arXiv subjects

H. A. Dabkowska

Publications and source records attributed to H. A. Dabkowska.

52 records · Page 3Linked to original sources

arXiv2008

Revisiting Static and Dynamic Spin Ice Correlations in Ho2Ti2O7

Elastic and inelastic neutron scattering studies have been carried out on the pyrochlore magnet Ho2Ti2O7. Measurements in zero applied magnetic field show that the disordered spin ice ground state of Ho2Ti2O7 is characterized by a pattern of rectangular diffuse elastic scattering within the [HHL] plane of reciprocal space, which closely resembles the zone boundary scattering seen in its sister compound Dy2Ti2O7. Well-defined peaks in the zone boundary scattering develop only within the spin ice ground state below ~ 2 K. In contrast, the overall diffuse scattering pattern evolves on a much higher temperature scale of ~ 17 K. The diffuse scattering at small wavevectors below [001] is found to vanish on going to Q=0, an explicit signature of expectations for dipolar spin ice. Very high energy-resolution inelastic measurements reveal that the spin ice ground state below ~ 2 K is also characterized by a transition from dynamic to static spin correlations on the time scale of 10^{-9} seconds. Measurements in a magnetic field applied along the [1${\bar1}$0] direction in zero-field cooled conditions show that the system can be broken up into orthogonal sets of polarized alpha chains along [1${\bar1}$0] and quasi-one-dimensional beta chains along [110]. Three dimensional correlations between beta chains are shown to be very sensitive to the precise alignment of the [1${\bar1}$0] externally applied magnetic field.

cond-mat.str-el↗

arXiv2008

Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er2Ti2O7

We report detailed measurements of the low temperature magnetic phase diagram of Er$_2$Ti$_2$O$_7$. Heat capacity and time-of-flight neutron scattering studies of single crystals, subject to magnetic fields applied along the crystallographic [110] direction, reveal unconventional low energy states. Er$^{3+}$ magnetic ions reside on a pyrochlore lattice in Er$_2$Ti$_2$O$_7$, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of non-collinear phases, divided by a zero temperature phase transition at $μ_0 H_c \sim$ 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.

cond-mat.str-el↗

arXiv2008

Fractalization drives crystalline states in a frustrated spin system

We measure a sequence of quantum Hall-like plateaux at 1/q: 9 >= q >= 2 and p/q = 2/9 fractions in the magnetisation with increasing magnetic field in the geometrically frustrated spin system SrCu2(BO3)2. We find that the entire observed sequence of plateaux is reproduced by solving the Hofstadter problem on the system lattice when short-range repulsive interactions are included, thus providing a sterling demonstration of bosons confined by a magnetic and lattice potential mimicking fermions in the extreme quantum limit.

cond-mat.str-el↗

arXiv2008

Critical X-ray Scattering Studies of Jahn-Teller Phase Transitions in TbV$_{1-x}$As$_{x}$O$_{4}$

The critical behaviour associated with cooperative Jahn-Teller phase transitions in TbV$_{1-x}$As$_{x}$O$_{4}$ (where \textit{x} = 0, 0.17, 1) single crystals have been studied using high resolution x-ray scattering. These materials undergo continuous tetragonal $\to$ orthorhombic structural phase transitions driven by Jahn-Teller physics at T$_C$ = 33.26(2) K, 30.32(2) K and 27.30(2) K for \textit{x} = 0, 0.17 and 1 respectively. The orthorhombic strain was measured close to the phase transition and is shown to display mean field behavior in all three samples. Pronounced fluctuation effects are manifest in the longitudinal width of the Bragg scattering, which diverges as a power law, with an exponent given by $x=0.45 \pm 0.04$, on approaching the transition from either above or below. All samples exhibited twinning; however the disordered x = 0.17 sample showed a broad distribution of twins which were stable to relatively low temperatures, well below T$_C$. This indicates that while the orthorhombic strain continues to develop in a conventional mean field manner in the presence of disorder, twin domains are easily pinned by the quenched impurities and their associated random strains.

cond-mat.str-el↗

arXiv2007

Diagonal and Collinear Incommensurate Spin Structures in underdoped La$_{2-x}$Ba$_{x}$CuO$_{4}$

