Superconductivity in amorphous and crystalline Re-Lu films

We report on magnetron deposition and superconducting properties of a novel superconducting material: rhenium-lutetium films on sapphire substrates. Different compositions of Re$_{x}$Lu binary are explored from $x\approx 3.8$ to close to pure Re stoichiometry. The highest critical temperature, up to $T_{c}\approx $ 6.95 K, is obtained for $x\approx 10.5$. Depending on the deposition conditions, polycrystalline or amorphous films are obtainable, both of which are interesting for practice. Crystalline structure of polycrystalline phase is identified using grazing incidence X-ray diffractometry as a non-centrosymmetric superconductor. Superconducting properties were characterized both resistively and magnetically. Demonstration of superconductivity in this material justifies the point of view that Lu plays a role of group 3 transition metal in period 6 of the Periodic table of elements. In analogy with Re$_{0.82}$Nb$_{0.18}$, Re$_{6}$Ti, Re$_{6}$Hf and Re$_{6}$Zr, one can expect that crystalline Re--Lu is also breaking the time-reversal symmetry (this still waits confirmation). Magnetoresistivity and AC/DC susceptibility measurements allowed us to determine H$_{c1}$ and H$_{c2}$ of these films, as well as estimate coherence length $\xi (0)$ and magnetic penetration depth $\lambda _{L}(0)$. We also provide information on surface morphology of these films.

Rhenium itself belongs to transition metals, and in bulk form at ambient conditions, it superconducts below T c ≈ 1.7 K [22][23][24].In a thin film form T c is higher [22,[25][26][27][28] and can reach values as high as 6 K. Compounds of Re with other transition metals allowed material scientists to achieve not only higher values of T c , but also, as was mentioned above to demonstrate broken TRS in addition to NCS.From this point of view it is interesting to explore superconductivity of Re-Lu substance, since there is a widespread opinion that the lanthanide Lu is closer to transition metals than La itself [29].However, Re-Lu material has not been explored in bulk form, or in thin films.
To close this gap, we report here on superconducting properties of amorphous and polycrystalline Re-Lu films with critical temperature up to about 7 K.We studied morphology of the films, magnetotransport and magnetic susceptibility which allowed us to estimate basic features of superconducting state, such as the critical fields, coherence length, and London penetration depth.

II. EXPERIMENTAL DETAILS
The Re-Lu films were prepared via magnetron sputtering in our ATC series UHV Hybrid deposition system (AJA International, Inc.) with a base pressure of 1 × 10 −8 Torr.The Re target (ACI Alloys, Inc., 99.99% purity) was accommodated inside of a 1.5" DC gun.The Lu target (Heeger Materials, Inc., purity Lu/TREM 99.99%) was placed inside of a 2" DC gun.The sapphire substrate (AdValue Technology, thickness 650 µm, C-cut) was cleaned thoroughly with isopropyl alcohol before it was mounted on the holder.In our chamber's configuration, the substrate holder is at the center of the chamber facing upwards, while the (five) sputtering guns are located at the top.The substrate is rotated in plane throughout the whole deposition process to ensure a homogeneous deposition layer over the whole surface.Our predeposition in-situ cleaning of the substrate typically involves heating it up to 900 • C for 10 min followed by a gentle bombardment of Ar + ions at 600 • C for 5-10 min using the Kauffman source at 45 degrees to the substrate surface.Then the temperature was raised to 900 • C and kept at that value for 30 min.Afterwards, the temperature was reduced to 600 • C and simultaneous deposition took place for 10 min, at pressure 3 -4 mTorr, with gun power 250-260 W and anode voltage 605-460 V for Re, and with gun power 90-45 W and anode voltage correspondingly 325-275 V for Lu.Keeping the Re gun power constant, and varying the sputtering power of Lu from case to case allowed us to vary the values of x in composition Re x Lu (see Table I).After the deposition, the temperature was raised back to 900 • C for in-situ annealing for 30 min and then cooled down to ambient temperature.All the heating/cooling protocols consistently used a 30 • C/min ramp rate.The substrate was oriented to face the ion gun squarely.

