M. Enayat, U.R. Singh, S. White and P. Wahl
Heavy fermion compounds are now known for almost 40 years, and still many of their prop-erties are only poorly understood. Among the most spectacular surprises encountered in these materials is the emergence of superconductiv-ity out of magnetically ordered phases. Though this happens often at low transition tempera-tures around 1 K and below, magnetic impuri-ties in conventional superconductors act as pair-breaking scatterers and suppress superconduc-tivity.
The investigation of the electronic structure of heavy fermion materials requires an extremely high energy resolution to be able to track the dispersion of the heavy electron bands and of-ten very low temperatures below 1 K to enter the interesting regimes of the phase diagram. Fur-thermore, the electronic structure is not inher-ently symmetric with respect to the Fermi en-ergy, as is for example in the case of many su-perconductors for energies sufficiently close to the Fermi energy. Therefore, a characterization
both in the occupied as well as the unoccupied states is a necessity to obtain a full picture of the electronic structure. The rich physics of the heavy fermion materials due to the interaction of delocalized conduction electrons with local-ized spins makes these systems ideal candidates for studies by spectroscopic imaging STM.
Spectroscopic Imaging STM potentially offers insight into the electronic structure via a char-acterization of quasi-particle interference pat-terns, which has recently been demonstrated for the first time for a heavy fermion mate-rial, URu2Si2, in studies performed at temper-atures down to 1.7 K [1,2]. The quasi-particle interference patterns, which could be detected in thorium-doped URu2Si2 revealed how the heavy electron bands interact with the hidden-order phase.
Most heavy fermion materials exhibit very rich low-temperature phase diagrams, showing
su-perconductivity in close vicinity of or even co-existing with magnetic phases. To explore the physics of heavy fermion materials, we have constructed a spectroscopic imaging STM for operation at temperatures below 100 mK, oper-ating in magnetic fields up to 14 T. First mea-surement results are shown. Besides the mate-rial properties specifically in the case of heavy fermion compounds emerging only at tempera-tures well below 1 K, the additional benefit of operating an STM at these temperatures is an improved energy resolution. The energy resolu-tion of tunneling spectroscopy depends on the temperature of the STM tip.
Our setup comprises a low-temperature scan-ning tunneling microscope (STM) operating at temperatures below 10 mK optimized for spec-troscopic mapping over extended periods of up to one week. Samples are cleaved in cryogenic vacuum in a home-built cryogenic cleaving mechanism and immediately transferred into
Figure 51: (a) Topography of a cleaved NbSe2surface measured at a Mixing Chamber temperature of 9 mK (raw data). Besides the surface atomic structure, a modulation due to formation of a charge density wave can be seen, (b) line cut along an atomic row, it can be seen that the residual noise is below 5 pm. (c) Tunneling spectrum showing the superconducting gap of NbSe2measured atTMXC= 9 mK.
Figure 52: Tunneling Spectroscopy of a gold-aluminum junction at a temperature of 9 mK. (a) Tunneling spectroscopy as a function of applied magnetic field. The superconducting gap disappears at magnetic fields on the order of 30 mT. (b) Tunneling spectra as a function of lock-in modulation. (c) Comparison of a spec-trum calculated for an electronic temperature of 150 mK to a measured specspec-trum. (d) Tunneling spectroscopy at different conductances down to point contact.
the STM head. To prevent thermal radiation from heating the STM head, the sample trans-fer is performed through two radiation shields which can be opened by a custom designed mechanism, the coldest one is at 1 K. Here we present first measurement results, showing topographic images obtained on NbSe2 at a Mixing chamber temperature of 9 mK as well as tunneling spectroscopy of a gold-aluminum junction to demonstrate the energy resolution.
Figure 51 shows a topography of a NbSe2 sur-face revealing atomic resolution as well as the known charge density wave order [3]. From line cuts, the vertical stability of the instrument can be estimated, which is better than 5 pm.
For testing the energy resolution of the instru-ment, we have measured an aluminum-gold tunneling junction. The energy resolution is limited by the width of the Fermi edge due to the finite temperature of the experiment, but contains also contributions due to electronic and RF noise.
Aluminum becomes superconducting below 1.2 K and its properties in the superconducting phase are well-described by BCS theory. From a comparison of the calculated gap structure with measured spectra, the extrinsic broadening can be determined. Figure 52 displays tunnel-ing spectra acquired at the base temperature of the instrument (below 10 mK) at various mag-netic fields. The coherence peaks and the
super-conducting gap can be clearly seen, the gap dis-appears for magnetic fields of≈30 mT, signifi-cantly larger than the critical fieldHC≈10 mT of bulk aluminum. We also find that the conduc-tance inside the gap comes close to zero. There is a small residual conductance inside the tun-neling gap, which can be either due to a detuned phase of the lock-in amplifier which is used to measure the conductance or due to a gold coat-ing of our aluminum tip, which would be super-conducting due to the proximity effect. In this case, the conductance inside the gap would also remain finite [4]. Comparison with the density of states as calculated from BCS theory shows that the resolution of our instrument currently is on the order of 30µV.
While the energy resolution of our instrument is not yet pushed to its ultimate limit, namely
the thermal broadening at 10 mK, it is already sufficient to perform tunneling spectroscopy of unconventional superconductors and heavy fermion compounds.
References:
[1] Schmidt, A.R., M.H. Hamidian, P. Wahl, F. Meier, A.V. Balatsky, T. Williams, G.M. Luke and J.C. Davis.
Nature465, 570–576 (2010).
[2] Aynajian, P., E.H. Da Silva Neto, C.V. Parker Y. Huang, A. Pasupathy, J. Mydosh and A. Yazdani.
Proceedings of the National Academy of Sciences of the United States of America107, 10383–10388 (2010).
[3] Giambattista, B., A. Johnson, R.V. Coleman, B. Drake and P.K. Hansma.Physical Review B37, 2741–2744 (1988).
[4] Belzig, W., C. Bruder and G. Sch¨on.Physical Review B54, 9443–9448 (1996).