We have studied incommensurate spin ordering in single crystal underdoped La_{2-x}Ba_{x}CuO_{4} with x~0.08, 0.05 and 0.025 using neutron scattering techniques. Static incommensurate magnetic order is observed in the La_{2-x}Ba_{x}CuO_{4} (x=0.05 and 0.025) compounds with ordering wavevectors which are rotated by 45 degree about the commensurate (0.5,0.5,0) position, with respect to that in the superconducting x=0.08 material. These spin modulations are one dimensional in the x=0.05 and 0.025 samples, with ordering wavevectors lying along the orthorhombic b* direction. Such a rotation in the orientation of the static spin ordering as a function of increasing Ba doping, from diagonal to collinear, is roughly coincident with the transition from an insulating to a superconducting ground state and is similar to that observed in the related La_{2-x}Sr_{x}CuO_{4} system. This phenomenon is therefore a generic property of underdoped La-214 cuprates.

cond-mat.str-el↗

arXiv2007

Neutron and X-ray Scattering Studies of the Lightly-Doped Spin-Peierls System Cu(1-x)Cd(x)GeO3

Single crystals of the lightly-doped spin-Peierls system Cu(1-x)Cd(x)GeO3 have been studied using bulk susceptibility, x-ray diffraction, and inelastic neutron scattering techniques. We investigate the triplet gap in the magnetic excitation spectrum of this quasi-one dimensional quantum antiferromagnet, and its relation to the spin-Peierls dimerisation order parameter. We employ two different theoretical forms to model the inelastic neutron scattering cross section and chi''(Q,omega), and show the sensitivity of the gap energy to the choice of chi''(Q,omega). We find that a finite gap exists at the spin-Peierls phase transition.

cond-mat.str-el↗

arXiv2007

High resolution X-ray scattering studies of structural phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$

We have studied structural phase transitions in high quality underdoped La$_{2-x}$Ba$_x$CuO$_4$ single crystals using high resolution x-ray scattering techniques. Critical properties associated with the continuous High Temperature Tetragonal (HTT, $I4/mmm$) to Middle Temperature Orthorhombic (MTO, $Cmca$) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08 and we find that all behavior is consistent with three dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the Low Temperature Tetragonal (LTT, $P4_{2}/ncm$) phase, below the strongly discontinuous MTO$\to$LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wavevectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases, or that it is orthorhombic with very small orthorhombic strain, consistent with the ``less orthorhombic" low temperature structure previously reported in mixed La$_{2-x}$Sr$_{x-y}$Ba$_y$CuO$_4$ single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$ and find good agreement with results obtained on polycrystalline samples.

cond-mat.supr-con↗

arXiv2007

Structural Fluctuations in the Spin Liquid State of Tb2Ti2O7

High resolution X-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin liquid, or cooperative paramagnetic state, at temperatures below ~ 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd$\bar{3}$m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at ~ 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures.

cond-mat.dis-nn↗

arXiv2007

Impurity-induced singlet breaking in SrCu2(BO3)2

We have performed $μ$SR studies on single crystals of SrCu$_2$(BO$_3$)$_2$, a quasi-two-dimensional spin system with a spin singlet ground state. We observe two different muon sites which we associate with muons located adjacent to the two inequivalent O sites. One site, presumed to be located in the Cu-O-Cu superexchange path, exhibits a large increase in the frequency shift with decreasing temperature which is unaffected by the singlet formation, indicating that the muon has locally broken the singlet bond. We have also performed $μ$SR on single crystals of SrMg$_{0.05}$Cu$_{1.95}$(BO$_3$)$_2$, Sr$_{0.96}$La$_{0.04}$Cu$_2$(BO$_3$)$_2$, and Sr$_{0.95}$Na$_{0.05}$Cu$_2$(BO$_3$)$_2$. We have found that the frequency shifts of these doped samples are equivalent and contain three branches at low temperatures. Two of these branches map on to the branches observed in the pure sample reasonably well and so we attribute the third branch to effect of the dopants. Specifically, this third branch represents the case when the muon sits at a site in the superexchange path lacking a corresponding singlet, due to it already being broken as a result of doping. This then leads to the conclusion that singlets are broken in this system when it is doped both in and out of the CuBO$_3$ planes.

cond-mat.str-el↗

arXiv2006

In-Gap Spin Excitations and Finite Triplet Lifetimes in the Dilute Singlet Ground State System SrCu(2-x)Mgx(BO3)2

