Neutron Scattering Study of a 

Triplet Superconductor

Mark Zic1,2, Paul M. Neves1,2, Kathryn Krycka1, Sheng Ran1,2, John 

Collini1,2, Wes Fuhrman1,2

, Johnpierre Paglione2, Nicholas P. Butch1,2

1NIST Center for Neutron Research, National Institute of Standards and Technology, 

Gaithersburg, MD 20899

2Center for Nanophysics and Advanced Materials, Department of Physics, University of 

Maryland, College Park, MD 20742
Outline

1. Introduction to superconductivity.

2. Superconductivity and magnetic fluctuations in UTe2.

3. Introduction to small angle neutron scattering (SANS).

4. How we used SANS to examine spin fluctuations in UTe2.

2
Superconductivity

The typical hallmarks of superconductivity are:

1. A transition to zero resistance.

2. Expulsion of a magnetic field.

3. An anomalous transition in heat capacity.

Too strong of a magnetic field can destroy the 

superconductivity and return the material to its normal 

state.

3
Exotic Superconductivity in UTe2

• Samples are grown using chemical vapor transport in an orthorhombic structure.

• When applying magnetic fields up to 20 T, the detrimental effects of the 

magnetic field appear to halt.

4

c

a

b

Spin Triplet

Spin Singlet

Superconducting pairs 

of electrons
Mediation of SC in UTe2

• Re-entrant superconductivity at magnetic fields up to 60 T.

• 15 T re-entrant superconductivity is not unusual for a ferromagnetic 

superconductor, but UTe2 never undergoes a ferromagnetic transition!

• Spin-fluctuations may influence superconductivity in UTe2.

5

Magnetic 

easy axis
• Charge density periodicity  X-Ray scattering

Nuclear periodicity

Magnetic periodicity

• Scattering takes the Fourier Transform of real 

space.

Introduction to SANS

6

}  Neutron scattering

II

xx qq

ρρ

Real space qq-space
SANS in UGe2

• SANS on triplet superconductor UGe2

showed that magnetic fluctuations exist 

above the Curie temperature. 

• Strong scattering only present along the 

magnetic easy axis.

• Fluctuations are present in II vs. qq as a 

Lorentzian:

aa1

aa2

2+1

where aa2 is the correlation length.

7 Spin x-coordinate

Spin y-coordinate

nn ∥ aa nn ⊥ aa
SANS in UTe2

• 90-milligram UTe2 sample aligned along 

the a-axis (easy axis) using the NIST Laue.

• Sample wrapped in Al foil with Cd mask 

and Al stand. 

• We used a horizontal split magnet to have 

the magnetic field along the beam.

8

Mounted UTe2

SANS sample

UTe2 pattern 

from Laue

0.5 in.

B-field

Sample

Neutron beam

Horizontal split magnet
SANS in UTe2

• Observed a weak signal and a flare from crystal facets:

9

0 T 4 T 8 T

• Solution: extract the magnetic component by using a polarized beam.

QQ

yy

(

Å

−

1

)

QQxx (Å−1)

(Right scale)

(Top scale)
Sample

Polarized Neutron Scattering

10

B Field

Selects up spin: 

or down spin:

Detector

Supermirror
SANS in UTe2

• Extract magnetic component with:

NNNN

NN2 ∝

dddddddd−uuuu

dddddddd+uuuu

∝

NN. 

• Signal does not increase with field as we expect!

11

MM

NN

QQ (10−2Å−1)

0 T 4 T 8 T

MM ≡ magnetic

NN ≡ nuclear
SANS in UTe2

• Taking a closer look at all fields we see that intensity “turns on” at about 3 T. 

12

0 2 4 6 8

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

( Down - Up ) / ( Down + Up ) Ratio

Applied Magnetic Field (T)
Future Steps

We would like to do more SANS measurements so we should:

• Grow larger samples/use an array of aligned samples.

(if possible, shape the sample to reduce flares)

• Have better temperature control; use a different cryostat.

• Use a cryostat able to go to lower temperatures.

• Use a more powerful magnet (> 8 T).

• Test for reproducibility.

13

Temperature (K)

Time (0.5 s)
Summary

• We introduced SANS as a useful technique for studying magnetic fluctuations.

• We proposed our understanding of the fundamental physics inside UTe2. 

• We observed some result of spin fluctuations in UTe2, but its temperature and 

field dependence are unclear.

• We need more measurements!

14

THANK YOU &
Sources

• Tinkham, Michael. Introduction to Superconductivity. Dover Publications, 2015.

• Kirshenbaum, Kevin, et al. “Superconductivity and Magnetism in Platinum-substituted SrFe2As2 single Crystals.” Physical 

Review B, vol. 82, no. 14, 2010, doi:10.1103/physrevb.82.144518.

• Ran, Sheng, et al. “Spontaneously polarized half-gapped superconductivity.” Preprint, arXiv:1811.11808

• Ran, Sheng, et al. “Extreme magnetic field-boosted superconductivity.” Preprint, arXiv:1905.04343

• Hutanu, V., et al. “Crystal structure of the unconventional spin-triplet superconductor UTe2 at low temperature by single 

crystal neutron diffraction.” Preprint, arXiv:1905.04377

• Bragg reflection picture: https://physics.stackexchange.com/questions/297554/braggs-diffraction-huygens-principle

• SANS picture: https://www.nist.gov/ncnr/ng7-sans-small-angle-neutron-scattering

• Fourier transform picture: http://cvarin.github.io/CSci-Survival-Guide/fft.html 

• Haslbeck, F., et al. “Ultrahigh-Resolution Neutron Spectroscopy of Low-Energy Spin Dynamics in UGe2.” Physical Review B, 

vol. 99, no. 1, 2019, doi:10.1103/physrevb.99.014429.

• Knife edge diffraction picture: http://www2.oberlin.edu/physics/catalog/demonstrations/optics/knifeedge.html

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