The Dynamic Mixed Metal–Insulator Phase in Self‐Heated Needle‐Like VO₂ Single Crystals

Abstract

Reported herein is the mixed metal–insulator phase induced in needle‐like VO2 single crystals by direct current (DC) electric currents applied at ambient temperature; it appears in the current‐controlled negative differential resistivity (CC‐NDR) regime of the nonlinear I–V characteristic governed by self‐heating. In this regime, the I–V characteristic is stable for RL ≥ |dV/dI|max, (RL—the load resistance); for lower RL, portions of I–V are unstable and switching occurs between initial and final steady states (SSs). In high‐quality needle‐like VO2 single crystals (cross‐sectional area < 2 × 10−5 cm2) the SS NDR regime is “unique,” whereas crossing from the insulating to the mixed phase I(V) is almost—or perfectly—smooth, the dissipation power (P = IV) versus current bends and this bend is accompanied by the appearance of insulating domains sliding along the metallic background in the sense of the current. The periodic domain emission, the variation of the dynamic parameters (domains’ frequency and sliding velocity) over the steady‐state NDR regime, and the energetics of the process are discussed. These results show that the process of domain emission and sliding is fairly expensive at low frequencies but there is some evidence that the process becomes more efficient at higher frequencies.

The Insulator–Insulator and the Insulator–Mixed‐Metal–Insulator Transitions in Self‐Heated AlxV1−xO2 Single Crystals and Their Visualization

“…No evidence was found in any Al‐doped sample (from 0.7% up to 2%) of sliding insulating domains within the metallic background, as in pure [ 3,4 ] or W‐doped VO 2. [ 18 ]…”

“…Self‐heating of VO 2 single crystals, induced by electric currents, is essential for the study of its mixed metal–insulator (M–I) state with its wide variety of dynamic or static domain structures. [ 3,4 ]…”

“…With increasing current, the temperature of the sample continues to increase (and the resistance to decrease) until T IMT is reached. Under steady‐state conditions, the onset of the mixed‐metal–insulator (mMI) state is smooth and is marked only by a bend in the P ( I ) trace (see, e.g., Figure in Fisher et al [ 3 ] ). For low R L , there is switching between an initial and a final steady state; the jump in the I – V characteristic is along the load line of the slope d I /d V = − R L −1 .…”

“…Instead, an mMI state sets in. The mMI phase in single crystals consists of static and/or sliding domains [ 3 ] and in films it consists of hot filaments.…”

“…Self-heating of VO 2 single crystals, induced by electric currents, is essential for the study of its mixed metal-insulator (M-I) state with its wide variety of dynamic or static domain structures. [3,4] Pure, unstrained single crystals of VO 2 are monoclinic (M1) and insulating at temperatures below T IMT ¼ 340 K, and tetragonal and metallic above this temperature. The phase diagram of strained [5] or doped insulating VO 2 (with Cr, [6,7] Al, [7][8][9][10] Mg, [11] or vanadium vacancies [12] ) consists, with increasing temperature above ambient, of a triclinic phase (T) followed by a monoclinic one (M2) prior to the metallic rutile phase (R).…”