Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M13: Focus Session: Fe-based Superconductors-Optical properties |
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Chair: David Tanner, University of Florida Room: 207 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M13.00001: Strong electronic correlations in iron pnictides: Comparison of the optical spectra for BaFe$_{2}$As$_{2}$-related compounds Invited Speaker: Masamichi Nakajima The role of electronic correlations in iron pnictides is one of the hottest issues in research of iron-based superconductors. Utilizing optical spectroscopy, we quantified the strength of electronic correlations in BaFe$_{2}$As$_{2}$-related compounds. For the parent compound BaFe$_{2}$As$_{2}$, the fraction of the coherent spectral weight in the low-energy optical conductivity spectrum is distinctly small. Such a spectral feature is also observed in KFe$_{2}$As$_{2}$, indicating that the charge dynamics is highly incoherent in iron arsenides. It is found that the strength of electronic correlations significantly changes by chemical substitution, either through changing the electron filling and/or the As-Fe-As bond angle. The present result indicates that superconductivity of the iron pnictides emerges when the materials possess adequate amount of electronic correlations, and that either too weak or too strong correlations are not favorable for high-$T_{\mathrm{c}}$ superconductivity. The degree of electronic correlations in iron arsenides turns out to be comparable to that in the hole-underdoped cuprate superconductors. In this sense, the iron arsenides are classified into strongly correlated systems, probably arising from the Hund's rule coupling. This work was done in collaboration with S. Ishida, K. Kihou, Y. Tomioka, C. H. Lee, A. Iyo, T. Ito, H. Eisaki (AIST), T. Tanaka, T. Kakeshita, S. Uchida (University of Tokyo), T. Saito, H. Fukazawa, and Y. Kohori (Chiba University). [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M13.00002: Optical conductivity of clean-limit superconductor LiFeAs R.P.S.M. Lobo, A.V. Pronin, G. Chanda, J. Wosnitza, S. Kasahara, T. Shibauchi, Y. Matsuda We present the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral weight loss and yields a penetration depth of 215 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the very clean limit. The normal state optical conductivity can be modeled through a Drude-Lorentz decomposition and allows to determine a quasiparticle scattering rate that evolves linearly with temperature. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M13.00003: Superfluid and quasiparticle behavior below Tc of strain introduced high-quality epitaxial thin films of Fe(Se,Te) Atsutaka Maeda, Fuyuki Nabeshima, Yoshnori Imai, Masafumi Hanawa, Ataru Ichinose, Ichiro Tsukada We succeeded in introducing compressive strain in epitaxial films of FeSe and Fe(Te,Se), leading to high Tcs' (1.5 times higher that in bulk crystals for FeSe)[1]. It is of great interest how the effect of strain shows up in properties in the superconducting state of these thin-film samples. We investigated superfluid- and quaiparticle response at THz frequencies. Structures characteristic of superconductivity was found clearly both in real part and imaginary part of the conductivity spectrum. Increase of quasiparticle scattering time below Tc was observed even in THz frequencies, which is connected with microwave data measured in bulk crystals consistently. Even in these high-quality, high Tc films, development of superfluid density with decreasing temperature is rather gradual, keeping a ``dirty'' feature. This might be related to possible excess Fe characteristic of this material, and further improvement of Tc is expected by additional heat treatment. Alternatively, the contribution of Legget mode is also considered. At present, any anomalous features related to strain have not been observed in these properties. The data at microwave frequencies taken by a dielectric resonator will also be discussed.\\[4pt] [1] F. Nabeshima et al.: APL 103 (2013) 172602. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M13.00004: Infrared spectroscopy of rare-earth-doped CaFe$_{2}$As$_{2}$ Zhen Xing, T.J. Huffman, Peng Xu, M.M. Qazilbash, S.R. Saha, Tyler Drye, J. Paglione Recently, rare-earth doping in CaFe$_{2}$As$_{2}$ has been used to tune its electronic, magnetic, and structural properties. The substitution of rare-earth ions at the alkaline-earth sites leads to the suppression of the spin-density wave (SDW) phase transition in CaFe$_{2}$As$_{2}$. For example, Pr substitution results in a paramagnetic metal in the tetragonal phase that is susceptible to a low temperature structural transition to a collapsed tetragonal phase. However, La-doped CaFe$_{2}$As$_{2}$ remains in the uncollapsed tetragonal structure down to the lowest measured temperatures. Both the uncollapsed and collapsed tetragonal structures exhibit superconductivity with maximum Tc reaching 47 K, the highest observed in inter-metallics albeit with a small superconducting volume fraction. In this work, we perform ab-plane infrared spectroscopy of rare-earth-doped CaFe$_{2}$As$_{2}$ at different cryogenic temperatures. Our aim is to ascertain the contributions of electron doping and chemical pressure to the charge and lattice dynamics of this iron-arsenide system. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M13.00005: Spin-density-wave–induced anomalies in the optical conductivity of AFe2As2, (A=Ca, Sr, Ba) Aliaksei Charnukha, Daniel Proepper, Timofei Larkin, Dunlu Sun, Z.W. Li, Chengtian Lin, Thomas Wolf, Bernhard Keimer, Alexander Boris We report the complex dielectric function of high-quality AFe2As2, (A=Ca,Sr,Ba) single crystals with TN=150K, 200K, and 138K, respectively, determined by broadband spectroscopic ellipsometry. In CaFe2As2 we identify the optical spin-density--wave gap $2\Delta_{\mathrm{SDW}}\approx1250\ \textrm{cm}^{-1}$. The $2\Delta_{\mathrm{SDW}}/(k_{\mathrm{B}}T_{\mathrm{N}})$ ratio amounts to 12 in CFA, significantly larger than the corresponding values for the SFA and BFA compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both SFA and CFA and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in CFA at a temperature T*=80K, well below the N\'eel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. Our results single out CFA in the class of 122 iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M13.00006: Negative transport times due to interband scattering Maxim Breitkreiz, Philip Brydon, Carsten Timm Negative transport times lead to unexpected transport behavior such as negative magnetoresistance, strongly enhanced Hall coefficient, and reduced resistivity. Within a semiclassical Boltzmann approach beyond the relaxation-time approximation, it is demonstrated that negative transport times generically arise due to anisotropic single-particle scattering between electronlike and holelike Fermi surfaces. This mechanism could be responsible for the anomalous transport properties of materials close to an excitonic instability. In particular we discuss the case of one circular hole pocket and two elliptical electron pockets, which is relevant for iron pnictides. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M13.00007: Low-energy quasiparticle excitations in $A$Fe$_{2}$As$_{2}$, $A=$Rb, Cs revealed by magnetic penetration depth measurements Yuta Mizukami, Yuta Kawamoto, Satoshi Kurata, Yusuke Shimoyama, Philipp Burger, Anna B{\"o}hmer, Frederic Hardy, Thomas Wolf, Christoph Meingast, Hilbert L{\"o}hneysen, Yuji Matsuda, Takasada Shibauchi In superconductors with strong correlations, clarifying the superfluid response in the superconducting state plays a crucial role to determine the symmetry and the structure of superconducting gap. In hole-doped iron-based pnictide superconductor KFe$_{2}$As$_{2}$ without electron pockets, highly unusual nodal structure in the superconducting gap has been reported. How this gap structure changes in related materials is important to understand its pairing mechanism. Here, we report on the magnetic penetration depth measurements in $A$Fe$_{2}$As$_{2}$, $A=$Rb, Cs. We observe strong temperature dependence of penetration depth at low temperatures, evidencing low-energy quasiparticle excitations. From detailed comparisons of the data between K, Rb, Cs systems, the gap structure in these materials will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M13.00008: Superconducting gap structure in over-doped (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ ($x=$0.35, 0.47, 0.56, and 0.64) from London penetration depth measurements Serafim Teknowijoyo, Kyuil Cho, Makariy A. Tanatar, Yong Liu, Thomas A. Lograsso, Ruslan Prozorov Single crystals of (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ were extensively studied from the optimal doping to the very underdoped regime. However, overdoped regime was out of reach due to various issues with the crystal growth. Here we report London penetration depth measured in high quality single crystals of (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ with ($x=$0.35, 0.47, 0.56, and 0.64) that have $T_c=$30 K, 39 K, 32 K and 22 K, respectively. The study of the evolution from the optimally doped composition toward the end member, KFe$_2$As$_2$, is especially important, since the former is clearly a full isotropic gap material, whereas the latter is a d-wave superconductor. Our results suggest a gradual evolution from the full gap to the nodal gap with doping. The results will be discussed in terms of competing s- and d- channels in the general $s_\pm$ framework. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M13.00009: Effect of electron irradiation on resistivity and London penetration depth of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Kyuil Cho, J. Murphy, H. Kim, M.A. Tanatar, R. Prozorov, M. Konczykowski, B. Shen, H.H. Wen The effect of electron irradiation on the in-plane resistivity and London penetration depth was studied in single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (x = 0.19, 0.24, and 0.34). The irradiation fluence varied between $8.7\times10^{18}$ and $5.2\times10^{19}$ electrons per cm$^{2}$. We found a profound decrease of the critical temperature, $T_c$, by 3 - 10 K depending on doping and the irradiation dose. Expectedly, the residual resistivity increases. The analysis of low-temperature part of London penetration depth shows that the superconducting gap becomes more anisotropic in under-doped (x = 0.19 and 0.24) crystals. Interestingly, however, the full - gap at the optimal doping (x = 0.34) remained at the same $\Delta(0)/T_c$ ratio after $5.2\times10^{19}$ e/cm$^2$ irradiation even though $T_c$ has decreased by almost 10 K (1/4 of the original value). The results will be discussed in a framework of s$_\pm$ pairing with a complex interplay between intra - and inter - band interactions and scattering. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M13.00010: Flux-flow resistivity and penetration depth measurements of BaFe$_2$(As,P)$_2$: semi-quantitative estimates of gap anisotropy Tatsunori Okada, Yoshinori Imai, Hideyuki Takahashi, Masamichi Nakajima, Akira Iyo, Hiroshi Eisaki, Atsutaka Maeda Flux-flow measured by using a microwave technique is unique method to investigate quasiparticles in the vortex core. By measuring the magnetic-field dependence of the flux-flow resistivity, $\rho_f$, of several Fe-based SCs, we found that $\rho_f(H)$ is expressed as $\rho_f/\rho_n=\alpha H/H_{\rm c2}$ with $\alpha$ strongly depends on materials, suggesting that $\rho_f(H)$ of Fe-based SCs is dominated by the gap structure. By comparing these with the penetration depth data, we also found that $\alpha$ becomes larger when the gap function is more anisotropic [1,2]. To make this gap-anisotropy scenario more convincing, we focused on BaFe$_2$(As,P)$_2$ (P=30, 45\%), and found that the penetration depth increased in proportion to $T^{1.5-1.7}$. The fractional exponent can be understood by assuming that these materials have gaps with lines of nodes and deeply-warped nodeless gaps. As for flux-flow, $\rho_f(H)$ showed a large gradient of $\alpha>2.5$, similar to that of SrFe$_2$(As,P)$_2$ [2], pointing to highly anisotropic gaps in a consistent manner. These results support the gap-anisotropy scenario. [1]T. Okada $et\ al.$, PRB {\bf 86}, 064516 (2012); Physica C {\bf 484}, 27 (2013); $ibid$ {\bf 494}, 109 (2013) [2]H. Takahashi $et\ al.$, PRB {\bf 86}, 144525 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M13.00011: Nodal superconducting state in clean single crystals of FeSe S. Kasahara, T. Mikami, Y. Mizukami, Y. Kawamoto, S. Kurata, D. Watanabe, T. Shibauchi, Y. Matsuda, A.E. B\"{o}hmer, T. Wolf, C. Meingast, H. v. L\"{o}hneysen Among iron-based superconductors, the binary ``11'' family offers the possibility to investigate systems consisting of just the iron arsenic/selenium layers without the intermediate layers which are present in the ``111'', ``122'' and ``1111'' families. This simplest iron based superconductor may therefore yield vital information about the origin of superconductivity in the iron pnictides/chalcogenides. Here we measured the penetration depth and thermal conductivity in very clean single crystals of FeSe~[1] with RRR $\textgreater$ 200. Presence of line nodes is evident by the quasi $T$-linear dependence of the penetration depth. Moreover, a large residual thermal conductivity, which is much larger than that expected for $d$-wave symmetry, suggests that nodes are accidental and nearly vanishing. The field dependence of thermal conductivity suggests a possible field induced phase transition in the superconducting state. \\ ~[1] A.E. B\"{o}hmer et al., Phys. Rev. B {\bf 87}, 180505(R) (2013). [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M13.00012: Ultrafast Critical Nematic Fluctuations and Giant Magnetoelastic Coupling in Iron Pnictide Aaron Patz, Tianqi Li, Sheng Ran, Rafael Fernandes, Joerg Schmalian, Sergey Bud'ko, Paul Canfield, Ilias Perakis, Jigang Wang A ubiquitous anisotropy in the normal state properties of many of the iron pnictides presents a crosscutting challenge important for understanding quantum magnetism and high-temperature superconductivity. Although an electronically-driven nematicity has been invoked, distinguishing this from spin and structural orders is challenging because they all couple together to break the same tetragonal symmetry. Here we use femtosecond-resolved polarimetry to reveal critical fluctuations of nematic correlation in unstrained Ba(Fe1-xCox)2As2. The ultrafast anisotropic response, which arises from the two-fold in-plane anisotropy of the refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase, with increasing relaxation time, indicative of critical nematic fluctuations approaching the structural transition temperature. The dynamics reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M13.00013: Coherent A$_{\mathrm{1g}}$ Phonon in thin Film Superconductor FeSe$_{0.5}$Te$_{0.5}$: $\pi $/2 Phase Difference over Superconducting Phase Transition Jimin Zhao, Yanling Wu, Minhui Hu, Yichao Tian, Lixin Cao, Rui Wang Coherent $A_{1g}$ phonon mode in a thin film superconductor FeSe$_{0.5}$Te$_{0.5}$ was generated and detected using ultrafast laser pulses. At below and above the transition temperature $T_{\mathrm{c}}$, the coherent lattice oscillation we observed exhibited a $\pi $/2 phase difference, manifesting a ``displacive limit $\sim$ impulsive limit'' transition upon crossing a phase transition within the same sample. We ascribe this $\pi $/2 phase difference to the different lifetimes ($\tau_{\mathrm{c}})$ of excited charge density components that couples to the fully symmetric $A_{1g}$ phonon mode, i.e. the different strength of electron-phonon couplings. In the superconducting and paramagnetic metallic states the lifetimes of such carrier excitations are largely different. Our investigation reveals possible correlation of superconducting electrons with zone-center optical phonons. Our 170nm thin film sample contains tension stress, which leads to enhanced $T_{\mathrm{c}}$ and thus facilitated our measurements. [Preview Abstract] |
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