Temperature Dependence

At 300K, the LCMO epitaxial thin films studied are above Tc and exist in the paramagnetic phase,

leading to significantly different spectral characteristics from those observed in the bulk ferromagnetic state atT = 77 K. This is reflected in Figures 5.4(a)-(b) and 5.4(d)-(e) for exemplified spectra taken at 300 K and with Pt/Ir and Cr-coated tips, respectively. Comparison of Figures 5.4(a)-(b) and 5.4(d)-(e) with Figures 5.4(a)-(b) and 5.4(e)-(f) indicate several important developments with the change in temperature. First, the largeU±peaks in the DOS for ferromagnetic LCMO became much

suppressed in the paramagnetic state. Second, the surface insulating gap found around the Fermi level at 77 K either evolved into a pseudogap (Figures 5.4(a) and 5.4(d)) or completely disappeared (Figures 5.4(b) and 5.4(e)). These observation are corroborated by the histograms of the insulating gap ∆± in Figures 5.4(c) and 5.4(f), where the large number of counts at both zero and the

pseudogap energiesh∆∗₊iare shown. The disappearing insulating gaps at 300 K for selected spectra cannot be accounted for by thermal smearing, and are therefore suggestive of a magnetic phase transition occurring at a mean transition temperature between 77 K and 300 K. In contrast, the nearly temperature independent pseudogap energies are suggestive of a completely separate physical origin for the pseudogap. Third, slight variations in the spectra were found between those taken with the Pt/Ir tip and those taken with the Cr-coated tip at 300 K, as manifested in Figures 5.4(a)-(b) and 5.4(d)-(e). The spectral differences between different tip types can be accounted for because spectra taken with the Pt/Ir were representative of the DOS of LCMO in the paramagnetic phase, whereas those taken with Cr coated consisted of convoluted DOS of paramagnetic LCMO and magnetic Cr-coated tip.

Spatial variations in the characteristics of the spectra with temperature were also accompanied by the temperature-dependent evolution of the spatial variation in the tunneling conductance. The tunneling conductance in the paramagnetic state was generally more homogeneous than that in

Figure 5.4: Comparison of the normalized tunnel conductance ( ¯G) spectral characteristics taken with Pt/Ir and Cr-coated tips at T = 300 K and H = 0. (a) A psuedogap-like spectrum taken with a Pt/Ir tip, showing greately suppressedU_±∗ values relative to the peak energies U± found in

the spectrum of Figure 5.2(a). The psuedogap values ∆∗_± are determined from the maximum of the (d2_I/dV2_{)-vs.-V spectrum, as displayed in the inset. (b) Another spectral type taken with a Pt/Ir} tip, showing a non zero conductance at the Fermi level and vanishing insulating gap, as shown in the inset. (c) Histograms of the insulating gap values ∆± and the peak energiesU±∗ obtained by using

a Pt/Ir tip over a (500×500) nm2 _{area at 300 K, showing greately decreased U}∗

± values relative

to theU± values found at 77 K, as well as large population of vanishing insulating gaps (shown by

the arrows at ω = 0) and pseudogaps at ω =h∆∗_±i. (d) A typical pseudogap-like spectrum taken with a Cr-coated tip, showing greately decreasedU_±∗ values relative to the peak energiesU± found

in the spectrum of Figure 5.2(e). The pseudogap values ∆∗_± are determined from the maximum of the (d2I/dV2)-vs.-V spectrum. (e) An additional typical type of spectra taken with a Cr tip, showing vanishing insulating gaps as detailed in the inset. (f) Histograms of the insulating gap values ∆± and the characteristic energiesU±∗ obtained by using a Cr-coated tip over a (500×500)

nm2area at 300 K, showing decreasedU_±∗ values relative to theU± values found at 77 K, as well as

a large population of vanishing insulating gaps (shown by the arrows atω= 0) and pseudogaps at ω=h∆∗_±i.

Figure 5.5: Comparison of the high-bias normalized tunnel conductance ( ¯G) spectral characteristics taken with Pt/Ir and Cr-coated tips atH= 0 for T=77K and 300K: (a) A (500×500) nm2_tunneling conductance map taken with a Pt/Ir tip atω =hU+i and 300 K. (b) Histograms of the tunneling conductance obtained by using a Pt/Ir tip and a Cr-coated tip at 300 K and evaluated forω=hU+i. (c) A (500×500) nm2 tunneling conductance map taken with a Cr-coated tip atω=hU+iand 300 K. (d) A (500×500) nm2tunneling conductance map taken over a different sample area as (a) with a Pt/Ir tip atω=hU+iand 77 K. (e) Histograms of the tunneling conductance obtained by using a Pt/Ir tip and a Cr-coated tip at 77 K and forω=hU+i.(f) A (500×500) nm2tunneling conductance map taken over a different sample area as in (b) with a Cr-coated tip at the characteristic energy ω=hU+iand 77 K.

