Electronic and magnetic properties of Lu and LuH$_2$

Clarifying the electronic and magnetic properties of lutetium, lutetium dihydride, and lutetium oxide is very helpful to understand the emergent phenomena in lutetium-based compounds (such as room-temperature superconductivity). However, this kind of study is still scarce at present. Here, we report on the electronic and magnetic properties of lutetium metals, lutetium dihydride powders, and lutetium oxide powders. Crystal structures and chemical compositions of these samples were characterized by X-ray diffraction and X-ray photoemission spectroscopy, respectively. Electrical transport measurements show that the resistance of lutetium has a linear behavior depending on temperature, whereas the resistance of lutetium dihydride powders is independent of temperature. More interestingly, paramagnetism-ferromagnetism-spin glass transitions were observed at near 240 and 200 K, respectively, in lutetium metals. Our work uncovered the complex magnetic properties of Lu-based compounds.

(Quantum Design) from the temperature 300 to 2 K. To detect magnetic impurities, inductively coupled plasma optical emission spectroscopy (ICP-OES, SPECTRO ARCOS II) was applied. For preparing the ICP-OES measurement, the samples (Lu and LuH 2 ) were digested in hot 36 % hydrochloric acid and 65 % nitric acid, and the solutions were subsequently diluted to appropriate concentrations after complete dissolution of samples.

III. RESULTS AND DISCUSSION
First, we characterized the crystal structures of these three samples. As shown in Fig. 1, the diffraction peaks of these three samples are totally different. The dominative feature of LuH 2 XRD data (see the red curve in Fig. 1) is the four main diffraction peaks in the range 20-70 degrees, which can be assigned to (111), (002), (220), and (311) diffraction peaks, respectively [13].
The crystal structure of LuH 2 is cubic (Fm3m space group) with an estimated lattice parameter a ∼ 5.02Å [32], whereas the lattice parameters of hexagonal Lu (P6 3 /mmc) are a = b ∼ 3.516Å and c ∼ 5.573Å [34]. The four main peaks of Lu XRD data (see blue curve in Fig. 1) [40]. It is noteworthy that several small peaks can be seen in the XRD curve of LuH 2 (see red curve in Fig. 1). Compared to the XRD lineshapes of Lu and Lu 2 O 3 (see blue and brown curves in Fig. 1), it is indicated that the LuH 2 sample has little Lu and Lu 2 O 3 compositions, which is consistent with the previous report [4].
To further investigate the chemical compositions of Lu, LuH 2 , and Lu 2 O 3 samples, we performed XPS at room temperature, which is a surface-sensitive technique to probe the valence states of compounds. As seen in Fig. 2, apart from the oxygen (O 1s ∼ 531 eV) and carbon (C 1s ∼ 284.8 eV) existed on sample surfaces, no distinct impurity is detected, verifying the purity of these samples. It is noted that the detection limits in XPS is 1%-0.1% generally [41]. Interestingly, a number of peaks can be observed in the XPS spectra, most of which are assigned to Lu core-level states such as Lu 4f (∼ 9 eV), 5p (∼ 28 eV), 4d 5/2 (∼ 196 eV), 4d 3/2 (∼ 207 eV), 4p 3/2 (∼ 360 eV), and 4p 1/2 (∼ 411 eV) states [42]. Here, we emphasize that the surfaces of both Lu and LuH 2 are easily oxidized.
Next, we measured the electronic properties of Lu and LuH 2 samples by electrical transport from 300 to 2 K. As shown in is noisy due to the measured LuH 2 sample in Fig. 3 (a) being a compressed tablet from LuH 2 powders.
At last, to investigate the magnetic properties of Lu, LuH 2 , and Lu 2 O 3 , we measured the zero-field cooling (ZFC) and field cooling (FC) magnetization curves by SQUID. During measurements, the cooling field was set at 1000 Oe. As seen in Fig. 4 Fig. 4 (a)) increases monotonously with decreasing the temperature to 20 K, whereas the magnetization has a broad bump for the ZFC protocol (see the solid red curve in Fig. 4 (a)). Comparing to LuH 2 in Fig. 4 (a), the behavior of temperaturedependent magnetization of Lu is analogous but more clear. As shown in Fig. 4 (b), there is a distinct separation between ZFC and FC curves, which can result from the spin glass transition [43][44][45]. With decreasing the temperature from 300 to 2 K, two critical features ( at ∼ 240 K and ∼ 200 K) can be observed. The first one ∼ 240 K can be interpreted as the paramagnetic to the ferromagnetic phase transition, whereas the second one ∼ 200 K is due to the spin glass transition. In contrast to Lu and LuH 2 , the magnetizations of Lu 2 O 3 are almost independent of temperature, indicating the diamagnetic nature of Lu 2 O 3 [49].
Then, we discuss the origins of complex magnetic transitions. Generally, there are two scenarios that can explain the paramagnetic-ferromagnetic-spin glass transitions in Lu and LuH 2 compounds, which are self-induced spin glass states and dilute magnetic impurities-induced spin glass states, respectively [43,46]. For the first scenario, it is noted that a self-induced spin glass state has been observed in crystalline neodymium very recently, resulting from valley-like pockets of degenerate mag- Here, Lu-1 and Lu-2 are the labels of two Lu samples, whereas LuH2-1 and LuH2-2 are the marks of two LuH2 specimens.
netic wave vectors [46]. For the second one, the spin glassy behavior in dilute magnetic alloys has been studied for a long time [47]. It has been reported that dilute Fe impurity even at the level of 10 atomic parts per million (ppm) can lead to the formation of spin glassy behavior [48]. Therefore, to detect the concentration of magnetic impurities (such as Mn, Fe, Co, Ni) in Lu and

A. Conflict of Interest
The authors have no conflicts to disclose. Writing -review & editing (equal).

VII. DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.