Gap states in insulating LaMnPO1−xFx(x=0

Abstract

Infrared transmission and electrical resistivity measurements reveal that single crystals of LaMnPO1−xFx (x ≤ 0.28) are insulating. The optical gap obtained from transmission measurements is nearly unaffected by doping, decreasing only slightly from 1.3 eV in undoped LaMnPO to 1.1 eV for x = 0.04. The activation gaps obtained from electrical resistivity measurements are smaller by at least an order of magnitude, signalling the presence of states within the optical gap. At low temperatures, the resistivity is described well by variable range hopping conduction between these localized gap states. Analysis of the hopping conduction suggests that the gap states become slightly more delocalized with fluorine content, although metallic conduction is not observed even for fluorine concentrations as large as x = 0.28.Correlation gap insulators are distinct from simple band insulators in that the former require an electronelectron interaction to open a band gap at the Fermi level. This interaction typically stems from strong Coulomb repulsion (U) between the charge carriers, which may or may not be accompanied by further restrictions on orbital occupancy that are derived from the often subtle interplay between crystal field splitting and Hund's rules. These correlations can be weakened by carrier doping or by the application of high pressures [1], in either case resulting in the transformation of the correlation gap insulator into a correlated metal at a Mott-like electronic delocalization transition (EDT). It is believed that the correlation gap itself remains essentially unchanged by this process and that the EDT involves the delocalization of intrinsic states located within the gap [2]. Initially these states are highly localized by strong Coulomb interactions. Their density increases as the EDT is approached, however, and a band of correlated states forms as the correlations become weaker. In this way, the ingap states found in a correlation gap insulator are the intrinsic precursors of the correlated metal that forms once the EDT occurs, although the gap itself remains almost unchanged. Qualitatively speaking, this scenario is consistent with the wealth of experimental information available on the cuprates, where charge doping drives the magnetically ordered and insulating parent compounds metallic and ultimately superconducting [3].The more recent discovery of high superconducting critical temperatures (T c ) in the iron pnictide family, most notably T c = 55 K in F-doped SmFeAsO with the ZrCuSiAs structure [4], often draws comparison to the cuprate superconductors [5,6]. Nearly all compounds of the iron pnictide family so far reported are * jsimonson@bnl.gov metals, however, falling on the itinerant side of a possible EDT. Two insulating exceptions have been reported, but neither provides the perfect analogue to La 2 CuO 4 : (K,Tl)Fe x Se 2 [7] is subject to phase segregation and chemical inhomogeneity [8], while a doping study of compounds within the R 2 O 2 Fe 2 O(Se, S) 2 (R = La, Ce, Pr, Nd, Sm) system [9,10] h...

Magnetic and structural phase diagram of CaMn2Sb2

“…We observe an unexpectedly small Curie-Weiss moment that cannot be explained by the variety of highspin/low-spin transition often observed in 3d transition metal systems such as MnAs, suggesting that it results solely from the ferromagnetic state and that the majority of the moments are hidden by strong exchange coupling that dominates even in the paramagnetic state. Doping with Na simultaneously decreases both the resistivity and the IR transmission, but the optical gap remains unchanged, as was observed to be the case in other layered Mn pnictide systems 8 . On the other hand, La doping results in an anomalous increase of the electrical resistivity as the gap states are eliminated from the IR transmission spectra with the partial collapse of the optical gap.…”

“…The downturn typically observed upon reaching T N is absent in the La-doped samples, even if its signature remains in the χ', χ", and heat capacity measurements. With Na-doping, the observation of enhanced conductivity, lack of metallization, and minimal change in T N are reminiscent of our previous report of Fdoped LaMnPO 8 . The enhanced resistivity of La-doped CaMn 2 Sb 2 , however, represents an anomalous effect previously unreported to our knowledge, at least in the case of Mn pnictides.…”

Antiferromagnetism in semiconductingSrMn2Sb2andBaMn2Sb2single crystals

“…A high-pressure measurement on LaMnPO showed a semiconductor to metal transition at a pressure of 10 GPa [38]. Fluorine-doped LaMnPO 1−x F x also exhibits an insulator-to-metal transition on doping which only weakly affects T N and μ [39], similar to La 1−x Sr x MnAsO [40]. On the other hand, hydrogen-and deuterium-substituted LaMnAsO 1−x (H/D) x exhibits insulator-to-metal transitions associated with the emergence of itinerant ferromagnetism [41].…”

First principles study of the magnetic properties of LaOMnAs

“…La, Nd, Sm) that present a similar crystal structure as the materials mentioned above have also been synthesized and studied experimentally. [6][7][8][9][10][11][12][13] In contrast to their Fe-based cousins, at least until now these Mn-based oxypnictides have not shown any evidence of superconductivity even in doped cases. Instead, giant or even colossal magnetoresistance effects at room temperature have been observed in these compounds, [10][11][12][13] which reminds us of the well-known phenomenology of the colossal magnetoresistive manganites (Mn-based oxides) with the perovskite structures.…”