110 research outputs found
The Anderson-Mott transition induced by hole-doping in Nd1-xTiO3
The insulator/metal transition induced by hole-doping due to neodymium
vacancies of the Mott- Hubbard antiferromagnetic insulator, Nd1-xTiO3, is
studied over the composition range 0.010(6) < x < 0.243(10). Insulating p-types
conduction is found for x < 0.071(10). Anderson localization in the presence of
a Mott-Hubbard gap, is the dominant localization mechanism for the range of
0.074(10) < x < 0.089(1) samples. For x < 0.089(1), n-type conduction is
observed and the activation energy extrapolates to zero by x < 0.1. The
0.095(8) < x < 0.203(10) samples are Fermi-liquid metals and the effects of
strong electronic correlations are evident near the metal-to-insulator
boundaries in features such as large Fermi liquid T2 coefficients. For 0.074(9)
< x < 0.112(4), a weak negative magnetoresistance is found below ~ 15 K and it
is attributed to the interaction of conduction electrons with Nd3+ magnetic
moments. Combining information from our companion study of the magnetic
properties of Nd1-xTiO3 solid solution, a phase diagram is proposed. The main
conclusions are that long range antiferromagnetic order disappears before the
onset of metallic behavior and that the Anderson-Mott transition occurs over a
finite range of doping levels. Our results differ from conclusions drawn from a
similar study on the hole doped Nd1-xCaxTiO3 system which found the
co-existence of antiferromagnetic order and metallic behavior and that the Mott
transition occurs at a discrete doping level
Field-induced Bose-Einstein Condensation of triplons up to 8 K in Sr3Cr2O8
Single crystals of the spin dimer system Sr3Cr2O8 have been grown for the
first time. Magnetization, heat capacity, and magnetocaloric effect data up to
65 T reveal magnetic order between applied fields of Hc1 ~ 30.4 T and Hc2 ~ 62
T. This field-induced order persists up to ~ 8 K at H ~ 44 T, the highest
observed in any quantum magnet where Hc2 is experimentally-accessible. We fit
the temperature-field phase diagram boundary close to Hc1 using the expression
Tc = A(H-Hc1)^v. The exponent v = 0.65(2), obtained at temperatures much
smaller than 8 K, is that of the 3D Bose-Einstein condensate (BEC) universality
class. This finding strongly suggests that Sr3Cr2O8 is a new realization of a
triplon BEC where the universal regimes corresponding to both Hc1 and Hc2 are
accessible at He-4 temperatures.Comment: 4 pages, 3 figures, accepted by PR
Magnetic Order and Fluctuations in the Presence of Quenched Disorder in the Kagome Staircase System (Co(1-x)Mg(x))3V2O8
Co3V2O8 is an orthorhombic magnet in which S=3/2 magnetic moments reside on
two crystallographically inequivalent Co2+ sites, which decorate a stacked,
buckled version of the two dimensional kagome lattice, the stacked kagome
staircase. The magnetic interactions between the Co2+ moments in this structure
lead to a complex magnetic phase diagram at low temperature, wherein it
exhibits a series of five transitions below 11 K that ultimately culminate in a
simple ferromagnetic ground state below T~6.2 K. Here we report magnetization
measurements on single and polycrystalline samples of (Co(1-x)Mg(x))3V2O8 for
x<0.23, as well as elastic and inelastic neutron scattering measurements on
single crystals of magnetically dilute (Co(1-x)Mg(x))3V2O8 for x=0.029 and
x=0.194, in which non-magnetic Mg2+ ions substitute for magnetic Co2+. We find
that a dilution of 2.9% leads to a suppression of the ferromagnetic transition
temperature by ~15% while a dilution level of 19.4% is sufficient to destroy
ferromagnetic long-range order in this material down to a temperature of at
least 1.5 K. The magnetic excitation spectrum is characterized by two spin-wave
branches in the ordered phase for (Co(1-x)Mg(x))3V2O8 (x=0.029), similar to
that of the pure x=0 material, and by broad diffuse scattering at temperatures
below 10 K in (Co(1-x)Mg(x))3V2O8 (x=0.194). Such a strong dependence of the
transition temperatures to long range order in the presence of quenched
non-magnetic impurities is consistent with two-dimensional physics driving the
transitions. We further provide a simple percolation model that
semi-quantitatively explains the inability of this system to establish
long-range magnetic order at the unusually-low dilution levels which we observe
in our experiments.Comment: 10 pages, 13 figure
Gapped and gapless short range ordered magnetic states with wavevectors in the pyrochlore magnet TbTiO
Recent low temperature heat capacity (C) measurements on polycrystalline
samples of the pyrochlore antiferromagnet TbTiO
have shown a strong sensitivity to the precise Tb concentration , with a
large anomaly exhibited for at K and no such
anomaly and corresponding phase transition for . We have grown single
crystal samples of TbTiO, with approximate
composition , and , where the single
crystal exhibits a large C anomaly at =0.45 K, but neither the
nor the single crystals display any such anomaly. We
present new time-of-flight neutron scattering measurements on the
and the samples which show strong
quasi-Bragg peaks at low
temperatures characteristic of short range antiferromagnetic spin ice (AFSI)
order at zero magnetic field but only under field-cooled conditions, as was
previously observed in our single crystal. These results show that
the strong quasi-Bragg peaks
and gapped AFSI state at low temperatures under field cooled conditions are
robust features of TbTiO, and are not correlated with the presence
or absence of the C anomaly and phase transition at low temperatures.
Further, these results show that the ordered state giving rise to the C
anomaly is confined to for
TbTiO, and is not obviously connected with
conventional order of magnetic dipole degrees of freedom.Comment: 7 pages, 3 figure
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