Effect of uranium defect in unconventional superconductor UTe2

Y. Haga, P. Opletal, Y. Tokiwa, E. Yamamoto, Y. Tokunaga, S. Kambe and H. Sakai1) : Research Gr. for material physics of heavy elements

Superconductivity is a fairly common phenomenon occurringin metals at low temperatures. However, in some class of metals,
the superconducting properties look different from ordinary
superconductors. Examples of such superconductors include theso-called high-temperature cupurates where superconductivity
takes place above 100 K. On the other hand, actinide andlanthanide
compoundsoftenexhibit
unconventional
superconductivity. Unlike ordinary metals, conduction electronsin actinide or lanthanide compounds often acquire large
effective mass originated from strong electron correlation. As aresult, superconductivity occurring in these compounds is called“heavy fermion” superconductivity.
UTe2, discovered in 2019, is the most recent member ofheavy fermion superconductors [1]. Its superconductivity arises
from enhanced electron mass due to many-body interaction
among conduction carriers. Crystal structure of UTe2 is shownin Fig. 1(a). Uranium ladders decorated with tellurium areelongated along the a-axis. Reflecting the orthorhombic
symmetry, superconducting upper critical field is highly
anisotropic. In particular, unusual reinforcement ofsuperconductivity occurs under magnetic field along the b-axis,
until a field-induced magnetic phase transition takes place at
higher field. Theoretical suggestions for possible use of thepeculiar superconductivity for quantum computing make thiscompound important even in the context of practicalapplications.
It is noticed that the superconductivity in UTe2 is extremely
sensitive to the sample quality. The first report on thesuperconducting sample showed the superconducting transition
temperature Tc of 1.5 K. However, some of the subsequent
reports showed higher Tc. In general, residual resistivity at low
temperature is a measure of ‘quality’ of metals because
resistivity appears through electron scattering with impurities
which break crystal periodicity. For elucidating thesuperconducting properties, it is crucial to prepare the highquality samples. To do this we first characterized the origin ofimpurities [2].
We employed two conventional methods. The first one is thesingle crystal x-ray diffraction where crystallographic
parameters including the occupancy for each crystallographic
site can be determined. In addition, chemical composition as
well as the possible spatial distribution of is determined semiquantitatively using electron-probe microanalyzer.
Figure 1(b) shows the comparison of the x-ray diffraction
intensity (Iobs) and calculated values (Icalc) from the crystal
structure model. When the model is correct, all points should
fall on the solid line corresponding to Iobs = Icalc. As seen in Fig.
1(b) left panel, agreement between Iobs and Icalc is poorer for NS
sample. In Fig. 1(b), the points significantly deviating from thecalculation are highlighted. It turned out that those reflections
consist of large contributions from uranium and tellurium sites
but canceling each other. Such reflections are sensitive to theoccupancy, in particular, of the uranium site in the present case.
Figure 1(b) right panel shows the result of optimization of theuranium site occupancy x. Iobs can well be reproduced withuranium deficiency model x = 0.962(2). In the superconducting
samples, on the other hand, the similar analysis results in fulloccupancy on both uranium and tellurium sites within anFig. 1 (a) Crystal structure of UTe2. (b) Results of x-ray diffraction
on non-superconducting sample of UTe2. Uranium deficiency model
well describes experimental data. (c) Temperature dependence ofelectronic specific heat of superconducting (SC) and nonsuperconducting (NS) samples of UTe2.

experimental accuracy. Examining crystallographic parameters
as well as chemical composition, the uranium deficiency is notsubstituted by tellurium but rather left vacant.
Figure 1(c) shows the comparison of the electronic
contribution to specific heat for both SC and NS samples. Whilea clear anomaly is observed at the superconducting transition
temperature (Tc = 1.8 K) in the SC sample, superconductivity is
not seen in the NS sample down to the lowest temperatureinvestigated. At higher temperatures, different behavior persists
even in the normal state. While the SC shows a marked
enhancement of specific heat at 15 K shown by an arrow, theNS sample only shows a broad feature at the corresponding
temperature. The peak at 15 K is generally interpreted as anenergy gap in electronic states. The corresponding anomaly isalso seen in the nuclear relaxation rate [3]. The present
comparison suggests that this energy gap is not completely
formed in the NS sample, leaving a large electronic specific heatat low temperatures, which might hinder the superconducting
transition. The sensitivity of electronic state on the uranium
defects is consistent with the one-dimensional magnetic
correlation within the uranium ladder reported previously; the4 % vacancy breaks every 13 ladder rungs.
Having established the origin of the defect, improvements on
the crystal growth condition have immediately been made.
Investigations using crystals with higher quality, namely withlow residual resistivity, are underway.
References[1] S. Ran et al., Science 365, 684 (2019).
[2] Y. Haga et al., J. Phys.: Condens. Matter 34, 175601 (2022).
[3] Y. Tokunaga et al., J. Phys. Soc. Jpn. 91, 023707 (2022)