge with the As chain character. These unique features open a new avenue to clarify the role of electronic nematicity and metallic spacer layer in affecting the superconductivity.  [Show abstract] [Hide abstract]
ABSTRACT: The London penetration depth λ(T) was measured in single crystals of Ce_{1x}R_{x}CoIn_{5}, R=La, Nd, and Yb down to T_{min}≈50 mK (T_{c}/T_{min}∼50) using a tunneldiode resonator. In the cleanest samples Δλ(T) is best described by the power law Δλ(T)∝T^{n}, with n∼1, consistent with the existence of line nodes in the superconducting gap. Substitutions of Ce with La, Nd, and Yb lead to similar monotonic suppressions of T_{c}; however, the effects on Δλ(T) differ. While La and Nd substitution leads to an increase in the exponent n and saturation at n∼2, as expected for a dirty nodal superconductor, Yb substitution leads to n>3, suggesting a change from nodal to nodeless superconductivity. This superconducting gap structure change happens in the same doping range where changes of the Fermisurface topology were reported, implying that the nodal structure and Fermisurface topology are closely linked.  [Show abstract] [Hide abstract]
ABSTRACT: A single crystal of isovalently substituted Ba(Fe1−xRux)2As2 (x=0.24) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1×1019 e−/cm2. The electrical resistivity is measured in situ at T=22 K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature Tc decreases and the residual resistivity ρ0 increases. We find that electron irradiation leads to the fastest suppression of Tc compared to other types of artificially introduced disorder, probably due to the strong shortrange potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed Tc versus ρ0 is fully compatible with s± pairing, in contrast to earlier claims that this model leads to a too rapid suppression of Tc with scattering.  [Show abstract] [Hide abstract]
ABSTRACT: The dopingevolution of the superconducting gap structure in ironbased superconductor (CaLax )10(Pt3As8)(Fe2As2)5 (x = 0.04, 0.06, 0.09, 0.11, and 0.18) was probed by high—resolution measurements of the London penetration depth, . The samples spanned compositions from underdoped to slightly overdoped with superconducting critical temperatures, Tc , from 12.7 K (x = 0.04) through (optimal) 23.3 K (x = 0.11) to 21.9 K (x = 0.18). The lowtemperature variation (up to 0.3 Tc ) of was analysed using a powerlaw function, . For compositions close to the optimal doping, (x = 0.09, 0.11, and 0.18), characterized by , shows a tendency to saturation, indicative of a full gap on the Fermi surface. Fitting over the lowest temperature range () gives n = 2.6. This value is well outside the range expected for the linenodal superconductor. The exponent n decreased to in the two most underdoped compositions x = 0.04 (Tc = 12.7 K) and 0.06 (Tc = 18.2 K), implying the development of a notable gap anisotropy revealed by the enhanced influence of pairbreaking scattering. This decrease is accompanied by a significant increase of the total variation of the penetration depth in a fixed temperature interval (e.g., ). Both the decrease of the exponent and the increase of the absolute value of in the underdoped regime are similar to the observations in other chargedoped ironbased superconductors, such as doped BaFe2As2 and NaFeAs, suggesting a universal behavior in ironbased superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: Temperaturedependent interplane resistivity ${$\rho${}}_{c}(T)$ was measured in an isovalent substituted ironarsenide compound $\text{Ba}({\mathrm{Fe}}_{1$${}x}{\mathrm{Ru}}_{x}){}_{2}{\mathrm{As}}_{2}$ over a substitution range from parent compound to slightly below optimal doping $x=0.29$. The feature of interest in the ${$\rho${}}_{c}(T)$, a broad resistivity crossover maximum found in the parent compound at ${T}_{\text{max}}$\approx${}200$ K, shifts to higher temperatures with Ru substitution, $$\sim${}340$ K for $x=0.161$ and goes out of the 400 K range for $x=0.29$. Nearly $T$linear dependence of interplane resistivity is found at the highest substitution level $x=0.29$. This temperaturedependent ${$\rho${}}_{c}$ and its evolution with substitution bear close similarity to another type of isovalent substituted system ${\mathrm{BaFe}}_{2}({\mathrm{As}}_{1$${}x}{\mathrm{P}}_{x}){}_{2}$. Similarly to the isovalent substitutions, the measurements of interplane resistivity in the parent ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ compound under pressures up to 20 kbar also revealed a rapid rise in ${T}_{\text{max}}$.  [Show abstract] [Hide abstract]
