arXiv subjects

R. Prozorov

Publications and source records attributed to R. Prozorov.

At least 109 records · Page 6Linked to original sources

arXiv2013

Upper critical field of isoelectron substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$

The upper critical field $H_{c2}$ of optimally doped iron-based superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$ = 0.35, $T_c$ = 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H \parallel c$ and $H \parallel a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion irradiated single crystals, with columnar defect density corresponding to a matching field $Bϕ$ = 25 T. The $H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for $H_{c2,a}(T)$, leading to a strongly temperature dependent anisotropy parameter $γ$. The linear shape of $H_{c2,c}(T)$ is very similar to that observed in nodal KFe$_2$As$_2$ but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on $H_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$, keeping their ratio constant.

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arXiv2013

Nodal superconductivity in isovalently substituted SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ pnictide superconductor at the optimal doping, $x=$0.35

Temperature-dependent London penetration depth, $λ(T)$, was measured in optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K, RRR=$ρ(300K)/ρ(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4) single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from $λ(0) = 300 \pm 10$ nm in as-grown sample to $λ(0) = 275 \pm 10$ nm. At low temperatures, $λ(T) \sim T$ indicating superconducting gap with line nodes. Analysis of the full - temperature range superfluid density is consistent with the line nodes, but differs from the simple single - gap $d-$wave. The observed behavior is very similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently substituted pnictides are inherently different from the charge - doped materials.

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arXiv2012

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1-xPx)2

In a superconductor, the ratio of the carrier density, $n$, to their effective mass, $m^*$, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, $λ_L$. In two dimensional systems, this ratio $n/m^*$ ($\sim 1/λ_L^2$) determines the effective Fermi temperature, $T_F$. We report a sharp peak in the $x$-dependence of $λ_L$ at zero temperature in clean samples of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ at the optimum composition $x = 0.30$, where the superconducting transition temperature $T_c$ reaches a maximum of 30\,K. This structure may arise from quantum fluctuations associated with a quantum critical point (QCP). The ratio of $T_c/T_F$ at $x = 0.30$ is enhanced, implying a possible crossover towards the Bose-Einstein condensate limit driven by quantum criticality.

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arXiv2012

Temperature-concentration phase diagram of (Ca1-xLax)10(Pt3As8)(Fe2As2)5 superconductors

Single crystals of (Ca1-xLax)10(Pt3As8)(Fe2As2)5 (x = 0 to 0.182) superconductors have been grown and characterized by X-ray, microprobe, transport and thermodynamic measurements. Features in the magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x = 0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a half-dome of superconductivity with a maximal Tc around 26 K is observed in the temperature-concentration phase diagram.

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arXiv2012

New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122

The electrical resistivity rho of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases linearly with pressure. At pressures above around 1.0 GPa, a second sharp anomaly is detected at a lower temperature T_0, which rises with pressure. We attribute this second anomaly to the onset of a phase that causes a reconstruction of the Fermi surface. This new phase expands with increasing x and it competes with superconductivity. We discuss the possibility that a second spin-density wave orders at T_0, with a Q vector distinct from that of the spin-density wave that sets in at T_N.

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arXiv2012

Anisotropic resistivity of Na$_{1-δ}$Fe$_{1-x}$Co$_x$As

Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-δ}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $ρ_a (T)$, and transverse, $ρ_c (T)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both $ρ_a(T)$ and $ρ_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $ρ_a (T)$ with $x$ follows a universal trend observed in other pnictide superconductors, exhibiting a $T$-linear temperature dependence close to the optimal doping and development of $T^2$ dependence upon further doping. $ρ_c (T)$ in parent compound shows a non - monotonic behavior with a crossover from non-metallic resistivity increase on cooling from room temperature down to $\sim$ 80 K to a metallic decrease below this temperature. Both $ρ_a (T)$ and $ρ_c (T)$ show several correlated crossover - like features at $T>$ 80 K. Despite a general trend towards more metallic behavior of inter - plane resistivity in Co-doped samples, the temperature of the crossover from insulating to metallic behavior (80 K) does not change much with doping.

