UPt3: More data after all these years

Boston College, Physics Department, Chestnut Hill, MA, USA C. P. Opeil, S.J., M. J. Graf

A. de Visser University of Amsterdam, Van der Waal-Zeeman Institute, Amsterdam, The Netherlands

The UPt3 Story Begins on a Friday Afternoon:

UPt3

UBe13

CeCu2Si

Fisher et al., PRL 62,(89).

van Dijk, Physica B 186 (93)

UPt3 exhibits Unconventional - SC (Tc ~ 0.5K):

UPt3 Spin Fluctuation Behavior 1) Crystal Structure HCP

2) Thermodynamic Signature of Spin Fluctuations - Specific Heat: T3ln(T/TSF) at low T, AFM correlations in Magnetism, TSF ~ 18 K, B // a-b

3) Neutron scattering [Aeppli, G. et al. PRB32(85)] -

AFM and FM spin fluctuations

B // a,b axis

B // c axis

Frings et al. PRB 31 (85)

u Stabilization of AFM correlations starting at ~ 30 K along c (Goldman, PRB,36 (1987))

u Low T, T3ln(T/TSF) typical of SF systems

u HF γ = 422 mJ/K2 mole

UPt3

UPt3

Stewart et al. PRL 52 (84)

TSF ~ 18 K

c axis

a-b plane

Ferromagnetic Correlations In-plane, exist up to T~150 K

Antiferromagnetic Correlations between planes, appear at T<30 K

Why not pressure? Drives system away from magnetic instability, has minimal effect on

superconductivity

Pd (or Au) for Pt, or Th for U strengthen AFM in UPt3 Opposite effect compared to pressure, presumably because all these

substitutions slightly reduce c/a ratio in this HCP system

U(Pt1-xPdx)3 is most studied x ≤ 0.005: size of ordered moment increases, but onset temperature is

constant, and it is still only observable by neutron scattering

x > 0.01: onset of conventional AFM with same magnetic structure, much larger ordered moment, and detectable by bulk and local

probes

Using impurities to study the evolution of magnetism and superconductivity

Keizer et al., PRB, 1999

de Visser et al., PRL, 2000

Magnetic Phase Diagram of U(Pt,Pd)3

Þ Small-moment AF

“order” for x ≤ 0.01

Þ Large-moment AF

“order” for 0.007 ≤ x ≤ 0.08

Þ Superconductivity suppressed at xSC ~ 0.006

0.0

0.4

0.8

1.2

1.6

0.0 0.2 0.4 0.6 0.8 1.0

Tem

pera

ture

(K)

Pd Concentration (at. %)

Phase diagram: detailed for x < xc

M. J. Graf et al., Physica B 284-8, 1281 (2000).

Weak AFM

SC AFM

Tem

pera

ture

(K) QCP?

DC Magnetization Measurements

!mol =µ0Mmol

B

TSF

χ(H, Tconstant)

χ(Hconstant, T)

Magnetization shown to be linear up to

B = 9 T

From Opeil et al., Physica B, June 2002

12

16

20

24

-0.1 0.0 0.1 0.2

Pd Th Pressure

T SF (

K)

10 3 !(c/a)/(c/a)

Variation of c/a tunes magnetism

Qualitative agreement: Pd (and Th) doping,

Hydrostatic Pressure and the change in the c/a ratio

Pressure Doping

Hydrostatic pressure increases c/a Pd, Th decrease c/a (Pd shrinks lattice, Th expands it)

Pd and Th provide “negative chemical pressure”

1

10

100

1 10 100

Rho-0.0%Rho-0.4% Rho-0.5%Rho-0.6%Rho-0.7%Rho-0.8%Rho-1.0%Rho-1.4%Rho-2.0%

Res

istiv

ity (µO

hm-c

m)

Temperature (K)

Normal State Resistivity of U(Pt1-xPdx)3 0.3 < T< 300 K

x = 0.020

TN

x = 0.000

U(Pt1-xPdx)3 B = 0 Tesla

Electronic Transport/Resistivity Direct Evidence

x=0.050 data van Sprang (Thesis UvA, unpublished) data scaled for comparison

TN vanishes abruptly between

x = 0.013 - 0.014

in resistivity, but µSR says no!

0.0

0.4

0.8

1.2

1.6

0.0 0.2 0.4 0.6 0.8 1.0

Tem

pera

ture

(K)

Pd Concentration (at. %)

Is there any evidence for quantum critical behavior near xc?

