A peculiar ground-state phase for superconductor NbSe2: Bose metal!

06/25/2019

A peculiar ground-state phase for superconductor NbSe2: Bose metal!

Applying large enough magnetic fields usually results in the disruption of superconducting states in materials, even at drastically low temperatures. This thereby changes them directly into insulators – or so was generally thought. Now, scientists from the Tokyo Institute of Technology, The University of Tokyo and Tohoku University have discovered the interesting occurrence of multi-state transitions of superconductors – going from superconductor, to special metal, and then to insulator.

Electrical transport measurements

Figure 1. Electrical transport measurements
A schematic figure of the four-point probe method used for analyzing the electron transport properties under ultrahigh vacuum. The superconducting sample is one-unit-layer (1UL) NbSe2 grown on bilayer graphene (BLG) on SiC substrate.

Because of their absolute zero electrical resistance and amazing ability to completely expel external magnetic fields, superconductors have attracted much attention. This is from the point of view from both fundamental physics and their application. One such example is a superconducting coil in magnetic field. It is well understood that superconductivity is realized by the highly-ordered coherence among electrons in material, where two electrons are bounded with each other to make a pair and flow collectively without collision (resistance) in material. This process, of course, does not suffer from energy dissipation. However, when a magnetic field is applied on the superconducting material, the paired electrons are no longer able to maintain their coherent relationship and as a result the superconducting state is broken. At a given temperature, the highest magnetic field under which a material remains superconducting is called the critical field.

This phenomenon has been intensively studied under the framework of phase transition. When the change is abrupt like in the case of melting ice, it is called the first-order transition. In contrast, when the transition takes place in a gradual and continuous manner, it is called the second-order transition. Studying the transition path of superconductors under a magnetic field yields insights into the quantum processes involved. Furthermore, it enables us to design smarter superconducting materials and devices for advanced technologies.

Two-dimensional superconductors (2DSCs) such as mono layer NbSe2 are the best candidates to study the process of phase transitions. The very thin nature of mono layer film constrains the inside electrons to form a pair with a limited number of counterparts, so that even a small perturbation can triger the phase transition. Furthermore, 2DSCs have a high potential for applications to small-scale electronics.

In 2DSCs, when we apply a magnetic field beyond the critical value, the material enters into a fuzzy state, where the magnetic field penetrates the material, but the resistance still remains minimal. Upon further increasing the magnetic field, the superconductivity is finally destroyed and the material changes to an ordinary insulator. This is known as the superconductor-to-insulator phase transition. Because this phenomenon is observed at very low temperatures, the quantum fluctuations in the material become comparable to or even larger than the classical thermal fluctuations. Therefore, this is called a “quantum” phase transition.

To understand the path-phase transition as well as the fuzzy or mixed state in ultrathin NbSe2, the present team measured the magnetoresistance of material (see Fig. 1), namely, the response of electrical resistivity under an external magnetic field. Prof. Ichinokura explains, “As shown in Fig. 2, we estimated the critical magnetic field at the respective quantum phase boundaries in mono-layer NbSe2 by the four-point-probe method”. In fact, they found that when a small magnetic field is applied to the sample, the coherent flow of electrons is broken, but the electron pairs seem to still remain. This indicates that the vortices still move even under the magnetic field and create a finite resistance. This peculiar (fuzzy) state with a minimal resistance is called the Bose metal state, where electron pairs do exist in material, but the coherence among them is lost. Upon further increase of the magnetic field, the material enters into an insulating state. The team also found that the transition between the normal and superconducting states around the critical temperature was driven by quantum fluctuations, reflecting a similar multi-transition pathway. Excited by the results, Prof. Ichinokura commented, “The scaling analysis based on the Bose-metal model explains well the two-step transition, indicating the existence of bosonic ground state.”

Superconductivity-related states

Figure 2. Superconductivity-related states
Schematically drawn phase diagram of superconductivity-related states in ultrathin NbSe2. SC; superconductor, BM; Bose metal, INS; insulator, B1, B2, Bc2; magnetic fields at boundaries between the phases.

Reference

Authors: Satoru Ichinokura,1 *Yuki Nakata,2 Katsuaki Sugawara,2,3,4 Yukihiro Endo,1 Akari Takayama,1 Takashi Takahashi,2,3,4 and Shuji Hasegawa1
Title of original paper: Vortex-induced quantum metallicity in the mono-unit-layer superconductor NbSe2
Journal: Physical Review B
DOI: 10.1103/PhysRevB.99.220501
Affiliations:
1Department of Physics, The University of Tokyo, Japan
2Department of Physics, Tohoku University, Japan
3Center for Spintronics Research Network, Tohoku University, Japan
4WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Japan
*Current address: Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan; ichinokura@phys.titech.ac.jp

*Corresponding author’s email: ichinokura@phys.titech.ac.jp

Contact

Katsuaki Sugawara
WPI-AIMR, Tohoku University, Japan

Email: k.sugawara@arpes.phys.tohoku.ac.jp
Tel: +81-3-5734-2975

Satoru Ichinokura
Department of Physics, The University of Tokyo, Japan

Email: ichinokura@phys.titech.ac.jp

Katsuaki Sugawara
WPI-AIMR, Tohoku University, Japan

Email: k.sugawara@arpes.phys.tohoku.ac.jp
Tel: +81-3-5734-2975