Session C14: Fe-based Superconductors -- Quantum Criticality and Quantum Phase Transitions

Scale-Invariant transport near quantum critical point in high-temperature superconductors.

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 zero-temperature collapse of a line of phase transitions inside superconducting dome. Then I will discuss high-magnetic-field 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 linear-in-field resistivity at very high magnetic fields (up to 93T) which exhibits a characteristic temperature-field competition as field is lowered. Such scale-invariant behavior of magneto-transport in a broad range of temperatures and magnetic fields is incompatible with quasiparticle transport in the strange metal state.   

The Hall Efffect at Ultra High Fields in Ba-122

To this day, the basic charge transport phenomenology of the high-Tc cuprates, including the T-linear resistivity near optimal doping and the T-dependent 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 high-Tc superconductivity in general, and gives new urgency to the problem. I will report Hall effect measurements up to 65 Tesla on samples of the iron-pnictide superconductor BaFe2(As1-xPx)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.   

Quantum critical nematic fluctuations and spin excitation anisotropy in the iron pnictides

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 Ginzburg-Landau theory involving spin and Ising-nematic degrees of freedom [1,2] in the presence of a small external C4-symmetry-breaking 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 Ni-doped BaFe₂As₂ under a uniaxial strain.

[1] J. Dai, Q. Si, J. X. Zhu, and E. Abrahams, PNAS 106, 4118, (2009).
[2] J. Wu, Q. Si and E. Abrahams, Phys. Rev. B 93, 104515 (2016).
[3] Y. Song, X. Lu, D. L. Abernathy, D. W. Tam, J. L. Niedziela, W. Tian, H. Luo, Q. Si, and P. Dai, Phys. Rev. B 92, 180504 (2015).
  

Magnetic and nematic quantum bicritical point in Ba(Fe0.97Cr0.03)2(As1-xPx)2

The disappearance of long-range 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 iron-based 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(As1-xPx)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 V-shaped region of T-linear resistance, which suggests the position of the QCP significantly shifts from that in the superconducting system.   

Lower critical field across the superconducting dome of Ba(Fe1-xCox)2As2 measured using optical NV-magnetometer

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 Nitrogen-Vacancy (NV) centers in diamond as a non-invasive, sensitive optical magnetometer to precisely measure H_c1 in single crystals of Ba(Fe_1-xCo_x)₂As₂ 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 P-doped BaFe₂As₂.   

Observation of Charge Density Waves in Superconducting Ba(Ni1-xCox)2As2

Nickel-pnictide compounds exhibit superconductivity at relatively low T_c, compared to isostructural iron-pnictide high-T_c superconductors. As an example, BaNi₂As₂, a non-magnetic analogue of BaFe₂As₂, has T_c of 0.7 K and Co-substitution raises it up to 2.5 K, whereas T_c of BaFe₂As₂ can be as high as 38 K upon doping. This strong suppression of the superconductivity in BaNi₂As₂ suggests the possible existence of competing phases. Here, we present the first observation of charge density wave (CDW) order in BaNi₂As₂. Using x-ray 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 Co-substitution, 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 Ni-based superconductors and support a competing order scenario for the coexistence of charge order and superconductivity in nickel-pnictide materials.   

Persistent short-range stripe-type magnetic order in overdoped Ba(Fe1-xCux)2As2 (0.145<=x<=0.553)

In the heavily Cu-doped regime where long-range stripe-type antiferromagnetic order in BaFe₂As₂ is suppressed, Ba(Fe_1-xCu_x)₂As₂ (0.145≤x≤0.553) samples exhibit spin-glass-like behavior in magnetic susceptibility and insulating-like temperature dependence in electrical transport. Using elastic neutron scattering, we find stripe-type short-range magnetic order in the spin-glass region identified by susceptibility measurements. The persistent stripe order in Ba(Fe_1-xCu_x)₂As₂ likely result from Cu acting effectively as vacancies, relieving magnetic frustration. The behaviors in Ba(Fe_1-xCu_x)₂As₂ are compared with those in NaFe_1-xCu_xAs and Fe_1-xCu_xTe, suggesting a common tendency towards stripe-type magnetic order in these materials.   

Nematic fluctuations in NaFe1-xNixAs

We report elastroresistivity measurements on single crystals of NaFe_1−xNi_xAs. The nematic susceptibility is defined as proportional to the change of resistivity under uniaxial pressure, whose temperature dependence can be well fitted by a Curie-Weiss-like 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_2-xNi_xAs₂ system, although the mean-field nematic transition temperature from the Curie-Weiss 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 zero-pressure resistivity.   

Quantum oscillations reveal the low temperature Fermi surfaces of FeSe1−xSx tuned by chemical and applied pressure across the nematic phase transition

We present the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1−xSx using Shubnikov-de 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 low-energy 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.   

Two Distinct Superconducting Pairing States Divided by the Nematic End Point in FeSe1-xSx

Nematicity in iron-based superconductors has attracted much attention but its relationship with superconductivity has been elusive. We have performed spectroscopic-imaging scanning tunneling microscopy on FeSe_1-xS_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 in-plane 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_1-xS_x depending on the presence or absence of nematicity.   

Abrupt Change of the Superconducting Gap Structure at the Nematic Quantum Critical Point in FeSe1-xSx

FeSe offers an ideal platform to investigate the role of nematicity on the electron pairing interaction in iron-based superconductors. To reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specfic heat of FeSe_1−xS_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 two-gap 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.   

Non-Fermi Liquid Transport in AFe2As2 (A=K,Rb,Cs)

We report milliKelvin heat and charge transport measurements in high-quality single crystals of the heavily hole-doped AFe₂As₂ (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 non-Fermi-liquid 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 non-Fermi-liquid temperature dependence of the resistivity was taken to indicate a field-tuned quantum critical point at the superconducting upper critical field.