ABSTRACT: We present a study of the crystal structure and physical properties of single crystals of a new Febased ternary compound, Zr2−xFe4Si16−y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric βFeSi2derived slabs are separated by ZrSi planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these ZrSi layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in xray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasiparticle masses are not particularly strongly enhanced. The onset of superconductivity at Tc 6 K results in a partial resistive transition and a small Meissner signal, although a bulklike transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nanosized superconducting grains are separated by a nonsuperconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the ZrFeSi ternary system could be a potentially rich source of new bulk superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: We investigated the occurrence and nature of superconductivity in single crystals of YFe$_2$Ge$_2$ grown out of Sn flux by employing xray diffraction, electrical resistivity, and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as $\sim$60, by decanting the crystals from the molten Sn at $\sim$350$^\circ$C and/or by annealing at temperatures between 550$^\circ$C and 600$^\circ$C. We found that samples with RRR $\gtrsim$ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition $T_c\approx1.4$ K. RRR values vary between 35 and 65 with, on average, no systematic change in $T_c$ value, indicating that systematic changes in RRR do not lead to comparable changes in $T_c$. Specific heat measurements on samples that showed clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain stabilized superconductivity associated with small amounts of stressed YFe$_2$Ge$_2$ (perhaps at twin boundaries or dislocations) or is a second crystallographic phase present at levels below detection capability of conventional powder xray techniques.  [Show abstract] [Hide abstract]
ABSTRACT: Irradiation with 2.5 MeV electrons at doses up to 5.2 $\times$10$^{19}$ electrons/cm$^2$ was used to introduce pointlike defects in single crystals of Ba$_{1x}$K$_x$Fe$_2$As$_2$ with $x=$ 0.19 ($T_c=$ 14 K), $x=$ 0.26 ($T_c=$ 32 K) and 0.34 ($T_c=$ 39 K) to study the superconducting gap structure by probing the effect of nonmagnetic scattering on electrical resistivity, $\rho(T)$, and London penetration depth, $ \lambda (T)$. For all compositions, the irradiation suppressed the superconducting transition temperature, $T_c$ and increased resistivity. The low  temperature behavior of $\lambda (T)$ is best described by the power  law function, $\Delta \lambda (T) = A(T/T_c)^n$. While substantial suppression of $T_c$ supports $s_{\pm}$ pairing mechanism, in samples close to the optimal doping, $x=$ 0.26 and 0.34, the exponent $n$ remained high ($n \geq 3$) indicating robust full superconducting gaps. For the $x=$ 0.19 composition, exhibiting coexistence of superconductivity and long  range magnetism, the suppression of $T_c$ was much more rapid and the exponent $n$ decreased toward dirty limit of $n$ = 2. In this sample, the irradiation also suppressed the temperature of structural/magnetic transition, $T_{sm}$, from 103 K to 98 K consistent with the itinerant nature of the magnetic order. Our results suggest that underdoped compositions, especially in the coexisting regime are most susceptible to nonmagnetic scattering and imply that in multiband Ba$_{1x}$K$_x$Fe$_2$As$_2$ superconductors, the ratio of the interband to intraband pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping.  [Show abstract] [Hide abstract]