Figure 5.6: Comparison of the low-bias normalized tunnel conductance ( ¯G) spectral characteristics taken with Pt/Ir and Cr-coated tips atH = 0: (a) A (500×500) nm2_{tunneling conductance map} taken with a Pt/Ir tip atω=h∆∗

+iand 300 K. (b) Histograms of the tunneling conductance obtained by using a Pt/Ir tip and a Cr-coated tip at 300 K forω =h∆∗₊i. (c) A (500×500) nm2 _tunneling conductance map taken with a Pt/Ir tip at ω = h∆∗₊i and for T = 77 K. (d) Histograms of the tunneling conductance obtained by using a Pt/Ir tip and a Cr-coated tip at 77 K and forω=h∆+i. (e) A (500×500) nm2 _{tunneling conductance map taken with a Cr-coated tip at the characteristic} energyω=h∆+iand forT = 77 K.

the ferromagnetic state due to the tendency toward phase separation in the ferromagnetic state of LCMO, as reflected by the constant-bias tunneling conductance maps in Figures 5.5(a) and 5.5(c) for room temperature spectra taken atω=hU+iwith the Pt/Ir and Cr-coated tips, respectively. In contrast, the tunneling conductance upon cooling the sample into the ferromagnetic state developed significantly more heterogenity, as shown in Figures 5.5(d) and 5.5(f) for tunneling conductance taken at 77 K and forω=hU+i. HerehU±iare defined as the most prevalentU± value observed in

the histograms in Figures 5.2(c)-(d) and 5.2(g)-(h). The statistical distributions of the conductance at ω =hU±i for 77 K and 300 K are summarized by the histograms in Figures 5.5(b) and 5.5(e)

respectively for spectra taken with both the Pt/Ir and Cr-coated tips. While the histograms at 300 K showed statistical similarity between spectra taken with Pt/Ir and Cr-coated tips as demonstrated in Figure 5.6(b), at 77 K the tunneling conductance distributions shift to higher values for spectra taken with the Cr-coated tip as compared with those measured with the Pt/Ir tip. The apparent differences between the histograms obtained with Pt/Ir and Cr-coated tips from LCMO at 77 K can be accounted for by the spin-polarized nature of the tunnel current from the Cr tip into spatially inhomogeneous LCMO in its ferromagnetic phase vs the non-polarized Pt/Ir tip tunnel current.

Similarly, the tunneling conductance maps forω=h∆+itaken at 300 K with Pt/Ir and Cr-coated tips are shown in Figures 5.6(a) and 5.6(c), respectively, whereas those for ω=h∆+itaken at 77 K with Pt/Ir and Cr-coated tips are shown in Figures 5.6(d) and 5.6(f). These maps again reveal more spatial inhomogeneity in the tunneling conductance in the ferromagnetic state as compared to the paramagnetic state. For completeness, the statistical distributions of the tunneling conductance at ω = h∆∗+i for T = 300 K and ω = h∆+i for T = 77 K are summarized by the histograms in Figures 5.6(b) and 5.6(e). Hereh∆+i denotes the most commonly occurring insulating gap value at positive bias from Figures 5.2(c)-(d) and 5.2(g)-(h) forT = 77 K, andh∆∗₊irepresents the most commonly found pseudogap values from Figures 5.4(c) and 5.4(f) forT = 300 K.

Figure 5.7: Comparison of magnetic field-dependent spectral characteristics taken over the same sample area with a Cr-coated tip at T = 6 K: (a) Normalized conductance (dI/dV)/(I/V) ( ¯G) vs. energy for H = 0. (b) Normalized conductance vs. energy for H =−0.3 T. (c) Normalized conductance vs. energy for H = 3.0 T. (d) Histograms of the characteristic energy U+ over the same (90×250) nm2 _{sample area for H = 0, −0.3 T and 3.0 T. (e) Histograms of ∆+ over the} same (90×250) nm2 _{sample area at H = 0,−0.3 T and 3.0 T. (f) Temperature evolution of the} histograms of ∆+ over a (90×250) nm2 _{sample area at H = 0, showing shift of insulating gap} values downward with increasing temperature. In particular, two types of gap values at 6 K may be attributed to an insulating surface gap and a pseudogap, while only the psuedogap persists at 300 K.