Lower Critical Field and Meissner Effect in UTe2

We report on low temperature susceptibility and magnetization measurements made on single
crystals of the recently discovered heavy-fermion superconductor UTe2 and compare the results with
the two ambient pressure ferromagnetic superconductors URhGe and UCoGe. Hysteresis curves in
the superconducting phase show a familiar diamond shape superimposed on a large paramagnetic
background. The Meissner state was measured by zero field cooling in small fields of a few Oe as
well as ac susceptibility measurements in small fields and resulted in 100% shielding, with a sharp
transition at 1.5K. However the field cooling Meissner-Ochsenfeld effect (expulsion of ux) was
negligible in fields greater than just a few Oe, but becomes nearly 30% of the perfect diamagnetic
signal when field was reduced to 0.01 Oe. The critical current due to
ux pinning was studied by ac
susceptibility techniques . The lower critical field Hc1 was found to be approximately 23 and 42 Oe
along the a and b axis, respectively. The upper critical field Hc2 along the a axis was measured
using bulk ac susceptibility measurements and found to be Hc2= 5.4 T in good agree with published
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resistivity measurements. Over the range in fields and temperature of this study, no signature of a
ferromagnetic transition could be discerned, certainly not above Tc, nor below Tc where one might
expect, for example, a break in the curves of Hc1 or Hc2 vs temperature to occur if a ferromagnetic
transition took place.