We present measurements of longitudinal ultrasonic velocity on single crystals of the heavy-fermion superconductor UPt3. The measurements show clear signatures of second-order phase transitions in the superconducting state, with the velocity anomalies well accounted for by Ginzburg-Landau theory. From these signatures we construct a phase diagram for UPt 3 that reveals all the boundary lines that have been identified as possible phase transitions. We are able to track the phase transition lines to a tetracritical point, located on the upper-critical-field curve, to within the width of the normalsuperconducting transition.PACS numbers: 74.30. Gn, 62.80.+f, 74.70.Tx This Letter presents the first complete phase diagram in the field-temperature plane for the heavy-fermion superconductor UPt3 derived from a single measurement technique. UPt3 has shown a variety of interesting properties in the superconducting state which strongly suggest unconventional superconductivity with a multicomponent order parameter. Power-law temperature dependences of ultrasonic attenuation 1 " 3 and heat capacity 4 have been interpreted in terms of a gap with line or point nodes on the Fermi surface. More convincingly, a series of experiments have revealed the presence of more than one superconducting phase: Initially, ultrasonic-attenuation measurements showed the presence of two superconducting phases depending on the magnetic field; 5 " 7 subsequently, heat-capacity measurements demonstrated the presence of multiple superconducting phases even in zero field. 8,9 A composite phase diagram assembled from different measurements (ultrasonic-attenuation, 5 heat-capacity, 9 and torsional-oscillator data 10 ) on different samples indicates that UPt3 has at least two and possibly three superconducting phases. However, this interpretation relies on data from a variety of measurement techniques on samples with transition temperatures ranging from 420 to 540 mK.Here we present measurements of ultrasonic velocity on two samples of UPt3. Our velocity measurements yield a more complete phase diagram for superconducting UPt3 than has previously been available. Earlier attenuation measurements on one of the samples 7 (sample No. 1) revealed a peak in the attenuation (the 7/FL peak) in field sweeps at constant temperature, as well as a peak (the X peak) in temperature sweeps in low field. From these attenuation studies it was not possible to follow Hf^(T) very close to the upper-critical-field line HdiT).Thus, it was not possible to answer with confidence the basic question of how many phases there are in UPt3. Theoretical proposals for the phase diagram of UPt 3 have been published by several authors. 11 ' 13 Our results bear directly on these theories:The phase diagrams obtained by a single measuring technique on both of the samples studied give strong support for a tetracritical point intersecting the uppercritical-field line, for fields both parallel and perpendicular to the c axis. In addition, the velocity anomalies at both zero-field transitio...

We describe the design, development and performance of a scanning probe microscopy (SPM) facility operating at a base temperature of 10 mK in magnetic fields up to 15 T. The microscope is cooled by a custom designed, fully ultra-high vacuum (UHV) compatible dilution refrigerator (DR) and is capable of in situ tip and sample exchange. Subpicometer stability at the tip-sample junction is achieved through three independent vibration isolation stages and careful design of the dilution refrigerator. The system can be connected to, or disconnected from, a network of interconnected auxiliary UHV chambers, which include growth chambers for metal and semiconductor samples, a field-ion microscope for tip characterization, and a fully independent additional quick access low temperature scanning tunneling microscope (STM) and atomic force microscope (AFM) system. To characterize the system, we present the cooling performance of the DR, vibrational, tunneling current, and tip-sample displacement noise measurements. In addition, we show the spectral resolution capabilities with tunneling spectroscopy results obtained on an epitaxial graphene sample resolving the quantum Landau levels in a magnetic field, including the sublevels corresponding to the lifting of the electron spin and valley degeneracies.