Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C14: Febased Superconductors  Quantum Criticality and Quantum Phase TransitionsFocus

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Sponsoring Units: DMP Chair: Makariy Tanatar, Iowa State University Room: LACC 304B 
Monday, March 5, 2018 2:30PM  3:06PM 
C14.00001: ScaleInvariant transport near quantum critical point in hightemperature superconductors. Invited Speaker: Arkady Shekhter The origin of the anomalous relaxation dynamics and the very nature of charge carriers in the "strange metal" state are among the most pressing open questions in copper and iron based superconductors. In this talk, I will review thermodynamic measurements that indicate the existence of quantum critical point in iron based superconductors, such as enhanced electronic correlations approaching critical doping as well as zerotemperature collapse of a line of phase transitions inside superconducting dome. Then I will discuss highmagneticfield transport measurements near critical doping which highlight the dynamic consequences of quantum criticality such as scale invariant transport. In both classes of high temperature superconductors these measurements reveal a linearinfield resistivity at very high magnetic fields (up to 93T) which exhibits a characteristic temperaturefield competition as field is lowered. Such scaleinvariant behavior of magnetotransport in a broad range of temperatures and magnetic fields is incompatible with quasiparticle transport in the strange metal state. 
Monday, March 5, 2018 3:06PM  3:18PM 
C14.00002: The Hall Efffect at Ultra High Fields in Ba122 Ian Hayes, Nikola Maksimovic, Mun Chan, Brad Ramshaw, Ross McDonald, James Analytis To this day, the basic charge transport phenomenology of the highTc cuprates, including the Tlinear resistivity near optimal doping and the Tdependent Hall effect, has remained mysterious. The observation of a very similar set of transport properties in the iron based superconductors supports the idea that these properties are closely tied to the physics of highTc superconductivity in general, and gives new urgency to the problem. I will report Hall effect measurements up to 65 Tesla on samples of the ironpnictide superconductor BaFe2(As1xPx)2 form across the superconducting dome, providing a comprehensive picture of the Hall effect in these materials. There is a strong enhancement of R_H centered around optimal doping that bears a striking resemblance to what is seen in the cuprates and in quantum critical metals and also appears to be intimately tied to the superconductivity. I will discuss the implications of these observations and the paths they open up for obtaining a clearer understanding of the Strange Metal state. 
Monday, March 5, 2018 3:18PM  3:30PM 
C14.00003: Quantum critical nematic fluctuations and spin excitation anisotropy in the iron pnictides Elihu Abrahams, ChiaChuan Liu, Qimiao Si Quantum criticality in the iron pnictides involves the nematic and antiferromagnetic channels in a concurrent way [1,2]. To elucidate the nematic correlations, we study the spin excitation anisotropy defined by the difference between the dynamical spin susceptibilities at (pi,0) and (0,pi) [3]. We consider an effective GinzburgLandau theory involving spin and Isingnematic degrees of freedom [1,2] in the presence of a small external C4symmetrybreaking potential. Our analysis is carried out in a large N limit [2]. We connect the spin excitation anisotropy to the dynamical spin susceptibility and the nematic susceptibility. This relationship provides the understanding of the singular energy dependences observed for the spin excitation anisotropy in the avoided quantum critical regime of the optimally Nidoped BaFe_{2}As_{2} under a uniaxial strain. 
Monday, March 5, 2018 3:30PM  3:42PM 
C14.00004: Magnetic and nematic quantum bicritical point in Ba(Fe0.97Cr0.03)2(As1xPx)2 Wenliang Zhang, Huiqian Luo, Shiliang Li The disappearance of longrange order in metallic system may result in a quantum critical point (QCP) that is believed to be crucial in understanding unconventional superconductivity in many materials. Both the antiferromagnetic and nematic quantum critical points have been suggested to present in ironbased superconductors, but they are always preempted by superconductivity. Here we show that just 3\% of Cr doping can fully suppress superconductivity and reveal a quantum critical point in the BaFe2(As1xPx)2 system, which corresponds to the disappearance of both the antiferromagnetic and nematic orders. We found that the antiferromagnetic transition temperature and the magnetic moment decrease linearly upon the phosphorus doping till the QCP, with a Vshaped region of Tlinear resistance, which suggests the position of the QCP significantly shifts from that in the superconducting system. 
