Research Group for Nanoscale Structure and Function of Advanced Materials
Group Leader: Shin-ichi Shamoto
グループ名, :Times
New Roman,
14 point, bold Yuki Fukaya, Takashi U. Ito, Yuki Uozumi, Masahiro
Members
WataruHigemoto,
GL、メンバー名, Times New Roman, 12points
Yano, Shota Suzuki, Eiko Torikai

The research objective of Nanoscale Structure and Function ofAdvanced Materials group is to study nanoscale structures offunctional devices and materials under external fields by using
high-intensity quantum beams of muons, positrons, neutrons,and synchrotron light. As a highlighted result in our group, aninterface height between graphene and metal substrate isexperimentally revealed to change depending on the metalelements by Total-Reflection High-Energy Positron Diffraction
method [1]. In this activity report, following two topics areintroduced. Quadrupole fluctuation is detected via a coupling
between muon and praseodymium (Pr) nuclear spin in PrV2Al20
by muon spin relaxation (SR) technique. Surface stress ofSi(111) 7×7 reconstruction is successfully measured by using
the surface-curvature and the reflection high-energy electrondiffraction (RHEED) apparatuses simultaneously.

surface-curvature and the RHEED apparatus during atomic
hydrogen adsorption at 380˚C and room temperature (RT).
Atomic hydrogen exposure of Si(111) 7×7 results in rapid
stress relaxation in the surface (Fig. 2). At the same time, thesurface structure is transformed from Si(111) 7×7 to H-Si(111)1×1. From the stress relaxation, the surface stress change isestimated to be 1.7 N/m at 5000 Langumuir (1 Langumuir =1.33 × 10-4 Pa･s) at 380˚C [5], whereas it is 2.1 N/m at RT. H-Si(111) 1×1 surface forms a high-quality surface structure at 5000
Langumuir at 380˚C. On the other hand, it has many defects(holes, stacking fault, and islands) at RT. In conclusion, we havesucceeded in the surface stress measurement of Si(111) 7×7
reconstruction during atomic hydrogen adsorption.

Perturbation on hyperfine-enhanced 141Pr nuclear spin
dynamics associated with quadrupolar order in PrV2Al20 [2]Recent discoveries of heavy fermion superconductivity inquadrupolar phases of 4f electrons in some Pr-based compounds
have stimulated renewed attention on quadrupolar quantum
criticality [3,4]. However, an experimental investigation ofquadrupolar correlations is difficult because experimental
techniques sensitive to quadrupolar moments are still quite
limited. Thus, it is important to develop new methodologies toprobe quadrupolar properties.
We have studied the influence of 4f quadrupolar correlations
on hyperfine-enhanced (HE) 141Pr nuclear magnetism by using
the SR method. SR is sensitive to slow spin dynamics andthus is suitable for probing HE 141Pr nuclear spin fluctuations ina MHz scale.
SR experiment has been performed on PrV2Al20 which shows
antiferroquadrupolar (AFQ) ordering at TQ ~ 0.6 K [2]. As
shown in Fig. 1, we observed an increase in the muon spinlattice relaxation rate 1/T1, below 1 K, slightly above TQ, withdecreasing temperature. This increase can be attributed to theperturbation on the strength of HE 141Pr nuclear spin-spin
interactions associated with the development of AFQ correlation.
This paves the way for a new microscopic approach toinvestigate quadrupolar correlations in Pr-based compounds
using local spin probes.

Fig. 1. Temperature dependence of 1/T1, in PrV2Al20 under zeromagnetic field (ZF).

In situ stress measurement of Si(111) 7×7 reconstruction
during atomic hydrogen adsorption [5]Surface stress of Si(111) 7×7 reconstruction has beentheoreticallystudied
but
not
been
demonstrated
experimentally[6]. We have measured the surface stress of theSi(111) 7×7 reconstruction by comparison with the hydrogen(H-) terminated Si(111) 1×1 surface.
The Si(111) 7×7 reconstruction surface was prepared byheating Si(111) wafer in a ultrahigh vacuum system after wet
etching process. To obtain information on both the stress andthe surface structure simultaneously, we have combined theFig. 2. Evolution of surface stresses during atomic hydrogen
exposure of Si(111) 7×7 surfaces at two substrate temperaturesof 380 ˚C (red line) and room temperature (blue line).
References[1] Y. Fukaya et al.,in this annual report.
[2] T. U. Ito et al., Phys. Rev. B 92, 125151 (2015).
[3] K. Matsubayashi et al., Phys. Rev. Lett. 109, 187004 (2012).
[4] M. Tsujimoto et al., Phys. Rev. Lett. 113, 267001 (2014).
[5] H. Asaoka, and Y. Uozumi, Thin Solid Film 591, 200 (2015).
[6] D. Vanderbilt, Phys. Rev. Lett. 59, 1456 (1987).