SUPERSOLID AND SUPERFLUID

Research Overview

LIST OF RESEARCHERS

Dr. Duk-Young Kim (06-10), Dr. Hyeongsoon Choi (07-11), Dr. Wonsuk Choi (06-13),

Prof. Evan S. Hyunkoo Kang (07-13), Dr. Jaeho Shin (09-15), Dr. Jaewon Choi (10-16)

WHAT IS SUPERSOLID?

Supersolid, or superfluidic solid, is a quantum state of matter in which both crystalline (solid) order and off-diagonal long range (superfluid) order exist simultaneously and spontaneously [1]. At temperatures below 2.18 K, liquid 4He enters into a superfluid state and flows without any friction. The onset of superfluidity is associated with Bose-Einstein condensation where the 4He atoms condense into a single momentum state and acquire quantum mechanical coherence over macroscopic length scales. Counter-intuitively superfluid-like behavior is thought possible even in solid helium. Several theorists such as Gross, Yang [2], and Leggett [3] anticipated that the off-diagonal long-range order can exist when atoms have a high degree of quantum delocalization, more simply, exchanges of adjacent atoms. Because of its quantum nature, solid helium-4 has been regarded as the most likely candidate to exhibit supersolidity. However, no successful obeservation of a supersolid phase was reported until 2004, despite of steady experimental efforts.

FIRST EXPERIMENTAL EVIDENCE: KIM & CHAN EXPERIMENT

In 2004, Eunseong Kim and Moses Chan reported a first possible evidence for supersolidity [4-5]. They put a solid helium into a sample space of a device called 'torsional oscillator', and induced an AC oscillation. The resonant period of the torsional motion was measured while the temperature is cooled down to 20mK. As a result, the period started to decrease anomalously at 200mK compared to the empty-cell data. They interpreted that part of solid helium became supersolid, so did not contribute to the rotational inertia, and made a drop in the TO period. This observation regarded as a first experimental evidence for supersolidity. At least nine different group successfully reproduced this result.

UNDERSTANDING A RELATIONSHIP BETWEEN TWO ANOMALIES

In 2007, Day and Beamish [6] at the University of Alberta reported that the shear modulus of solid helium anomalously increases at 200mK. In a series of experiments, they concluded that there is a remarkable similarity between shear modulus and TO anomaly. In spite of a quantitative mismatch, some physicitsts raised a doubt that the TO period anomaly is actually originated from the change in shear modulus, not from a supersolid transition. To elucidate the origin of a TO anomaly and its relationship with a shear modulus anomaly, we designed a torsional oscillator with a pair of piezo-electric transducers which allows us to measure a period of TO and shear modulus simultaneously. In a series of experiments, we did not only test a direct causal relationship between two anomalies but also understood the property of shear modulus anomaly in a framework of Granato-Lucke theory.

- Researchers:

Dr. Duk Young Kim, Prof. Hyeongsoon Choi, Dr. Wonsuk Choi, Dr. Jaeho Shin, and Prof. Evan S. Hyunkoo Kang

- Results:

H. Choi, S. Kwon, D. Y. Kim, and E. Kim, Nature Physics 6, 424 (2010)

D. Y. Kim, H. Choi, W. Choi, S. Kwon, E. Kim and H. C. Kim, Physical Review B 83, 052503 (2011)

E. S. H. Kang, D. Y. Kim, H. C. Kim and E. Kim, Physical Review B 87, 094512 (2013)

E. S. H. Kang, H. Yoon, and E. Kim, Journal of Physical Society of Japan 84, 034602 (2015)

J. Shin, J. Choi, K. Shirahama and E. Kim, Physical Review B 93, 214512 (2016)

SUPERSOLID HELIUM UNDER DC ROTATION

In the collaboration with the Quantum Condensed Phase Research Team at RIKEN, we measured both period of a TO and shear modulus of solid helium inside the TO under a DC rotation. In contrast to an AC oscillation, a constant DC rotation cannot induce any shear stress to solid helium. Therefore, the TO period should be suppressed by increasing a rotation velocity while the shear modulus of solid helium should not be affected. We confirmed this hypothesis from our experiments. Moreover, we found a staircase-like periodic suppression of the TO period in a velocity-sweep data, which might be related with a quantum vortex injection. The above observations were regarded as strong evidences for supersolidity.

- Researchers: Dr. Hyeongsoon Choi, Dr. Wonsuk Choi

- Results:

H. Choi, D. Takahashi, K. Kono, and E. Kim, Science 330, 1512 (2010)

H. Choi, D. Takahashi, K. Kono, and E. Kim, Physical Review Letters 108, 105302 (2012)

W. Choi, D. Takahashi, D. Y. Kim, H. Choi, K. Kono and E. Kim, Physical Review B 86, 174505 (2012)

RIGID DOUBLE-PENDULUM TORSIONAL OSCILLATORS

Recently a number of experimental and theoretical efforts have indicated that the anomalous period drop in TO response, originally interpreted as the evidence for supersolid phase, can be explained by the shear modulus change. The contribution from the change in shear modulus can be substantially amplified to explain the entire signal if TOs are not desigend to be 'rigid'. Almost all previous TO experiments cannot free from this 'rigidity' criteria. Motivated by these arguments, we have designed and built the KAIST rigid double-torus TO to eliminate the various elastic effect and to find a genuine supersolid signal. As a result, a majority of TO period change, previously interpreted as the evidence for supersolid phase, is actually attributed to the change in shear modulus of solid helium. We found very small frequency-independent period drop, as large as 4 ppm, which can be interpreted as the supersolid density, if it exists. The previous rotating solid helium experiment is also revisited with the rigid TO. Three important results in the previous experiment are not reproducible. Instead, we find very small suppression of frequency-independent (or superfluidic) period drop as large as 2 ppm, which cannot be reconciled with the elastic stiffening model.

- Researchers: Dr. Jaewon Choi, Dr. Jaeho Shin

- Preliminary results:

J. Choi, J. Shin and E. Kim, Physical Review B 92, 144505 (2015)

J. Shin, J. Choi, K. Shirahama and E. Kim, Physical Review B 93, 214512 (2016)

J. Choi, T. Tsuiki, D. Takahashi, K. Kono, K. Shirahama, H. Choi and E. Kim, arXiv:1701.07190

COLLABORATORS

Prof. Moses H. W. Chan (Department of Physics, Pennsylvania State University, USA)

Prof. Kimitoshi Kono (Quantum Condensed Phase Research Team, RIKEN, Japan)

Prof. Keiya Shirahama (Department of Physics, Keio University, Japan)

Prof. Norbert Mulders (Department of Physics, University of Delaware, USA)

Prof. Daisuke Takahashi (Asahikawa Institute of Technology, Japan)

Dr. Hyoung Chan Kim (National Fusion Research Institute, Republic of Korea)

REFERENCES

[1] M. Boninsegni and M. Prokofev, Reviews of Modern Physics 84, 759 (2012)

[2] C. N. Yang, Reviews of Modern Physics 34, 694 (1962)

[3] A. J. Leggett, Physical Review Letters 25, 1543 (1970)

[4] E. Kim and M. H. W. Chan, Nature 427, 225 (2004)

[5] E. Kim and M. H. W. Chan, Science 305, 1941 (2004)

[6] J. Day and J. Beamish, Nature 450, 853 (2007)