High resolution neutron scattering measurements on a single crystal of SrCu(2-x)Mgx(BO3)2 with x=0.05 reveal the presence of new spin excitations within the gap of this quasi-two dimensional, singlet ground state system. Application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu2(BO3)2. Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.

cond-mat.str-el↗

arXiv2006

Field Induced Order and Spin Waves in the Pyrochlore Antiferromagnet Tb2Ti2O7

High resolution time-of-flight neutron scattering measurements on Tb2Ti2O7 reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T=0.4 K, terbium titanate is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like spin degrees of freedom.

cond-mat.str-el↗

arXiv2005

Ordering and Spin Waves in NaNiO2 : A Stacked Quantum Ferromagnet

Neutron scattering measurements on powder NaNiO2 reveal magnetic Bragg peaks and spin waves characteristic of strongly correlated s=1/2 magnetic moments arranged in ferromagnetic layers which are stacked antiferromagnetically. This structure lends itself to stacking sequence frustration in the presence of mixing between nickel and alkali metal sites, possibly providing a natural explanation for the enigmatic spin glass state of the isostructural compound, LiNiO2.

cond-mat.str-el↗

arXiv2005

Two and Three Dimensional Incommensurate Modulation in Optimally-Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

X-ray scattering measurements on optimally-doped single crystal samples of the high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ reveal the presence of three distinct incommensurate charge modulations, each involving a roughly fivefold increase in the unit cell dimension along the {\bf b}-direction. The strongest scattering comes from the well known (H, K$\pm$ 0.21, L) modulation and its harmonics. However, we also observe broad diffraction which peak up at the L values complementary to those which characterize the known modulated structure. These diffraction features correspond to correlation lengths of roughly a unit cell dimension, $ξ_c$$\sim$20 $Å$ in the {\bf c} direction, and of $ξ_b$$\sim$ 185 $Å$ parallel to the incommensurate wavevector. We interpret these features as arising from three dimensional incommensurate domains and the interfaces between them, respectively. In addition we investigate the recently discovered incommensuate modulations which peak up at (1/2, K$\pm$ 0.21, L) and related wavevectors. Here we explicitly study the L-dependence of this scattering and see that these charge modulations are two dimensional in nature with weak correlations on the scale of a bilayer thickness, and that they correspond to short range, isotropic correlation lengths within the basal plane. We relate these new incommensurate modulations to the electronic nanostructure observed in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ using STM topography.

cond-mat.supr-con↗

arXiv2004

High Resolution Study of Spin Excitations in the Shastry-Sutherland Singlet Ground State of SrCu2(BO3)2

High resolution, inelastic neutron scattering measurements on SrCu2(BO3)2 reveal the dispersion of the three single triplet excitations continuously across the (H,0) direction within its tetragonal basal plane. These measurements also show distinct Q dependencies for the single and multiple triplet excitations, and that these excitations are largely dispersionless perpendicular to this plane. The temperature dependence of the intensities of these excitations is well described as the complement of the dc-susceptibility of SrCu2(BO3)2.

cond-mat.str-el↗

arXiv2004

High Field Specific Heat of 2D Quantum Spin System SrCu2(BO3)2

We report measurements of the specific heat of the quantum spin liquid system SrCu2(BO3)2 in continuous magnetic fields H of up to 33 T. The specific heat vs temperature at zero field shows an anomaly at 8 K, marking the opening of a gap in the spin singlet excitations. At fields H~12 T, we clearly see a second anomaly that shifts to lower temperatures as H is increased. We attribute its origin to single triplet excitations of the singlet dimer ground state. This conclusion is supported by calculations of the specific heat, which reproduce the experimental data, made using the finite temperature Lanczos method to solve a Shastry-Sutherland Hamiltonian including nearest and next-nearest neighbor Dzyaloshinsky-Moriya interactions. The parameters used to fit the data are the exchange constants J = 74 K and J'/J = 0.62, and the Dzyaloshinsky-Moriya coupling constants |D|=6.1K, and $|D'|=2.2K.

cond-mat.str-el↗

arXiv1997

Temperature Dependence of the FIR Reflectance of LaSrGaO4

The reflectance of single crystal LaSrGaO4 has been measured from approx 50 to 40000 cm^-1 along the "a" and "c" axis. The optical properties have been calculated from a Kramers-Kronig analysis of the reflectance for both polarizations. The reflectance curves have been fit using a product of Lorentzian oscillators.

cond-mat.mtrl-sci↗