Bose Metal

March 4, 2025

GS Paper III Science & Tech

Niobium Diselenide (NbSe2) has shown strong possibilities containing properties of a Bose Metal, as found out by a team of researchers from China and Japan in a recent report.

About the Research

Published In: The research is published in the journal Physical Review Letters
Subject: To characterize how the Anomalous Metallic State (AMS) in NbSe2 emerges as the material becomes thinner.
Technology: Raman spectroscopy was used to find the thin NbSe2 had Cooper pairs without entering a superconducting state and the material’s Hall resistance vanished as the team increased its thickness.
Findings:
- Evidence consistent with a Bose metal in atomically thin films of niobium diselenide (NbSe2) has been found.
  - Mixed State: NbSe2 becomes a superconductor at low temperature but with additional ‘abilities’ ie. superconductivity along with the presence of magnetic field
- Type -II Superconductors: NbSe2 enters a mixed state when the field strength is slowly increased and remains superconducting but also allows the magnetic field to enter its body in small, isolated pockets without spreading through its bulk.
  - Superconductivity and Magnetic field cannot exist simultaneously in a metal.
- Superconducting Collapse: When the magnetic field continues to strengthen, beyond an upper threshold the superconducting state will collapse and NbSe2 will revert to its pre-superconducting state.
- Vanishing Hall Resistance: The Hall resistance vanished in NbSe2 signaling that its charge-carriers are Cooper pairs rather than electrons.
  - Hall Resistance: It is the resistance associated with the voltage which develops in the perpendicular direction when a piece of regular metal is placed in a magnetic field and a current is passed through it
Significance:
- The findings impose “limitations on theories centred around” pockets of superconductivity in a non-superconducting material and the coexistence of superconducting and non-superconducting phases in the same material.
- To Understand Disordered Metals: Studying AMSs is important to understand Disordered metals.
  - They have deviant properties which are not fully understood and studying them will help scientists probe a variety of quantum processes.

About Bose Metal

A Bose metal is a theoretical concept that could, in principle, account for the observed properties of a so-called anomalous metallic state (AMS), which differ from those of the regular metallic state
- Bose metals have only been predicted to exist in specific materials without any physical evidence of their existence found yet.
Formation: Bose metals are made of Cooper pairs, which form at very low temperatures in superconducting films as an intermediate phase between superconductivity and superinsulation.
Characteristics:
- Anomalous Metallic State (AMS): Bose metals are a type of AMS, where the behavior deviates from typical metallic properties.
- Cooper Pairs: These are pairs of electrons that can form in certain materials at very low temperatures, and are crucial for superconductivity.
- No Superconductivity: In a Bose metal, although Cooper pairs form, but they don’t establish long-range superconducting coherence ie. they don’t transition into a superconducting state where electricity can flow with zero resistance.
- Intermediate State: Bose metals exist as an intermediate state between a normal metal and a superconductor, challenging traditional theories of condensed matter physics.
- Conductivity: Unlike traditional metals, Bose metals exhibit conductivity that remains between zero (insulation) and infinity (superconductivity) at near absolute zero temperature.
Limitations of Bose Metals:
- No Practical Applications: Bose metals exist just in theory with no direct industrial or technological applications found yet.
- Experimental Challenges: To realize the existence of bose metals requires huge investments in experiments which conducts precise control over temperature, thickness, and magnetic fields, which can be difficult to achieve.
- Not Defined Yet: Scientists debate whether Bose metals are distinct quantum states or transitional phases, leading to uncertainty in their characterization.
- Disorder and Localization: Bose metals can exist in disordered systems if the disorder is strong enough to localize some pairs, and a metallic state can be produced on top of a transition.

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