m1 colloquium presentation arora varun 29a13106 (shimizu lab) high pressure study of na x tincl and...

M1 Colloquium Presentation

Arora Varun

29A13106 (Shimizu Lab)

High Pressure Study of NaxTiNCl and CeFe2

Contents

• Introduction• Motivation• Experimental Method• Results• Summary

Intercalated MNX Compound(NaxTiNCl)

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)

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

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

Ti

Cl

N

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

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 )

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

Previous Research onNa0.16TiNCl

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

Motivation

On applying pressure, distance between the layers “d” decreases, and so Tc should also

increase ?

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

Check for Superconductivity in Na0.16TiNCl at High Pressure

Resistivity Measurement Sharp Drop in Resistivity

Meisner Effect Sharp change in AC Susceptibility ( χ )

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

AC - Susceptibility Experiment

AC Susceptibility and Superconductivity

Sample can be considered as a solenoid

Bin = μ0nI = μ0MBout = 0

If such a sample is placed in a external magnetic field, Ha, thenBa = μ0 Ha

Thus, Bin(total) = μ0 (Ha + M)

In case of paramagnetic material, in the absence of any external magnetic field, M = 0. M is directly proportional to the external magnetic field. Thus,

M = χHa (χ : AC Susceptibility )

Thus, Bin(total) = μ0 (1+χ) Ha

In Case of Superconductivity , Bin(total) = 0, therefore, χ = -1

Principle of AC Susceptibility Measurement

B (Magnetic field by primary coil) : Kept small, on the order of few Oe, so as not to avoid local heating

f (Frequency of the field) : Increasing frequency increases not only the signal size but also the noise.

n, r (No. of turns , radius of pick up coil) : Signal size is directly proportional to number of turns and radius of coil, but there is a limitation to both of these parameter

when we are dealing with high pressure experiment.

Vs, Vc (Volume of sample and coil) : Increasing the ratio of Vs/Vc will make the signal stronger

Setting(AC Susceptibility Measurement)

Pick up Coil Cu wire (diameter = 16 μm)Diameter of coil = 300 μm

Primary Coil Cu wire (diameter = 100 μm) Number of turns = 350

Results

Setting 1Pressure Medium : Fluorinert 70 + 77 (1:1)Sample : Na0.16TiNCl and Pb

Pressure : 0.1GPa

Pb

6 8 10 12 14 16 18 20 22

EM

F (

V)

Temperature (K)

Fluctuations due to change in the heater

range!

Setting 2Pressure Medium : Fluorinert 70 + 77 (1:1)Sample : Na0.16TiNCl and Pb

Pressure : 0.1GPa

8 10 12 14 16 18 20 22

EM

F (

V)

Temperature (K)

Pb

Setting 3 ( No pressure medium )Sample : Na0.16TiNCl and Pb

Pressure : 0.1GPa

8 10 12 14 16 18 20 22 24

EM

F (

V)

Temperature (K)

Pb

6 8 10 12 14

EM

F (

V)

Temperature (K)

7 8 9 10 11 12 13 14 15

EM

F (

V)

Temperature (K)

Heating (1V)Heating (0.5V)

Pb

Pb

Resistivity Measurement( 4 probe Method) ‐

Sample

500 mm

Ruby

Gasket

Insulating Layer

0 50 100 150 200 250 3000

100

200

300

400

500

600

700

800

Resi

stan

ce (

Ω)

Temperature (K)

• 1GPa• 1.5 GPa

Summary

• Na0.16TiNCl shows

superconductivity at Tc = 18.1 K

at ambient pressure.

• Exact reason for no superconductivity observed when pressure is applied is not known, but possible reasons which can be given are :

– Sample is sensitive to air/moisture, so there is a chance that sample was contaminated while installing it between the diamonds.

– Maybe on applying pressure, it undergoes some structural change, and Na0.16TiNCl does not show superconductivity.

Next Plan

• Try to load sample inside the cell with even more care, so that is there is absolutely no contamination with air/moisture.

• Next time see the crystal structure of Na0.16TiNCl under pressure and see if there is any change in structure.

CeFe2

Introduction・ Lanthanide La Ce Pr Nd Sm Eu Gd Tb Dy Ho Tm Yb Lu

1s2

2s2 2p6

3s2 3p6 3d10

4s2 4p6 4d10 4f 1

5s2 5p6 5d1

6s2

n(r)

r/rB

Ce

Pm

Localized or delocalized

• Cubic C15 Laves Structure (MgCu2)

• Strongly hybridized compound between Fe 3d electrons and Ce 4f electrons.

Ce

Fe

CeFe2 v/s other REFe2 compounds

• CeFe2 has lower Tc (around 230K) than other REFe2 compounds(around 600K).

• The ferromagnetic ordering temperature is higher in the REFe2

compounds because of strong Fe-Fe interaction.

• In case of CeFe2 , the Fe3d - Fe3d interactions compete with Fe3d - Ce4f interactions, and thus has lower Tc.

Temperature Dependence of the Electrical Resistivity of REFe2, Gratz et al. , Solid State Communication, Vol.69,No. 10 (2011)

Competing Ferro-Antiferromagnetic interaction in CeFe2

Ferromagnetism in CeFe2 is unstable because of competition between Fe3d- Fe3d interaction (Ferromagnetic) and Fe3d - Ce4f interaction (Anti-

ferromagnetic).

Presence of such a delicate balance between Ferromagnetism and Anti-ferromagnetism in CeFe2 has been proved by two methods :

1. Doping CeFe2 with Co or Al

2. High Pressure

Antiferromagnetic order in pure CeFe2 under pressure, Braithwaite et al, Phys Rev B 76 (2007)

Previous Research(High Pressure)

On applying pressure, distance between Ce

and Fe decreases

3d - 4f interactions increases

Anti-ferromagnetic becomesprominent

Antiferromagnetic order in pure CeFe2 under pressure, Braithwaite et al, Phys Rev B 76 (2007)

Motivation

To know how this system behave under pressure when the 4f-3d hybridization is enhanced in CeFe2

Experiment Setting

100 mm

Gasket

My Results

120 140 160 180 200 220 240 260

0.40

0.45

0.50

0.55

0.60

0.65

Res

ista

nce

(Ohm

)

Temperature (K)

Tc

Tc ( My Experiment )

@ 0.25 GPa

Summary• CeFe2 is a strongly hybridized in which Anti-ferromagnetic state is unstable.

• Anti-ferromagnetic stability can be achieved by applying pressure or by doping pure CeFe2 with Co or Al

Next Plan

Apply higher pressure and study the effect of increased 3d-4f interactions in CeFe2