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|Title: ||Novel Metallic States at Low Temperatures in Strongly Correlated Systems|
|Authors: ||Wu, Wenlong|
|Advisor: ||Julian, Stephen R.|
|Keywords: ||Quantum Criticality|
|Issue Date: ||2-Sep-2010|
|Abstract: ||This thesis describes experiments carried out on two novel strongly correlated electron
systems. The first, FeCrAs, is a new material that has not been studied before, while the
second, Sr3Ru2O7, has been previously shown to have a very novel so-called ‘nematic’
phase around the metamagnetic quantum critical end point (QCEP).
For these studies, a new variation on an established method for measuring the field
dependence of susceptibility in a BeCu clamp cell has been developed, and is described,
as is a relaxation heat capacity cell that works from 4 K down to 300 mK.
A method of growing stoichiometric crystals of the hexagonal iron-pnictide FeCrAs
has been developed, and transport and thermodynamic measurements carried out. The
in-plane resistivity shows an unusual “non-metallic” dependence on temperature T, rising
continuously with decreasing T from ∼800 K to below 100 mK. The c-axis resistivity is
similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N ∼ 125 K.
Below 10 K the resistivity follows a non-Fermi-liquid power law, ρ(T) = ρ_0 − AT^x with
x < 1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld
coefficient, γ ∼ 30 mJ/molK^2. The high temperature properties are reminiscent of those
of the parent compounds of the new layered iron-pnictide superconductors, however the
T → 0 K properties suggest a new class of non-Fermi liquid.
The metamagnetic critical end point temperature T^∗ in Sr3Ru2O7 as a function of
hydrostatic pressure with H//ab has been studied using the ac susceptibility. It is found that T^∗ falls monotonically with increasing pressure, going to zero at Pc = 14±0.3 kbar.
One sign of the nematic phase observed in the field-angle tuning, i.e. T^∗ rises as the
novel phase emerges, has not been seen in our study. However, we see a slope change
in T^∗ vs P at ∼12.8 kbar, and a shoulder at the upper field side of the peak in χ′ from
∼12.8 kbar to ∼16.7 kbar. These new features indicate that some new physics sets in
near the pressure-tuned QCEP.|
|Appears in Collections:||Doctoral|
Department of Physics - Doctoral theses
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