We describe the design and development of a scanning tunneling microscope (STM) which can operate at temperatures down to 240 mK and in magnetic fields up to 7 T with high spatial and energy resolution The compact and rigid STM head is mounted directly on a low vibration, single shot, 3He refrigerator This refrigerator can be operated at its base temperature continuously for several days before the 3He needs to be recondensed The system is equipped with a sample transport manipulator from room temperature, and a cleavage device at low temperature, so that the cryogenic ultrahigh vacuum condition inside the cryostat can be utilized A superconducting magnet provides a magnetic field of up to 7 T at the sample along the STM tip direction Test results have shown that, at the base temperature, this instrument has better than 05 pm z-direction resolution in imaging mode, and better than 20 μV energy resolution in spectroscopy mode

/pdf/3he-refrigerator-based-very-low-temperature-scanning-4jjd7np7yg.pdf

3He refrigerator based very low temperature scanning tunneling microscope

Charge- and spin-density waves in existing superconductors: competition between Cooper pairing and Peierls or excitonic instabilities

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-Tc) superconductivity is nucleated in ‘stripes’ aligned along one of the axes1,2,3. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-Tc material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on ‘untwinned’ YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim4, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order5. The debate about stripes has therefore been reopened.

https://arxiv.org/pdf/cond-mat/0408379.pdf

Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x.

This review deals with the properties of superconductors with competing electron spectrum instabilities, namely, charge-density waves (CDWs) and spin-density waves (SDWs). The underlying reasons of the electron spectrum instability may be either Fermi surface nesting or the existence of Van Hove saddle points for lower dimensionalities. CDW superconductors include layered dichalcogenides, NbSe3, and compounds with the A15 and C15 structures among others. There is much evidence to show that high-Tc oxides may also belong to this group of materials. The SDW superconductors include URu2Si2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. We review the experimental evidence for CDW and SDW instabilities in a wide range of different superconductors, and assess the competition between these instabilities of the Fermi surface and the superconducting gap. Issues concerning the superconducting order parameter symmetry are also touched upon. The accent is put on establishing a universal framework for further theoretical discussions and experimental investigations based on an extensive list of available and up-to-date references.

http://iopscience.iop.org/article/10.1088/0953-2048/14/4/201/pdf

Charge- and spin-density-wave superconductors

We survey recent experimental results including quantum oscillations and complementary measurements probing the electronic structure of underdoped cuprates, and theoretical proposals to explain them. We discuss quantum oscillations measured at high magnetic fields in the underdoped cuprates that reveal a small Fermi surface section, comprising quasiparticles that obey Fermi-Dirac statistics, unaccompanied by other states of comparable thermodynamic mass at the Fermi level. The location of the observed Fermi surface section at the nodes is indicated by a body of evidence including the collapse in Fermi velocity measured by quantum oscillations, which is found to be associated with the nodal density of states observed in angular resolved photoemission, the persistence of quantum oscillations down to low fields in the vortex state, the small value of density of states from heat capacity and the multiple frequency quantum oscillation pattern consistent with nodal magnetic breakdown of bilayer-split pockets. A nodal Fermi surface pocket is further consistent with the observation of a density of states at the Fermi level concentrated at the nodes in photoemission experiments, and the antinodal pseudogap observed by photoemission, optical conductivity, nuclear magnetic resonance (NMR) Knight shift, as well as other complementary diffraction, transport and thermodynamic measurements. One of the possibilities considered is that the small Fermi surface pockets observed at high magnetic fields can be understood in terms of Fermi surface reconstruction by a form of small wavevector charge order, observed over long lengthscales in experiments such as NMR and x-ray scattering, potentially accompanied by an additional mechanism to gap the antinodal density of states.

https://iopscience.iop.org/article/10.1088/0034-4885/75/10/102501/pdf

Towards resolution of the Fermi surface in underdoped high-Tc superconductors.

Nuclear quadrupole resonance (NQR) measurements have been performed on Cu(2) plane sites and Cu(1) chain sites in fully doped YBa2Cu3O7 between 300 and 4.2 K. The sharp increase of the Cu(1) NQR linewidth across the superconducting transition and the T dependence of the Cu(1) spin lattice relaxation rate confirm the existence of a charge-density-wave state (CDW) in the chains. The simultaneous broadening of the Cu(2) linewidth below T(c) and the anomalous T dependence of Cu(1) and Cu(2) NQR parameters indicate that these in-chain CDW correlations are strongly involved in the appearance of an in-plane charge modulated structure below T(c).

In-plane charge modulation below T(c) and charge-density-wave correlations in the chain layer in YBa2Cu3O7

Evidence for Charge Instability in the CuO 3 Chains of PrBa 2 Cu 3 O 7 from 63,65 Cu NMR

Superconducting clusters and phase separation in Pr 1+x Ba 2-x Cu 3 O 7+δ : A 63,65 Cu nuclear quadrupole resonance and zero-field NMR study

Comment on "Low-temperature charge ordering in the superconducting state of YBa2Cu3O(7-delta)"

Spin and charge fluctuations are investigated along the three lattice directions on the Cu(1) site in a nonsuperconducting PrBa2Cu3O7 sample using 63,65Cu NMR/NQR. Our data give evidence for charge instability (CDW) involving charge fluctuations along the O(4)-Cu(l)-O(4) apical axis. In view of these findings, we reanalyzed the situation of chains and planes in YBa2Cu3O7 using Cu(1) and Cu(2) NQR. We confirm the important role of charge/lattice effects in these cuprates and we discuss the consequences of these mechanisms on the interaction between Cu-O chains and CuO2 planes.

Y. Berthier

Papers

In-plane charge modulation below T(c) and charge-density-wave correlations in the chain layer in YBa2Cu3O7

Evidence for Charge Instability in the CuO 3 Chains of PrBa 2 Cu 3 O 7 from 63,65 Cu NMR

Superconducting clusters and phase separation in Pr 1+x Ba 2-x Cu 3 O 7+δ : A 63,65 Cu nuclear quadrupole resonance and zero-field NMR study

Comment on "Low-temperature charge ordering in the superconducting state of YBa2Cu3O(7-delta)"

Charge and Spin Dynamics of Cu-O Chains in REBa2Cu3O7 Cuprates (RE=Pr, Y): An NMR/NQR Study