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arXiv2012

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

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arXiv2012

Orbital upper critical field and its anisotropy of clean one- and two-band superconductors

The Helfand-Werthamer (HW) scheme\cite{HW} of evaluating the orbital upper critical field is generalized to anisotropic superconductors in general, and to two-band clean materials, in particular. Our formal procedure differs from those in the literature; it reproduces not only the isotropic HW limit, but also the results of calculations for the two-band superconducting MgB$_2$\cite{MMK,DS} along with the existing data on $H_{c2}(T)$ and its anisotropy $γ(T)=H_{c2,ab}(T)/H_{c2,c}(T)$ ($a,c$ are the principal directions of a uniaxial crystal). Using rotational ellipsoids as model Fermi surfaces we apply the formalism developed to study $γ(T)$ for a few different anisotropies of the Fermi surface and of the order parameters. We find that even for a single band d-wave order parameter $γ(T)$ decreases on warming, however, relatively weakly. For order parameters of the form $ Δ(k_z) = Δ_0(1+η\cos k_za)$,\cite{Xu} according to our simulations $γ(T)$ may either increase or decrease on warming even for a single band depending on the sign of $η$. Hence, the common belief that the multi-band Fermi surface is responsible for the temperature variation of $γ$ is proven incorrect.

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arXiv2012

Infrared Measurement of the Pseudogap in P-Doped and Co-Doped BaFe2As2 High-Temperature Superconductors

We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

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arXiv2012

Reply to "Comment on \emph{"Precision global measurements of London penetration depth in Fe(Te$_{0.58}$Se$_{0.42}$)"}"

We reply to the Comment by T. Klein, P. Rodière and C. Marcenat (arXiv:1205.2566), on our paper, "Precision global measurements of London penetration depth in FeTe$_{0.58}$Se$_{0.42}$", Phys. Rev. B 84, 174502 (2011). Our work was motivated by Klein \emph{et. al}, "Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals in fields up to 28 Tesla", Phys. Rev. B 82, 184506 (2010). In their paper, Klein et al. have attributed a factor of five difference in the value of the London penetration depth obtained from their tunnel diode resonator (TDR) measurements and calculated from the "field of first penetration" to the surface roughness, although they have not verified it experimentally. In our paper, we have studied the effects of deliberately introduced surface roughness and found that its effects are minor and cannot be responsible for the difference of such magnitude. Instead, we suggest that the value of the "field of first penetration" measured with Hall - arrays cannot be used to extract a true lower critical field due to several reasons outlined in our Reply. We emphasize that the accuracy of the calibration procedure of the TDR technique has been carefully verified in several prior studies and our work on FeTe$_{0.58}$Se$_{0.42}$ further confirms it. We show that in their Comment, Klein \emph{et. al} use wrong arguments of the universal behavior of the superfluid density in the gapless limit, because it is inapplicable for the multi - band superconductors. We also discuss the applicability of the clean - limit $γ-$ model and the influence of the disorder on the obtained results.

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arXiv2012

Doping - dependent anisotropic superconducting gap in Na$_{1-δ}$(Fe$_{1-x}$Co$_x$)As

The London penetration depth was measured in single crystals of self-doped Na$_{1-δ}$FeAs (from under - to optimal - doping, $T_c$ from 14 to 27 K) and electron-doped Na(Fe$_{1-x}$Co$_{x}$)As with $x$ ranging from undoped, $x=0$ to overdoped $x= 0.1$. In all samples, the low - temperature variation of the penetration depth exhibits a power-law dependence, $Δλ(T) = AT^n$, with the exponent that varies in a dome - like fashion from $n \sim 1.1$ in the underdoped, reaching a maximum of $n \sim 1.9$ in the optimally doped and decreasing again to $n \sim 1.3$ on the overdoped side. While the anisotropy of the gap structure follows universal dome-like evolution, the exponent at the optimal doping, $n \sim 1.9$, is lower than in other charge - doped Fe-based superconductors (FeSCs). The full - temperature range superfluid density, $ρ_s(T) = (λ(0)/λ(T))^2$, at the optimal doping is also distinctly different from other charge - doped FeSCs but is similar to isovalently - substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$, believed to be a nodal pnictide at the optimal doping. These results suggest that the superconducting gap in Na(Fe$_{1-x}$Co$_{x}$)As is highly anisotropic even at the optimal doping.

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arXiv2012

Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state

The thermal conductivity of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of the RLT in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity was ten times larger. This independence of the RLT on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate for suppressing Tc to zero; the magnitude of the RLT, and its dependence on current direction and on magnetic field; the temperature dependence of the thermal conductivity.

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arXiv2012

Type I Superconductivity in YbSb2 Single Crystals

We present evidence of type I superconductivity in YbSb2 single crystals, from DC and AC magnetization, heat capacity and resistivity measurements. The critical temperature and critical field are determined to be $T_c\approx$ 1.3 K and $H_c\approx$ 55 Oe. A small Ginzburg-Landau parameter κ= 0.05, together with typical magnetization isotherms of type I superconductors, small critical field values, a strong Differential Paramagnetic Effect (DPE) signal, and a field-induced change from second to first order phase transition, confirm the type I nature of the superconductivity in YbSb2. A possible second superconducting state is observed in the radiofrequency (RF) susceptibility measurements, with $T_{c}^{(2)}\approx$ 0.41 K and $H_{c}^{(2)}\approx$ 430 Oe.