Power law resistivity: ρ = ρ0 + AT α

From Opeil et al., Physica B (2002) αeff = dln(ρ-ρ0)/dlnT constant at low T -- true power law

1.4

1.6

1.8

2.0

0 0.005 0.010 0.015 0.020Ex

pone

nt !

xPd

xc

Exponent reaches minimum value near 1.5 at concentration near xc

FL α = 2

NFL α ≠ 2, 3Dim - AFM → α = 3/2 Look for deviations from T2 behaviors

A. Rosch, Physical Review Letters, 82, 4280 (1999). Opeil and Graf, Physica B (2002)

NFL Behavior? Disorder & Impurity Effects

AFM 3D, αEFF = 3/2

Can we infer a QCP from resistivity data? Evidence for QCP 1) Decrease of temperature dependent exponent α to near 1.5 2) TN → 0 K at xc = 0.006 3) Power law behavior of xc = 0.006 consistent with a strongly disordered NFL (Rosch). No bump upon entering the FL regime. Evidence against QCP 1) Exponent for pure UPt3 is only 1.75, varies from sample to sample 2) No perfect agreement with Temperature dependence in Rosch model 3) Disorder can cause loss of coherence of spin fluctuations α = 2, α → 3/2

Riseborough, PRB 29(84): SF scattering and disorder interplay...loss of SF coherence Lui et al., PRB 61(00): NFL arises from disorder...e.g. CeRhRuSi2 Conclusion: Evidence is ambiguous from Electronic Transport!

U(Pt,Pd)3 Polycrystal (6T) SCM- Measurements

TN vanishes between x = 0.014 and 0.010

graphs scaled

for clarity

U(Pt,Pd)3 Single Crystal

SCM- Measurements

Except by neutrons, No TN for x = 0.010 single crystal B = 6 T

Opeil et al., JMMM 272-276 (2004).

NFL behavior in xc = 0.006 ?

NFL behavior associated with a QCP manifests itself by an upturn in the magnetic susceptibility i.e. χ(T) ∝ - βT1/2 (3D behavior) or - β lnT (2D behavior) as TN → 0 K.

Known NFL/QCP behavior in CeCu5.9Au0.1

Field B = 3 T

Field B = 0 T

Specific Heat of U(Pt1-xPdx)3

Nothing special happens at x = 0.006

Specific Heat of U(Pt1-xPdx)3

Nothing special happens at x = 0.006

AF QCP (3D) Divergence as T→0 K γ ~ -ln(T/T0) in Specific Heat

Based on the SCM and specific heat data, we believe the on set of LMAF is a crossover behavior not the result of a true phase transition.

Detection depends on the timescale of the probe

Y. Okuno and K. Miyake, J. Phys. Soc. Japan 67 (1998).

Very broad transition into conventional AFM

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8

Frac

tiona

l Mag

netic

Sig

nal

Temperature (K)

x = 0.05

x = 0.02

x = 0.01

1% Pd 1% Th

G(t) = A123e!"1 t cos # t( ) +

13e!"1

' t$ % &

' ( ) + A2GKL (t) + A3GKT (t)

Magnetic Paramagnetic or weak AFM

Graf et al., Phys. Rev. B 68, 224421 (2003)

Fractional magnetic signal = (A1+A2)/(A1+A2+ A3)

Th - solid lines

Pd - dashed lines

Why is the transition region so broad? Speculation: Th substitution slows down magnetic fluctuations of weak AFM

Observable in µSR at 7 K Fluctuation

Timescale [Sec]

mag. susc. NMR neutron specific heat ZF-µSR scattering

10-3 10-11 10-7 100

Pure UPt3 1% Pd

1% Th

time domains Y. Okuno and K. Miyake, J. Phys. Soc. Japan 67 (1998).

Conclusions:

1) The change in TSF quantitatively correlates to the change c/a ratio.

2) Resistivity behavior is consistent with NFL, but not distinguishable from disorder effects.

3) No confirmation of NFL nor proposed QCP found by Cantilever Magnetometry or Specific Heat at x = 0.006. NFL behavior arised from magnetic instability and SF scattering.

4) AFM ordering (LMAF) at TN is not observed for x = 0.010 by bulk magnetization for U(Pt,Pd)3 single and polycrystals.

5) We believe that the transition from SMAF to LMAF is a crossover behavior. Pd slows down the temporal fluctuations of the SMAF phase.

Future Work: (Graf Dilution Refrigerator: Boston College)

1) Specific Heat and Cantilever Magnetometry (0.010 < T < 1 K) on U(Pt1-xPdx)3 0 < x < 0.020 to examine the affect of doping on the new AFM ordering reported at (~ 18 mK) by Schubeth et. al. (c/T vs T) and Schöttl et al.(τ vs T).

2) Hydrostatic Pressure studies on U(Pt1-xPdx)3 0.005 < x < 0.010 single crystals to examine the possibility of re-entrant (unconventional) SC and impurity effects about the AF magnetic instability.

With special thanks to J. L. Smith:

Happy

65th Birthday!