Electronic Response of a Mott Insulator at a Current-Induced Insulator-to-Metal Transition
Result of the Month
Schematic of transport-ARPES experiment. Band structure of Ca2RuO4 for the XM cut without and with current; the ruthenium (Ru) bands from EF to -2.5eV show a change in dispersion. Energy distribution curves (EDCs) taken across the displayed momentum space show that the gap has indeed been reduced in the L* (current-induced phase). Note, S, S*, and L* stand for insulating with no current, insulating with current, metallic with current. The dotted line shows that the sample spectra returns to the original pre-current flow. Valence bands are taken with pass energy 100, photon energy 74eV with σ-polarized light.
ARPES under current. (a) Experimental configuration for transport-ARPES. (b) I–V curve taken during the transport-ARPES experiment showing the negative differential resistance characteristic of the current-induced IMT. Inset shows contacts on the back side of the sample. (c) Spatial map of the cleaved section of the sample, measured by moving a 15 μm spot size across the surface in zero-current conditions. False colour represents the peak intensity of the Ca 3p core level. d–f, Variations of the Ca 3p core-level energy with sample position along the direction of current flow (y-axis) for the S (d), S* (e) and L* (f) phase; position range is indicated by the purple box in c. g,h, Correction of the energy shifts in the spectra: Ca 3p core levels (g) and valence bands (h) as measured for the three structurally distinct phases at (x, y) = (−1.37, 0.75). i,j, The same Ca 3p core levels (i) and valence bands (j) presented in g and h after correction for the energy shift by alignment of the Ca 3p core levels to that of the S phase. All spectra are taken at 74 eV with σ-polarized light. (k) Effect of broadening on the Ca 3p core level due to the electric potential present across a 15 μm (purple) and 400 μm (black) diameter beamspot: core levels displayed are integrated over the specified diameters from data shown in f; in addition to the broadening of the 400 μm curve, the shift in energy induced by the electric field can also be observed. Note, a.u. stands for arbitrary units.
To date, the widespread use of transport-ARPES has been precluded due to the difficulty of disassociating the effect of stray electric and magnetic fields from real current- or field-driven changes. In this image we see the potential gradient that develops across the sample as a consequence of current flow causes a deviation in energy and broadens spectral features; this gradient shifts the kinetic energy of emitted photoelectrons by the magnitude of the local potential at the emission position. We illustrate this in (d-f) by tracking the Ca 3p core-level binding energy: the peak shifts as the beam is physically moved along the direction of current flow. Note that the S* phase exhibits a larger shift, demonstrating that it experiences a larger local electric field than the L* phase at higher current (e,f), consistent with the negative differential resistance of the current-induced IMT. Although changes in the low-energy electronic states are expected to occur due to the IMT, core levels will be considerably less affected. As neither the core-level lineshapes nor the Ca 3p–O 2s energy difference changes evidently across the IMT (i), we conclude that the dominant cause of the energy shift is the electrostatic potential, rather than chemical shifts. This allows us to apply the extracted core-level shifts of 4.5 eV between the S and S* phases and 1.6 eV between the S and L* phases to the valence band spectra obtained at the same spot on the sample to expose the intrinsic current-induced modifications close to the chemical potential (h,j, and Image 1).
Comparison of current-induced metallic phase to high temperature metallic phase. (a)MDCs integrated over 0.5 eV for the L* phase along XM and YM (yellow) cuts, compared to the high-temperature L phase (red) cut. Note: the L phase MDC is labelled as both XM and YM because the same Fermi surface is measured when either axis is aligned parallel to the slit. (b) Fermi surface in the L* phase measured with the analyser slit oriented along the a-axis of the sample (top) and the b-axis of the sample (bottom). (c) Zero-current L phase Fermi surface measured at T = 384 K: that is, above TIMT = 357 K.
Distinct electronic signatures between the non-equilibrium current-induced metallic L* phase and the equilibrium high temperature metallic L phase are observed. In other words, the L* phase is not simply a product of Joule heating. Mean field theory calculations in our study show how a new pathway to achieve a metallic phase becomes accessible when current forces a directional dependence on the electron population in the Ru orbitals.