We describe the construction and performance of a scanning tunneling microscope (STM) capable of taking maps of the tunneling density of states with sub-atomic spatial resolution at dilution refrigerator temperatures and high (14 T) magnetic fields. The fully ultra-high vacuum system features visual access to a two-sample microscope stage at the end of a bottom-loading dilution refrigerator, which facilitates the transfer of in situ prepared tips and samples. The two-sample stage enables location of the best area of the sample under study and extends the experiment lifetime. The successful thermal anchoring of the microscope, described in detail, is confirmed through a base temperature reading of 20 mK, along with a measured electron temperature of 250 mK. Atomically-resolved images, along with complementary vibration measurements, are presented to confirm the effectiveness of the vibration isolation scheme in this instrument. Finally, we demonstrate that the microscope is capable of the same level of performance as typical machines with more modest refrigeration by measuring spectroscopic maps at base temperature both at zero field and in an applied magnetic field.PACS numbers: 07.79.Cz,07.79.Fc Scanning tunneling microscopy (STM), since its development almost 30 years ago, has become a powerful technique in condensed matter physics, providing not only structural information about surfaces, but also spectroscopic measurements of the electronic density of states at the atomic length scale. However, most instruments operate at temperatures above 1 K, limiting access to exotic electronic phases and quantum effects expected at lower temperatures, which are studied as a matter of routine by other techniques. Generally, very little spectroscopic information about the electronic density of states is known at dilution refrigerator temperatures, usually being limited to what can be learned using either point contact spectroscopy or planar tunnel junctions. Moreover, STM can make such measurements on the atomic length scale, allowing it to probe systems, such as single spins and atomic chains, which are not directly accessible any other way.While the integration of STM with a dilution refrigerator can be conceptually reduced to simply attaching the microscope to the end of a mixing chamber in lieu of some other cryogenic refrigerator, the technical requirements for sub-Angstrom positioning of an STM tip above an atomically clean surface are often at odds with those for cooling a sample to milli-Kelvin temperatures. For example, when attaching the microscope to the refrigerator, the former would favor the use of a soft mechanical joint using springs, which would isolate vibrations, while a) Electronic mail: yazdani@princeton.edu the latter would favor the use of a rigid mechanical joint with a metal rod, which would provide a strong thermal contact. Nevertheless, a number of STM instruments have been developed that cool the sample using a dilution refrigerator 1-8 . However, among these, few feature ultra-high vacuum (UHV...

We have investigated the superfluid transition of 3 He in different samples of silica aerogel. By comparing new measurements on a 99.5% sample with previous observations on the behaviour of 3 He in 98% porous aerogel we have found evidence for universal behaviour of 3 He in aerogel. We relate both the transition temperature and superfluid density to the correlation length of the aerogel.The properties of bulk 3 He are well understood. The extreme purity of 3 He at low temperatures makes it an ideal system to study the agreement between theoretical and experimental results on non-conventional Cooper pairing in the absence of disorder. Disorder plays a crucial role in suppressing the pairing interaction in high T c superconductors, the other well established non swave paired system. The superfluid transition of 3 He confined to a sample of very porous silica aerogel was first reported four years ago [1,2]. The aerogel provides a structural disorder background [4] to the liquid. 3 He is compressible, and the density can be continuously tuned by ≈30% while maintaining a fixed disorder. The 3 He zero temperature coherence length ξ 0 , defined as ξ 0 =hv f /k B T c , varies from 180Å to over 700Å as a function of density. Because the Cooper pairs in 3 He form in a p-wave state, quasiparticle scattering from the aerogel strands is pairbreaking [3]. Thus the 3 He-in-aerogel system is well suited to the exploration of the effect of impurity scattering and disorder on the superfluid transition and phase diagram.The superfluidity of 3 He in silica aerogel has been studied using torsional oscillators [1,[4][5][6], NMR [2,6-9] and sound propagation [10,11] techniques. These measurements show that both the superfluid transition temperature (T c ) and superfluid density (ρ s ) of the 3 He are suppressed by the disorder, but that the transition remains sharp [1]. This suppression is sensitive to both the density and the microstructure of the aerogel sample.The simplest model for the effect of impurity scattering on the 3 He superfluid transition is the homogeneous scattering model (HSM) which is based on the AbrikosovGorkov model for a superconductor with magnetic impurities that induce pair-breaking via spinflip scattering [12]. This mechanism is similar to that of diffuse scattering of Cooper paired 3 He from a surface [13], and is unable to explain the observed behaviour. Specifically, the observed suppression of the superfluid density is much greater than predicted by this model. More sophisticated models, such as the isotropic inhomogeneous scattering model (IISM) [14][15][16] proposed by Thuneberg and coworkers are able to quantitatively predict the superfluid transition temperature of 3 He in aerogel (for small suppressions) and have had success at qualitatively explaining the observed superfluid densities.In this Letter we present data from several different experiments on 3 He in aerogel, including new results on 3 He confined to a 99.5% porosity sample. This sample is a factor of four more dilute than any previously i...

We describe the design, construction, and performance of an ultra-high vacuum (UHV) scanning tunneling microscope (STM) capable of imaging at dilution-refrigerator temperatures and equipped with a vector magnet. The primary objective of our design is to achieve a high level of modularity by partitioning the STM system into a set of easily separable, interchangeable components. This naturally segregates the UHV needs of STM instrumentation from the typically non-UHV construction of a dilution refrigerator, facilitating the usage of non-UHV materials while maintaining a fully bakeable UHV chamber that houses the STM. The modular design also permits speedy removal of the microscope head from the rest of the system, allowing for repairs, modifications, and even replacement of the entire microscope head to be made at any time without warming the cryostat or compromising the vacuum. Without using cryogenic filters, we measured an electron temperature of 184 mK on a superconducting Al(100) single crystal.

No abstract

No abstract