ABSTRACT: Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration 0.2 have been found in the system. These phenomena have no discernable effect on the unconventional superconductivity and normalstate nonFermi liquid behaviour that occur over a broad range of up to 0.8. However, the variation of the coherence temperature and the superconducting critical temperature with nominal Yb concentration for bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function of to determine the actual composition of bulk single crystals. This analysis is supported by energydispersive Xray spectroscopy, wavelengthdispersive Xray spectroscopy, and transmission Xray absorption edge spectroscopy measurements. The actual composition is found to be about onethird of the nominal concentration up to 0.5, and resolves the discrepancy between the variation of the physical properties of single crystals and thin films with Yb concentration.  [Show abstract] [Hide abstract]
ABSTRACT: The inplane London penetration depth, $\Delta\lambda(T)$, was measured using a tunnel diode resonator technique in single crystals of Ba$_{1x}$K$_{x}$Fe$_{2}$As$_{2}$ with doping levels $x$ ranging from heavily underdoped, $x$=0.16 ($T_{c}$=7~K) to nearly optimally doped, $x$= 0.34 ($T_{c}=$39 K). Exponential saturation of $\Delta\lambda(T)$ in the $T\to0$ limit is found in optimally doped samples, with the superfluid density $\rho_{s}(T)\equiv(\lambda(0)/\lambda(T))^{2}$ quantitatively described by a selfconsistent $\gamma$model with two nodeless isotropic superconducting gaps. As the doping level is decreased towards the extreme end of the superconducting dome at $x$=0.16, the lowtemperature behavior of $\Delta\lambda(T)$ becomes nonexponential and best described by the powerlaw $\Delta\lambda(T)\propto T^{2}$, characteristic of strongly anisotropic gaps. The change between the two regimes happens within the range of coexisting magnetic/nematic order and superconductivity, $x<0.25$, and is accompanied by a rapid rise in the absolute value of $\Delta\lambda(T)$ with underdoping. This effect, characteristic of the competition between superconductivity and other ordered states, is very similar to but of significantly smaller magnitude than what is observed in the electrondoped Ba(Fe$_{1x}$Co$_{x}$)$_{2}$As$_{2}$ compounds. Our study suggests that the competition between superconductivity and magnetic/nematic order in holedoped compounds is weaker than in electrondoped compounds, and that the anisotropy of the superconducting state in the underdoped iron pnictides is a consequence of the anisotropic changes in the pairing interaction and in the gap function promoted by both magnetic and nematic longrange order.  [Show abstract] [Hide abstract]
ABSTRACT: London penetration depth, $\lambda(T)$, was measured in single crystals of SrFe$_2$(As$_{1x}$P$_x$)$_2$ ($x=$0.35) iron  based superconductor. The influence of disorder on the transition temperature, $T_c$, and on $\lambda(T)$ was investigated. The effects of scattering controlled by the annealing of asgrown crystals was compared with the effects of artificial disorder introduced by 2.5~MeV electron irradiation. The low temperature behavior of $\lambda(T)$ can be described by a powerlaw function, $\Delta \lambda (T)=AT^n$, with the exponent $n$ close to one in pristine annealed samples, as expected for superconducting gap with line nodes. Upon $1.2 \times 10^{19}$ \ecm irradiation, the exponent $n$ increases rapidly exceeding a dirty limit value of $n=$ 2 implying that the nodes in the superconducting gap are accidental and can be lifted by the disorder. The variation of the exponent $n$ with $T_c$ is much stronger in the irradiated crystals compared to the crystals in which disorder was controlled by the annealing of the growth defects. We discuss the results in terms of different influence of different types of disorder on intra and inter band scattering.  [Show abstract] [Hide abstract]
ABSTRACT: London penetration depth, $\lambda (T)$, was measured in single crystals of K$_{1x}$Na$_x$Fe$_2$As$_2$, $x$=0 and 0.07, down to temperatures of 50~mK, $\sim T_c/50$. Isovalent substitution of Na for K significantly increases impurity scattering, with $\rho(T_c)$ rising from 0.2 to 2.2 $\mu \Omega$cm, and leads to a suppression of $T_c$ from 3.5~K to 2.8~K. At the same time, a close to $T$linear $\Delta \lambda (T)$ in pure samples changes to almost $T^2$ in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to $T^2$ dependence in superconductors with symmetry imposed line nodes. The superfluid density in the full temperature range follows a simple clean and dirty $d$wave dependence, for pure and substituted samples, respectively. This result contradicts suggestions of multiband scenarios with strongly different gap structure on four sheets of the Fermi surface.  [Show abstract] [Hide abstract]
