M1 Colloquium Presentation

Arora Varun

29A13106 (Shimizu Lab)

Superconductivity in MNX type compounds

( TiNCl )

Superconducting Nitride Halides (MNX), Christian M. Schurz et al. , Zeitschrift for Kristallographie 226, 395-416 (2011)

Outline

• Introduction• Motivation• Experimental Findings• Conclusion• Next Plan

Introduction• What is Kyokugen? （極限）

　 High Pressure　　　　高圧

Low Temperature 低温

Strong Magnetic Field強磁場

pressure

Pressure

• Pressure is one of the important physical quantity.

• Volume decreases!

• Inter-atomic distance decreases!

• Interactions between atoms and electrons change => Changes in electronic states!

• What happens when we apply pressure?

High Pressure!! ( ～ 350 GPa)

R = 6400 km M = 5.97 x 10 24 kgCentre of Earth ～ 350 GPa

3 cm

6 cm

Diamond Anvil Cell (DAC) 1 GPa = 109Pa ～ 10000 atm (1 万気

圧）

DAC

Why use Diamond?• Hard• Transparent

What happens at Low Temperature!!

Resistance in a metal stems from scattering of electrons because of thermal vibrations!

Critical Temperature

Superconductivity!!

Resistance = 0 Meissner Effect

Decrease in Temperature

Decrease in energy and thermal vibrations

Decrease in resistance !!!

How to estimate pressure

Ruby produces fluorescence when irradiated by laser. The wavelength of the peak changes with pressure.

For P < 100 GPa : Ruby Fluorescence （ルビー蛍光法）

For P > 100 GPa : Raman Spectroscopy ( ラマン分光法）

　　 P = 66.9 - 0.5281ν + 3.585×10-4 ν2

Irradiate diamond with laser. On applying pressure, the vibration of C-C bond in diamond changes, the

wavelength of scattered light becomes small.

MNX Compound(TiNCl)

Introduction• MNX

M : Group IV metal ( Ti, Zr, Hf ) N : Nitrogen X : Halogen ( Cl, Br, I )

• Two types of layer-structured polymorphs of MNX ：

– α-form 　 ( Orthorhombic )– β-form ( Rhombohedra )

• Superconductivity upto Tc = 25.5 K in β-form :- Lithium doped ZrNCl : Superconductivity at 13 K (1996) Lithium doped HfNCl : Superconductivity at 25.5 K (1998)

TiNCl• α-MNX type structure

Layer Structure[MN]+ layer is sandwiched between two sheets of halide anions [X]-

• Orthorhombic Structure• Sensitive to moisture• Energy Band = 1.7 eV

(Semi-conductor)

BasalSpacing

Motivation for TiNCl• Potentiality of Intercalation

– Since TiNCl is a layer structured compund, metal atoms/organic molecules can be inter-calated in the Van-der Walls gap between the layers

• Similar structure as FeOCl– FeOCl also forms intercalation compounds with alkali metals as well

as amines– Upon intercalation with pyridines, the electrical conductivity has

been seen to increase drastically in FeOCl

What is Inter-calation?• To insert something between elements or layers.• Dopants can be introduced through the interlayer space without

substituting or disturbing the original network for electron transport.• Two types of layer structured crystals :

Molecular- Van der Walls force( eg : Graphite )

Ion – exchangeable- Redox reaction( eg : clay minerals )

Experimental Findings(a) Na

Tc = 18.1 K

Superconductivity in electron-doped layered TiNCL, Shuai Zhang et al, Physical Review B 86 (2012)

Experimental Findings(b) K

Tc = 16.1 K

Structure and superconductivity of the intercalation compunds of TiNCl, Shoji Yamanaka et al, Journals of Material Chemistry (2008)

Why Inter-calated TiNCl exhibits superconductivity

• NOT a conventional BCS superconductor since the electron carrier concentration is very low, and the electron-phonon coupling constant observed is also too small to explain the relatively high Tc

Tc exp∝ [ -1 / N(0)V ]

• Proposed reason is “Structural changes due to Inter-calation”

Conclusions• Like other layer structures, α-MNX also has the potential for high Tc

superconductivity

• β- MNX type compounds have been found to exhibit superconductivity, so there is a vast scope possible for superconductivity in α-MNX type compounds as well

• TiNCl contain versatile and flexile metal nitride layers for chemical modifications via Inter-calation

• TiNCl inter-calated with various metals or organic compunds exhibits moderately high Tc superconductivity (around 16.3 K). Experimental findings have also supported theoretical values

Next Plan• By applying pressure as well, we can expect a change in Tc for such an

intercalated compound.

• By controlling doping concentration or by using different kinds of dopants with TiNCl, we have can get even more results for such unconventional superconductors