Spin-Polarized Scanning Tunneling Microscopy


Ilija Zeljkovic, Can-Li Song, Dennis Huang


Cuprate superconductors

Structural distortion in Bi-based cuprates

A complicating factor in unraveling the theory of high-temperature (high-Tc) superconductivity is the presence of a pseudogap in the density of states, whose origin has been debated since its discovery. Some believe the pseudogap is a broken symmetry state distinct from superconductivity, while others believe it arises from short-range correlations without symmetry breaking. A number of broken symmetries have been imaged and identified with the pseudogap state, but it remains crucial to disentangle any electronic symmetry breaking from pre-existing structural symmetry of the crystal. We use scanning tunneling microscopy (STM) to observe an orthorhombic structural distortion across the cuprate superconducting Bi2Sr2Can-1CunO2n+4+x (BSCCO) family tree, which breaks two-dimensional inversion symmetry in the surface BiO layer. Although this inversion symmetry breaking structure can impact electronic measurements, we show from its insensitivity to temperature, magnetic field, and doping, that it cannot be the long-sought pseudogap state. To detect this picometer-scale variation in lattice structure, we have implemented a new algorithm to create high resolution images of the average supercell structure, which can be generally useful for future STM studies.

structural distortion in BSCCO
Fig. 1: STM imaging of the orthorhombic structural distortion in Bi2+ySr2-yCaCu2O8+x (Bi-2212). (a) Topographic image of 30 nm square region of the BiO lattice from overdoped Tc = 80K Bi-2212 with supermodulation. (b) Fourier transform of (a), showing peaks at the tetragonal Bragg vectors Qx and Qy, incommensurate peak at Qsm corresponding to supermodulation, as well as at the orthorhombic Qa=(Qx+Qy)/2 (but not at the equivalent Qb). (c) We have mapped the structural distortion throughout the Bi-2201 phase diagram, inside and outside the superconducting state, and at a wide range of pseudogap energies, at all the red points shown. We observe no dependence of the orthorhombic distortion on temperature, doping, or magnetic field.

Past Researchers

Adam Pivonka, Liz Main


Hiroshi Ikuta, Nagoya University (Bi2+ySr2-yCuO6)
Genda Gu, Brookhaven National Lab (Bi2+ySr2-yCaCu2O8+x)
Eric Hudson, Pennsylvania State University

Oxygen dopants in Bi2+ySr2-yCaCu2O8+x

High-Tc cuprate superconductors display startling nanoscale inhomogeneity in essential properties such as pseudogap energy, Fermi surface, and even superconducting critical temperature. The direct cause of this inhomogeneity has remained mysterious; theoretical explanations have ranged from chemical disorder to spontaneous electronic phase separation. We extend the energy range of scanning tunneling spectroscopy on Bi2+ySr2-yCaCu2O8+x, allowing the first complete mapping of two types of interstitial oxygen dopants and vacancies at the apical oxygen site. We show that nanoscale spatial variations in electronic ordered states are governed by disorder in dopant concentrations, particularly vacancies at the apical oxygen site.

Oxygen dopants in BSCCO
Fig. 2: Imaging apical oxygen vacancies in Bi2+ySr2-yCaCu2O8+x with Tc=55K. (a) Topograph acquired at Vsample=+1V and Iset=150pA over a 35nm area (inset: 3x magnification). (b) dI/dV image acquired at +1V bias over the same areas as in (a), showing atomic-scale features we have identified as apical oxygen vacancies. (c) Apical oxygen vacancies superimposed as light-blue circles on top of a pseudogap map. (d) Average pseudogap as a function of distance from the nearest dopant of each type.


Genda Gu, Brookhaven National Lab


We use scanning tunneling microscopy to investigate the electron-doped superconductor Ca1-xPrxFe2As2 with Tc up to 49K. The low-temperature cleaved surface shows two different morphologies, one web-like and one with clean 2x1 reconstruction. We study the local effect of the Pr dopants on the superconducting gap. We also investigate the scattering mechanisms as a function of temperature and field, by studying trends in the zero bias conductance, and the quasiparticle interference images.

Fig. 3: STM topographies of 18nm field of view of (a) 2x1, and (b) web-like surface reconstructions observed in cold-cleaved Ca1-xPrxFe2As2 with Tc=44K.


Paul Chu & Bing Lv, University of Houston


We have constructed a custom variable temperature scanning tunneling microscope with the following features optimized for spin-polarized tunneling:

STM body

labeled photo of SP-STM

Fig. 4: Diagram and photo of microscope.
Flow Cryostat

labeled photo of flow cryostat

Fig. 5: Diagram and photo of cryostat.
Vacuum Chamber

labeled photo of vacuum chamber

Fig. 6: Diagram and photo of vacuum chamber.

Spin-polarized tunneling

We use a bulk Cr tip, sharpened by the process of field emission on a Cr single crystal taget, to detect a spin-signal in a material known to be a layered antiferromagnet such as La1-xSrxMnO3 (LSMO) with x~0.3 doping. In figure below, we show the difference in consecutive step heights measured by STM, demonstarting the spin-sensitivity of our tip.
LSMO step edges
Fig. 7: (a) Topograph of step-edges on LSMO surface over a 30nm area acquired at 15pA and -250mV bias. (b) Line-cut through the red line drawn in panel (a) across the stepedges, showing the apparent height differences due to the spin-polarized layers.


John Mitchell, Argonne National Lab


NSF CAREER DMR-0847433, AFOSR PECASE FA9550-06-1-0531, AFOSR DURIP FA9550-06-1-0359