Superconductivity. Tuning order in cuprate superconductors

Charge order driven by multiple-Q spin fluctuations in heavily electron-doped iron selenide superconductors

Intertwined spin and charge orders have been widely studied in high-temperature superconductors, since their fluctuations may facilitate electron pairing; however, they are rarely identified in heavily electron-doped iron selenides. Here, using scanning tunneling microscopy, we show that when the superconductivity of (Li_0.84Fe_0.16OH)Fe_1-xSe is suppressed by introducing Fe-site defects, a short-ranged checkerboard charge order emerges, propagating along the Fe-Fe directions with an approximately 2a_Fe period. It persists throughout the whole phase space tuned by Fe-site defect density, from a defect-pinned local pattern in optimally doped samples to an extended order in samples with lower T_c or non-superconducting. Intriguingly, our simulations indicate that the charge order is likely driven by multiple-Q spin density waves originating from the spin fluctuations observed by inelastic neutron scattering. Our study proves the presence of a competing order in heavily electron-doped iron selenides, and demonstrates the potential of charge order as a tool to detect spin fluctuations.

Spin-density wave near the vortex cores in the high-temperature superconductor Bi2Sr2CaCu2O8+y

Competition with magnetism is at the heart of high-temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe based upon NMR spin-lattice-relaxation spectroscopy. With this approach we have found a spin-density wave associated with the vortex core in Bi(2)Sr(2)CaCu(2)O(8+y), similar to checkerboard patterns in the local density of electronic states reported from scanning tunneling microscope experiments. We have determined both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30  T.

Towards a two-dimensional superconducting state of La(2-x)Sr(x)CuO4 in a moderate external magnetic field

We report a novel aspect of the competition and coexistence between magnetism and superconductivity in the high-T(c) cuprate La(2-x)Sr(x)CuO4 (La214). With a modest magnetic field applied H parallel c axis, we monitored the infrared signature of pair tunneling between the CuO2 planes and discovered the complete suppression of interlayer coupling in a series of underdoped La214 single crystals. We find that the in-plane superconducting properties remain intact, in spite of enhanced magnetism in the planes.

Field dependence of the ground state in the exotic superconductor CeCoIn5: a nuclear magnetic resonance investigation

We report 115In nuclear magnetic resonance (NMR) measurements in CeCoIn5 at low temperature (T approximately 70 mK) as a function of the magnetic field (H0) from 2 to 13.5 T applied perpendicular to the c axis. A NMR line shift reveals that below 10 T the spin susceptibility increases as sqrt[H0]. We associate this with an increase of the density of states due to the Zeeman and Doppler-shifted quasiparticles extended outside the vortex cores in a d-wave superconductor. Above 10 T a new superconducting state is stabilized, possibly the modulated phase predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. This phase is clearly identified by a strong and linear increase of the NMR shift with the field, before a jump at the first order transition to the normal state.

Emergence of Fermi pockets in a new excitonic charge-density-wave melted superconductor

A superconducting state (T(c) approximately 4.2 K) has very recently been observed upon successful doping of the charge-density-wave (CDW) ordered triangular lattice TiSe(2), with copper. Using state-of-the-art photoemission spectroscopy we identify, for the first time, momentum-space locations of doped electrons that form the Fermi sea of the superconductor. With doping, we find that kinematic nesting volume increases, whereas coherence of the CDW collective order sharply drops. In superconducting doping, as chemical potential rises, we observe the emergence of a large density of states in the form of a narrow electron pocket near the L point of the Brillouin zone with d-like character. The k-space spectral evolution directly demonstrates, for the first time, that the CDW order parameter microscopically competes with superconductivity in the same band.

Magic doping fractions for high-temperature superconductors

We report hole-doping dependence of the in-plane resistivity rho(ab) in a cuprate superconductor La(2-x)Sr(x)CuO4, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole-doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m+1)/2n, with integers m and n.

