Photon Factory Activity Report 2017 #35 (2018)

BL-8B/2017G671

Hydrostatic pressure effects on severely-strained Nb
Masaki MITO1,*, Takayuki TAJIRI2, Terukazu NISHIZAKI3, Kaveh EDALATI4,5, andZenji HORITA4,5
1
Graduate School of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
2
Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan3
Department of Electrical Engineering, Kyushu Sangyo University, Fukuoka 813-8503, Japan
4
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University,Fukuoka 819-0395, Japan
5
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,Fukuoka 819-0395, Japan
We investigated the effects of hydrostatic pressure (HP) on the crystal structure of severely strained Nb samples, in which
severe plastic deformation by high-pressure torsion (HPT) treatment brought about strain at the unit-cell level as well as
reduction in the grain size. The HP effects on non-strained Nb have already been investigated in the pressure regime over
100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and Tc reportedly exhibited an increase from 9.2 to 9.9 K
at approximately 10 GPa. In the present study, the pressure scale of the pressure response observed by Struzhkin et al.
was reduced down to approximately one-fifth at maximum: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), Tc
exceeds 9.9 K at HP of approximately 2 GPa. We observed pronounced anisotropic change of the unit cell there.
According to our first-principle calculations, the reduction in structural symmetry affords an increase in the density ofstates at the Fermi energy, thereby yielding a prominent increase in Tc at low pressures.
1 Introduction
In the periodic table, twenty-nine elements exhibit
superconductivity at ambient pressure. Among them,eleven elements show the increase in the superconducting
transition temperature Tc at high pressures [1]. The study
on superconductivity of pure single elements has no moreroom for meaningful progress. For instance, the highest
Tc of single element superconductors at ambient pressureis 9.20 K for Nb, and it exhibits the maximum value of9.90 K at around P = 10 GPa [2].
Recently, it has been observed that enormous residualstrain sometimes plays a positive role in stabilizing
superconductivity, as observed in Nb [3], NbTi [4], andRe [5]. We expect any positive influence of residual strain
stored in materials from the perspective of desiring highTc under pressures. Indeed, in the severely strained Nb,
we observed Tc of more than 9.90 K at around 2 GPa as
seen in Fig.1. Now, the investigation on the crystal
structure as a function of pressure is desirable in order tounderstand quite sensitive pressure response.
2 ExperimentHigh-purity (99.9\%) Nb discs were subjected to HPTprocessing at room temperature under a pressure of P = 6
GPa and revolutions of N = 0, 1, 2, 5, 10, and 20 [6, 7].
For N < 2, dislocations are stored, and the grain size
largely changes [8]. The small-angle grain boundaries
formed in these cases gradually transform to large-angle
grain boundaries with increasing N. On the other hand,for N ≥ 2, only the large-angle grain-boundaries arepresent, and the grain size hardly changes with further
increase in N.
Further, at ambient pressure, Tc for AR isapproximately 9.25 K, and Tc exhibits a maximum ofapproximately 9.37 K at N = 2. In addition, Tc remains
unchanged at 9.35 K for N > 10 [3].
We performed X-ray diffraction (XRD) analyses underhigh pressure of up to P = 5.1 GPa at room temperatureusing a synchrotron radiation XRD system with acylindrical imaging plate at the Photon Factory at theInstitute of Materials Structure Science, High Energy
Accelerator Research Organization [9].
The energy of the incident X-rays was 16 keV. Pressure
was applied using a DAC that consisted of two diamondanvils with flat tips having a diameter of 0.8 mm and a
0.3-mm-thick CuBe gasket. The small pieces of HPT-Nb
were placed in a randomly oriented manner in a sample
cavity with a diameter of 0.4 mm along with a ruby ofmanometer and transparent pressure-transmittingmediums (PTMs) such as fluorinated oil (FC77,Sumitomo 3M Ltd.) or MEW.
All the atomic positions in the unit cell are special
positions, and therefore, the structural parameters wereevaluated with the use of the diffraction peak angle andfull-width at half-maximum (FWHM) values.
3 Results and Discussion
Prior to the experiments on hydrostatic pressure, wemention the crystal structure in the unit cell level for thetargeted samples. The position of the diffraction peak ofthe (110) plane slightly shifts toward high angle sides
with increasing the revolution number N in the turn as asreceived (AR) → N = 1 →2 → 5. However, between N =5 and 10, there occurs the sudden shift toward low angle
side. We assumed the reduction of structural symmetry
such as the change from cubic to orthorhombic between N= 5 and 10.Photon Factory Activity Report 2017 #35 (2018)

Indeed, as seen in Fig. 2, each lattice parameter inorthorhombic system exhibits independent pressure
dependence each other. At around P = 2 GPa, theelongation along b- and c-axes suggesting prominent
structural deformation occurs. According to our firstprinciple calculations, the reduction in structuralsymmetry affords an increase in the density of states at
the Fermi energy, thereby yielding a prominent increasein Tc at low pressures.
Thus, we can control Tc positively by realizing any
quasi-stable structure, which would be realized by bothlarge shear strain and hydrostatic pressure. The firstprinciple calculations suggest that the reduction in
structural symmetry affords an increase in the density ofstates at the Fermi energy.

109.9
N = 109.8

Tc [K]

9.7
9.6

AR

9.5
9.4
9.30

5
10
Pressure [GPa]15Fig. 1: Pressure dependence of superconducting transition
temperature Tc for HPT-Nb with N = 10. Here, theApiezon-J oil was used as the PTM. For reference, theguide curve corresponding to Tc(P) of the sample ARusing the Apiezon-J oil as PTM is also presented.

Lattice parameter [Å]

3.31
abc

3.3
3.29
3.28
3.27
3.26
3.25

N = 1001234Pressure [GPa]Fig. 2: Pressure dependence of lattice constants inorthorhombic system for HPT-Nb with N = 10.

5Acknowledgement
This work was supported by MEXT KAKENHI, aGrant-in-Aid for Scientific Research on Innovative Areas''Bulk Nanostructured Metals'' No. 25102709, and Grantin-Aid for Scientific Research (S) (No. 26220909). Thiswork was also supported by JSPS KAKENHI Grant
Number 16H04338, 16K05460, and 17H03379. The HPTprocess was carried out at the International ResearchCenter on Giant Straining for Advanced Materials (IRCGSAM) in Kyushu University.
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*mitoh@mns.kyutech.ac.jp