ABSTRACT: The thermal conductivity kappa of the heavyfermion metal CeCoIn5 was measured in the normal and superconducting states as a function of temperature T and magnetic field H, for a current and field parallel to the [100] direction. Inside the superconducting state, when the field is lower than the upper critical field Hc2, kappa/T is found to increase as T approaches absolute zero, just as in a metal and in contrast to the behavior of all known superconductors. This is due to unpaired electrons on part of the Fermi surface, which dominate the transport above a certain field. The evolution of kappa/T with field reveals that the electronelectron scattering (or transport mass m^*) of those unpaired electrons diverges as H approaches Hc2 from below, in the same way that it does in the normal state as H approaches Hc2 from above. This shows that the unpaired electrons sense the proximity of the fieldtuned quantum critical point of CeCoIn5 at H^* = Hc2 even from inside the superconducting state. The fact that the quantum critical scattering of the unpaired electrons is much weaker than the average scattering of all electrons in the normal state reveals a kspace correlation between the strength of pairing and the strength of scattering, pointing to a common mechanism, presumably antiferromagnetic fluctuations.  [Show abstract] [Hide abstract]
ABSTRACT: A single crystal of isovalently substituted Ba(Fe$_{1x}$Ru$_{x}$)$_2$As$_2$ ($x=0.24$) was sequentially irradiated with 2.5 MeV electrons up to a maximum dose of $2.1 \times 10^{19}$ electrons/cm^2. The electrical resistivity was measured \textit{in  situ} at $T=$22 K during the irradiation and \textit{ex  situ} as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature, $T_c$, decreases and the residual resistivity, $\rho_0$, increases. We find that electron irradiation leads to the fastest suppression of $T_c$ compared to other types of artificially introduced disorder, probably due to the strong shortrange potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed $T_c$ vs. $\rho_0$ is fully compatible with $s_\pm$ pairing, in contrast to earlier claims that this model leads to a too rapid a suppression of $T_c$ with scattering.  [Show abstract] [Hide abstract]
ABSTRACT: A single crystal of isovalently substituted Ba(Fe$_{1x}$Ru$_{x}$)$_2$As$_2$ ($x=0.24$) was sequentially irradiated with 2.5 MeV electrons up to a maximum dose of $2.1 \times 10^{19}$ electrons/cm^2. The electrical resistivity was measured \textit{in  situ} at $T=$22 K during the irradiation and \textit{ex  situ} as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature, $T_c$, decreases and the residual resistivity, $\rho_0$, increases. We find that electron irradiation leads to the fastest suppression of $T_c$ compared to other types of artificially introduced disorder, probably due to the strong shortrange potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed $T_c$ vs. $\rho_0$ is fully compatible with $s_\pm$ pairing, in contrast to earlier claims that this model leads to a too rapid a suppression of $T_c$ with scattering.
Publication Stats
5k  Citations  
693.27  Total Impact Points  
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Institutions

20082015

Iowa State University
 • Department of Physics and Astronomy
 • Ames Laboratory
Ames, Iowa, United States


20062010

Université de Sherbrooke
 Department of Physics
Sherbrooke, Quebec, Canada


20022007

University of Toronto
 Department of Physics
Toronto, Ontario, Canada


2005

Gakushuin University
 Department of Physics
Edo, Tokyo, Japan


2003

Le Centre de Recherche en Économie et Statistique
Malakoff, ÎledeFrance, France


19972003

Kyoto University
 Department of Physics II
Kioto, Kyōto, Japan


19951999

National Academy of Sciences of Ukraine
Kievo, Kyiv City, Ukraine


1998

Institute of Molecular Physics, Polish Academy of Sciences
Posen, Greater Poland Voivodeship, Poland


19931994

Ukrainian Academy of Agrarian Sciences
Kievo, Kyiv City, Ukraine


1989

Institute of Semiconductor Physics
NovoNikolaevsk, Novosibirsk, Russia