Monday, March 5, 2018 3:42PM  3:54PM 
C14.00005: Lower critical field across the superconducting dome of Ba(Fe_{1x}Co_{x})_{2}As_{2} measured using optical NVmagnetometer Kamal Joshi, Naufer Nusran, Kyuil Cho, Makariy Tanatar, Sergey Budko, Paul Canfield, Ruslan Prozorov The lower critical field of superconductors (H_{c1}) gives direct access to the absolute value of London penetration depth which is needed to obtain the superfluid density. H_{c1} is notoriously difficult to measure due to a variety of obstacles such as demagnetization, several types of surface barriers, imperfections, and anisotropies. Some of the most accurate techniques involve direct measurements of the magnetic field at which first vortices start penetrating the sample, and require outstanding sensitivity and good spatial resolution. Here we employ NitrogenVacancy (NV) centers in diamond as a noninvasive, sensitive optical magnetometer to precisely measure H_{c1} in single crystals of Ba(Fe_{1x}Co_{x})_{2}As_{2} and track it as a function of cobalt concentration (x). We discover an anomalous decrease of superfluid stiffness near x=0.057, the composition at the very edge of the magnetic phase where magnetic order becomes incommensurate. We compare our findings with similar behavior in Pdoped BaFe_{2}As_{2}. 
Monday, March 5, 2018 3:54PM  4:06PM 
C14.00006: Observation of Charge Density Waves in Superconducting Ba(Ni_{1x}Co_{x})_{2}As_{2} Sangjun Lee, Gilberto De La Pena, Xiaolan Sun, Chris Eckberg, Daniel Campbell, Tyler Drye, Hyunsoo Kim, Peter Zavalij, Phil Piccoli, Jeffrey Lynn, Johnpierre Paglione, Peter Abbamonte Nickelpnictide compounds exhibit superconductivity at relatively low T_{c}, compared to isostructural ironpnictide highT_{c} superconductors. As an example, BaNi_{2}As_{2}, a nonmagnetic analogue of BaFe_{2}As_{2}, has T_{c} of 0.7 K and Cosubstitution raises it up to 2.5 K, whereas T_{c} of BaFe_{2}As_{2} can be as high as 38 K upon doping. This strong suppression of the superconductivity in BaNi_{2}As_{2} suggests the possible existence of competing phases. Here, we present the first observation of charge density wave (CDW) order in BaNi_{2}As_{2}. Using xray reciprocal space mapping, we observed an incommensurate CDW below 148 K in tetragonal phase and a commensurate CDW below 128 K in triclinic phase. On Cosubstitution, the onset of the CDW is suppressed to 70 K at x=0.07 and 0.08, and completely suppressed at x=0.12. We also observe that CDW order is strongly tied to the tetragonal to triclinic phase transition, implying a role of the structural distortion in stabilizing the electronic ordering. These results prove the existence of novel ordered phases in Nibased superconductors and support a competing order scenario for the coexistence of charge order and superconductivity in nickelpnictide materials. 
Monday, March 5, 2018 4:06PM  4:18PM 
C14.00007: Persistent shortrange stripetype magnetic order in overdoped Ba(Fe_{1x}Cu_{x})_{2}As_{2} (0.145<=x<=0.553) Weiyi Wang, Yu Song, Ding Hu, Yu Li, Rui Zhang, Leland Harriger, Wei Tian, Huibo Cao, Pengcheng Dai In the heavily Cudoped regime where longrange stripetype antiferromagnetic order in BaFe_{2}As_{2} is suppressed, Ba(Fe_{1x}Cu_{x})_{2}As_{2} (0.145≤x≤0.553) samples exhibit spinglasslike behavior in magnetic susceptibility and insulatinglike temperature dependence in electrical transport. Using elastic neutron scattering, we find stripetype shortrange magnetic order in the spinglass region identified by susceptibility measurements. The persistent stripe order in Ba(Fe_{1x}Cu_{x})_{2}As_{2} likely result from Cu acting effectively as vacancies, relieving magnetic frustration. The behaviors in Ba(Fe_{1x}Cu_{x})_{2}As_{2} are compared with those in NaFe_{1x}Cu_{x}As and Fe_{1x}Cu_{x}Te, suggesting a common tendency towards stripetype magnetic order in these materials. 
