m1 colloquium presentation arora varun 29a13106 (shimizu lab) high pressure study of na x tincl and...
TRANSCRIPT
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M1 Colloquium Presentation
Arora Varun
29A13106 (Shimizu Lab)
High Pressure Study of NaxTiNCl and CeFe2
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Contents
• Introduction• Motivation• Experimental Method• Results• Summary
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Intercalated MNX Compound(NaxTiNCl)
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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)
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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)
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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)
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Previous Research onNa0.16TiNCl
Superconducting Nitride Halides (MNX), Christian M. Schurz et al. Zeitschrift for Kristallographie 226, 395-416 (2011)
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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)
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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)
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AC - Susceptibility Experiment
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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
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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
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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
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Results
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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!
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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
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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
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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
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Resistivity Measurement( 4 probe Method) ‐
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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
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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.
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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.
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CeFe2
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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
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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)
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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)
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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)
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Motivation
To know how this system behave under pressure when the 4f-3d hybridization is enhanced in CeFe2
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Experiment Setting
100 mm
Gasket
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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
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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
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Next Plan
Apply higher pressure and study the effect of increased 3d-4f interactions in CeFe2