Delocalized fermions in underdoped cuprate superconductors

Low-temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly doped cuprate YBa2Cu3O6.33. Samples were measured with doping concentrations on either side of the superconducting phase boundary. We report the observation of delocalized fermionic excitations at zero energy in the nonsuperconducting state, which shows that the ground state of underdoped cuprates is a thermal metal. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e., nodal. The insulating ground state observed in underdoped La2-xSrxCuO4 is attributed to the competing spin-density-wave order.

Broken particle-hole symmetry at atomically flat a-axis YBa2Cu3O7-delta interfaces

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa(2)Cu(3)O(7-delta) and DyBa(2)Cu(3)O(7-delta) through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor-insulator interface. Below T(c), the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero-bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.

Quantum phase transitions in the cuprate superconductor Bi2Sr2-xLaxCuO6+delta

To elucidate a quantum phase transition (QPT) in Bi(2)Sr(2-x)La(x)CuO(6+delta), we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity kappa(ab), and the magnetic-field dependence of kappa(ab). It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.

Localization and interaction effects in strongly underdoped La2-xSrxCuO4

The in-plane magnetoresistance (MR) in La(2-x)SrxCuO4 films with 0.03< x <0.05 has been studied in the temperature range 1.6 to 100 K, and in magnetic fields up to 14 T, parallel and perpendicular to the CuO2 planes. The behavior of the MR is consistent with a predominant influence of interaction effects at high temperatures, switching gradually to a regime dominated by spin scattering at low T. Weak localization effects are absent. A positive orbital MR appears close to the boundary between the antiferromagnetic and the spin-glass phase, suggesting the onset of Maki-Thompson superconducting fluctuations deep inside the insulating phase.

Quantitative test of SO(5) symmetry in the vortex state of Nd(1.85)Ce(0.15)CuO4

By numerically solving models with competing superconducting and antiferromagnetic orders, we study the magnetic field dependence of the antiferromagnetic moment in both the weak and strong field regimes. Through a comparison with the neutron scattering results of Kang et al. and Matsuura et al. on Nd(1.85)Ce(0.15)CuO4, we conclude that this system is close to a SO(5) symmetric critical point. We also make a quantitative prediction on increasing the upper critical field B(c2) and the superconducting transition temperature T(c) by applying an in-plane magnetic field.

Antiferromagnetic order as the competing ground state in electron-doped Nd1.85Ce0.15CuO4

Superconductivity in the high-transition-temperature (high-T(c)) copper oxides competes with other possible ground states. The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-T(c) materials. Antiferromagnetism has been theoretically predicted to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields. None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field B(c2). Here we report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4 (refs 13, 14), where B(c2) can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at B(c2). This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-T(c) copper oxide materials, irrespective of electron or hole doping.

Beliaev damping and Kelvin mode spectroscopy of a Bose-Einstein condensate in the presence of a vortex line

It is demonstrated theoretically that the counter-rotating quadrupole mode in a vortex of Bose-Einstein condensates can decay into a pair of Kelvin modes via the Beliaev process. We calculate the spectral weight of a density-response function within the Bogoliubov framework, taking account of both Beliaev and Landau processes. Good agreement with experiment on 87 Rb by Bretin et al. [Phys. Rev. Lett., 100403 (2003)]] allows us to unambiguously identify the decayed mode as the Kelvin wave propagating along a vortex line.

Magnetic-field-induced localization of quasiparticles in underdoped La(2-x)SrxCuO4 single crystals

Magnetic-field-induced ordering of electrons around vortices is a striking phenomenon recently found in high-T(c) cuprates. To identify its consequence in the quasiparticle dynamics, the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa of La(2-x)SrxCuO4 crystals is studied for a wide doping range. It is found that the behavior of kappa(H) in the subkelvin region changes drastically across optimum doping, and the data for underdoped samples are indicative of unusual magnetic-field-induced localization of quasiparticles; this localization phenomenon is probably responsible for the unusual "insulating normal state" under high magnetic fields.

Antiferromagnetism and hole pair checkerboard in the vortex state of high T(c) superconductors

We propose a microscopic state for the vortex phase of BSCO superconductors. Around the vortex core or above H(c2), the d wave hole pairs form a checkerboard localized in the antiferromagnetic background. We discuss this theory in connection with recent STM experiments.