Monday, March 5, 2018 4:18PM  4:30PM 
C14.00008: Nematic fluctuations in NaFe_{1x}Ni_{x}As Yanhong Gu, Zhuang Xu, Huiqian Luo, Shiliang Li We report elastroresistivity measurements on single crystals of NaFe_{1−x}Ni_{x}As. The nematic susceptibility is defined as proportional to the change of resistivity under uniaxial pressure, whose temperature dependence can be well fitted by a CurieWeisslike function. Surprisingly, the sign of nematic susceptibility changes from positive to negative with just slight Ni doping, which is contrary to the expectation that electron doping should always lead to positive nematic susceptibility. Compared to our previous results on BaFe_{2x}Ni_{x}As_{2} system, although the meanfield nematic transition temperature from the CurieWeiss fitting of the nematic susceptibility becomes zero around optimal doping, our results do not support the presence of a nematic quantum critical point in this system. We will also provide discussions on the possible influence of nematic fluctuations on the zeropressure resistivity. 
Monday, March 5, 2018 4:30PM  4:42PM 
C14.00009: Quantum oscillations reveal the low temperature Fermi surfaces of FeSe1−xSx tuned by chemical and applied pressure across the nematic phase transition Amalia Coldea, Pascal Reiss, David Graf, Amir Haghighirad We present the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1−xSx using Shubnikovde Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. We will compare the effect of the chemical and applied hydrostatic pressure on the lowenergy electronic structure and discuss the role of Lifshitz transitions and the changes in electronic correlations across the nematic phase transition. Our results can shed light on the interplay between competing electronic orders in FeSe1−xSx which can be finely tuned by combing chemical and applied pressure. 
Monday, March 5, 2018 4:42PM  4:54PM 
C14.00010: Two Distinct Superconducting Pairing States Divided by the Nematic End Point in FeSe_{1x}S_{x} Tetsuo Hanaguri, Katsuya Iwaya, Yuhki Kohsaka, Tadashi Machida, Tatsuya Watashige, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda Nematicity in ironbased superconductors has attracted much attention but its relationship with superconductivity has been elusive. We have performed spectroscopicimaging scanning tunneling microscopy on FeSe_{1x}S_{x} to concurrently study evolutions of the nematic band structure and the superconducting gap. Sulfur substation systematically suppress nematicity that diminishes above the namatic end point at x=0.17. We have found that anisotropy of the inplane band structure diminishes with increasing x but there is little change in the band parameters at the namatic end point. Superconducting gap is hardly affected by sulfur substitution in the nematic phase but suddenly shrinks above the nematic end point. This means that there are two distinguished superconducting pairing states in FeSe_{1x}S_{x} depending on the presence or absence of nematicity. 
Monday, March 5, 2018 4:54PM  5:06PM 
C14.00011: Abrupt Change of the Superconducting Gap Structure at the Nematic Quantum Critical Point in FeSe_{1x}S_{x} Yuki Sato, Shigeru Kasahara, Tomoya Taniguchi, Xiangzhuo Xing, Yuichi Kasahara, Yoshifumi Tokiwa, Youichi Yamakawa, Hiroshi Kontani, Takasada Shibauchi, Yuji Matsuda FeSe offers an ideal platform to investigate the role of nematicity on the electron pairing interaction in ironbased superconductors. To reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specfic heat of FeSe_{1−x}S_{x}, in which the nematicity is suppressed by isoelectronic sulfur substitution and a nematic quantum critical point (NQCP) appears at x_{c} ≈ 0.17. In the whole nematic regime, we find twogap behavior; one is small but highly anisotropic with deep minima or line nodes, and the other is larger and more isotropic. In stark contrast, in the tetragonal regime, the larger gap becomes strongly anisotropic, demonstrating an abrupt change of the superconducting gap structure at the NQCP. As charge fluctuations of d_{xz} and d_{yz} orbitals are fundamentally different on the each side of NQCP, our observation directly implies that the orbital dependent nature of the nematic fluctuations has a strong impact on the superconducting gap structure and hence on the pairing interaction. 
Monday, March 5, 2018 5:06PM  5:18PM 
C14.00012: NonFermi Liquid Transport in AFe_{2}As_{2} (A=K,Rb,Cs) Tristin Metz, Makariy Tanatar, Erik Timmons, Xiangfeng Wang, T. A. Lograsso, Yong Liu, Ruslan Prozorov, Johnpierre Paglione We report milliKelvin heat and charge transport measurements in highquality single crystals of the heavily holedoped AFe_{2}As_{2} (A=K,Rb,Cs) superconductors as a function of magnetic field applied both perpendicular and parallel to the basal plane. The temperature dependence of both electrical and thermal resistivities exhibit nonFermiliquid behavior that is dependent on magnetic field. We compare results for all A species to each other, as well as previous results in which the nonFermiliquid temperature dependence of the resistivity was taken to indicate a fieldtuned quantum critical point at the superconducting upper critical field. 
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