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arXiv2012

Synthesis and physical properties of the new potassium iron selenide superconductor K0.80Fe1.76Se2

In this article we review our studies of the K0.80Fe1.76Se2 superconductor, with an attempt to elucidate the crystal growth details and basic physical properties over a wide range of temperatures and applied magnetic field, including anisotropic magnetic and electrical transport properties, thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer measurements. We find that: (i) Single crystals of similar stoichiometry can be grown both by furnace-cooled and decanted methods; (ii) Single crystalline K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and electrical resistivity and a small modulation of stoichiometry of the crystal, which gives rise to broadened transitions; (iii) The upper critical field, Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits magnetic order, probably of the first order, from well below Tc to its Neel temperature of Tn = 532 +/- 2 K. It is very important to note that, although, at first glance there is an apparent dilemma posed by these data: high Tc superconductivity in a near insulating, large ordered moment material, analysis indicates that the sample may well consist of two phases with the minority superconducting phase (that does not exhibit magnetic order) being finely distributed, but connected with in an antiferromagnetic, poorly conducting, matrix, essentially making a superconducting aerogel.

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arXiv2011

Anisotropy of the coherence length from critical currents in the stoichiometric superconductor LiFeAs

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, ε_ξ. This allows extraction of ε_ξ at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.

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arXiv2011

Doping - dependent superconducting gap anisotropy in the two-dimensional 10-3-8 pnictide Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$]$_5$

The characteristic features of Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ("10-3-8") superconductor are relatively high anisotropy and a clear separation of superconductivity and structural/magnetic transitions, which allows studying the superconducting gap without complications due to the coexisting order parameters. The London penetration depth, measured in underdoped single crystals of 10-3-8 ($x =$ 0.028, 0.041, 0.042, and 0.097), shows behavior remarkably similar to other Fe-based superconductors, exhibiting robust power-law, $Δλ(T) = A T^n$. The exponent $n$ decreases from 2.36 ($x =$ 0.097, close to optimal doping) to 1.7 ($x =$ 0.028, a heavily underdoped composition), suggesting that the superconducting gap becomes more anisotropic at the dome edge. A similar trend is found in low-anisotropy superconductors based on BaFe$_2$As$_2$ ("122"), implying that it is an intrinsic property of superconductivity in iron pnictides, unrelated to the coexistence of magnetic order and superconductivity or the anisotropy of the normal state. Overall this doping dependence is consistent with $s_{\pm}$ pairing competing with intra-band repulsion.

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arXiv2011

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor, BaFe$_2$As$_2$, is characterized by temperature-dependent electrical resistivity, x-ray diffraction and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe$_2$As$_2$, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plain resistivity anisotropy above the transition in BaFe$_2$As$_2$. This conclusion is supported by the Ginzburg-Landau - type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy.

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arXiv2011

Evolution of normal and superconducting properties of single crystals of Na$_{1-δ}$FeAs upon interaction with environment

Iron-arsenide superconductor Na$_{1-δ}$FeAs is highly reactive with the environment. Due to the high mobility of Na ions, this reaction affects the entire bulk of the crystals and leads an to effective stoichiometry change. Here we use this effect to study the doping evolution of normal and superconducting properties of \emph{the same} single crystals. Controlled reaction with air increases the superconducting transition temperature, $T_c$, from the initial value of 12 K to 27 K as probed by transport and magnetic measurements. Similar effects are observed in samples reacted with Apiezon N-grease, which slows down the reaction rate and results in more homogeneous samples. In both cases the temperature dependent resistivity, $ρ_a(T)$, shows a dramatic change with exposure time. In freshly prepared samples, $ρ_a(T)$ reveals clear features at the tetragonal-to-orthorhombic ($T_s$ $\approx$ 60 K) and antiferromagnetic ($T_m$=45 K) transitions and superconductivity with onset $T_{c,ons}$=16 K and offset $T_{c,off}$=12 K. The exposed samples show $T-$linear variation of $ρ_a(T)$ above $T_c,ons$=30 K ($T_{c,off}$=26 K), suggesting bulk character of the observed doping evolution and implying the existence of a quantum critical point at the optimal doping. The resistivity for different doping levels is affected below $\sim$200 K suggesting the existence of a characteristic energy scale that terminates the $T-$linear regime, which could be identified with a pseudogap.

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