III. RESULTS
Our initial choice of x for examining Re x Lu composition was x ∼ 4, in analogy with the well-known NCS superconductor Re 0.82 Nb 0.18 , known for its breaking of TRS.The composition with x ≈ 3.8 indeed turned out to be an amorphous superconductor with T c ≈ 5.3 K, see Fig. 1(a).
Lowering the relative concentration of the co-deposited Lu first increased and then decreased the T c , with the optimum T c ≈ 7 K corresponding to x ∼ 10 − 11 (shown also in Fig. 1).Though the normal state resistivities of these two compositions are not much different, they have very different surface morphologies, Fig. 2.
Comparative characteristics our films with various values of x are in Table I.To characterize the crystalline structure of our films we used grazing incidence X-ray diffractometry which excludes the reflections of the substrate.In this way it was recognized that the films with x = 3.8 are amorphous, and those with x ≈ 10 are polycrystalline.In this report we will mainly focus on these two compositions.In the latter case, using the diffractogram (Fig. 3) it is possible to determine the lattice parameters of this novel substance.They are detailed in Table II.Magnetic and magnetotransport characterization of these films was also performed (Quantum Design PPMS), Fig. 4 and Fig. 5.

IV. DISCUSSION
As follows from Table I, both stoichiometric ratio and substrate's temperature affect the crystalline properties of this material.Moreover, the stoichiometric ratio itself depends on the substrate temperature.The last entry in the table corresponds to less than 1%(at.) of Lu in the composition -we reached here the resolution limit of our energy-dispersive spectrometer (Oxford Instruments X-Max N ).Meanwhile, as the special investigation revealed [28], pure Re films grown in the same conditions (600 • C) are amorphous and do not superconduct down to 1.8 K, while being grown on 30 • C they do at 3.6 K.The role of the substrate-film interplay is also important; for example, bulk Re never superconducts above 1.8 K [22].
Our samples' H c2 (T) curves show different behavior compared to the conventional BCS dependence . Therefore, the curves can instead be fitted using the expression H c2 (T ) = H c2 (0)[1−(T /T c ) p ] q following [30,31] where the exponents p = q were chosen to be 3/2.A slightly better fit to the data can be obtained when the constraint on p and q are removed by choosing p = 1.8, q = 1.2.This fit is shown in Fig. 5(b) which yields H c2 ≈ 13 T. Using the Ginzburg-Landau relation H c2 = ϕ 0 /(2πξ 2 ) , where ϕ 0 = 2.068 × 10 −15 Wb is the flux quantum [32], the estimated coherence length of our film with T c = 6.95K is ξ(0) = 5.05 nm.
V. SUMMARY Thus, the idea that Lu can successfully play the role of a transition element in Re-Lu compound is confirmed by this research.We obtained a new material, Re x Lu (3.8 ≤ x ≤ 99+).In particular, Re 10.5 Lu exceeds the critical temperature of known Re 6 Hf, Re 6 Zr and Re 6 Ti.While these superconductors have never been reported having T c > 6 K, either in bulk or thin film form, Re 10.5 Lu demonstrated T c ≈ 7 K.By analogy, one can expect that this NCS material will also break TRS.It will be very interesting to explore that property, though that goal is beyond this paper.The indirect proof of broken TRS may be obtained by effects related to nonreciprocal current control devices made of this material: demonstration of nonreciprocity in absence of applied magnetic field may serve as such a proof.
The simplicity of the described deposition method may facilitate the application of this material for wide range of devices mentioned in Introduction.Also, the information obtained by our research may provide grounds for further fundamental studies based on band-structure computations of superconducting state in Re-Lu materials to quantitatively explain the discovered features.Finally, the parameters λ L and ξ estimated above may be used for modeling of phenomena in Re-Lu-based superconducting devices.used, and the parabola is enforced to go through through 2 points: Hc1(Tc) = 0 and Hc1(2.5K)≈ 3 Oe.This method is less accurate, however, it is satisfactory for estimates.

FIG. 5 .
FIG. 5. (a) Magnetotransport measurements for determining critical field Hc2 vs. temperature in case of x = 3.8 (amorphous) film.Similar measurements were performed in case of x = 10.5 (polycrystalline) film.(b) Determining Hc2(T = 0) for polycrystalline and amorphous films.(c) Virgin curves of polycrystalline film at various temperatures for determining the value of Hc1 (panel d).This value of Hc1 is constructed in (d) using the linear part of experimental data (one such line is shown in panel c -dashed line for T = 6 K data) via modeling in accordance with the relation Hc1(T ) = Hc1(0)[1 − (T /Tc) 2 ].

TABLE II .
Crystal FIG. 4. Characteristic "butterfly" pattern of polycrystalline film Re10.5Lu.A noticeable clockwise rotation of the butterfly is caused by the substrate diamagnetism (detailed in inset (a)).Inset (b) demonstrates the "butterfly" of Re3.8Lu amorphous film.Its virgin curve is shown in inset c (dashed lines are guides for eyes only).