arXiv subjects

Thomas T. M. Palstra

Publications and source records attributed to Thomas T. M. Palstra.

At least 19 recordsLinked to original sources

arXiv2023

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor

The conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state relies on the Zeeman effect of an external magnetic field to break time-reversal symmetry, forming a state of finite-momentum Cooper pairing. In superconductors with broken inversion symmetries, the Rashba or Ising-type spin-orbit coupling (SOC) can interact with either the Zeeman or the orbital effect of magnetic fields, extending the range of possible FFLO states, though evidence for these more exotic forms of FFLO pairing has been lacking. Here we report the discovery of an unconventional FFLO state induced by coupling the Ising SOC and the orbital effect in multilayer 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite momentum cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of normal metal, uniform Ising superconducting phase, and a six-fold orbital FFLO state. This study highlights an alternative route to finite-momentum superconductivity and provides a universal mechanism to prepare orbital FFLO states in similar materials with broken inversion symmetries.

cond-mat.supr-con↗

arXiv2017

Helimagnon resonances in an intrinsic chiral magnonic crystal

We experimentally study magnetic resonances in the helical and conical magnetic phases of the chiral magnetic insulator Cu$_2$OSeO$_3$ at the temperature $T$=5 K. Using a broadband microwave spectroscopy technique based on vector network analysis, we identify three distinct sets of helimagnon resonances in the frequency range 2 GHz $\leq f \leq$ 20 GHz. The extracted resonance frequencies are in accordance with calculations of the helimagnon bandstructure found in an intrinsic chiral magnonic crystal. The periodic modulation of the equilibrium spin direction that leads to the formation of the magnonic crystal is a direct consequence of the chiral magnetic ordering caused by the Dzyaloshinskii-Moriya interaction. The opening of magnon band-gaps allows for excitation of helimagnons with wave vectors that are multiples of the spiral wave vector.

cond-mat.str-el↗

arXiv2017

Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO

We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on the antiferromagnetic NiO(111) single crystal. An SMR with a sign opposite of conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnetic field within the easy plane (111) of the antiferromagnet. This correlation of magnetic moment alignment and external magnetic field direction is realized just by the easy-plane nature of the material without the need of any exchange coupling to an additional ferromagnet. The SMR signal strength decreases with increasing temperature, primarily due to the decrease in Néel order by including fluctuations. An increasing magnetic field increases the SMR signal strength as there are less domains and the magnetic moments are more strongly manipulated at high magnetic fields. The SMR is saturated at an applied magnetic field of $6$~T resulting in a spin-mixing conductance of $\sim10^{18}~ Ω^{-1}$m$^{-2}$, which is comparable to that of Pt on insulating ferrimagnets such as yttrium iron garnet. An argon plasma treatment doubles the spin-mixing conductance.

cond-mat.mes-hall↗

arXiv2015

Spin-Hall magnetoresistance and spin Seebeck effect in spin-spiral and paramagnetic phases of multiferroic CoCr2O4 films

We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts = 28 K) and the spin lock-in transitions (T_{lock_in} = 14 K). The SMR and SSE response in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc = 94 K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the non-collinear magnetic state below Ts and T_{lock-in}. At T > Tc, magnetic field-induced SMR and SSE signals are observed, which can be explained by a high interface susceptibility. Our results show that the spin transport at the Pt|CCO interface is sensitive to the magnetic phases but cannot be explained solely by the bulk magnetization.

cond-mat.mtrl-sci↗

arXiv2014

Surface sensitivity of the spin Seebeck effect

We have investigated the influence of the interface quality on the spin Seebeck effect (SSE) of the bilayer system yttrium iron garnet (YIG) - platinum (Pt). The magnitude and shape of the SSE is strongly influenced by mechanical treatment of the YIG single crystal surface. We observe that the saturation magnetic field H_{sat} for the SSE signal increases from 55.3 mT to 72.8 mT with mechanical treatment. The change in the magnitude of H_{sat} can be attributed to the presence of a perpendicular magnetic anisotropy due to the treatment induced surface strain or shape anisotropy in the Pt/YIG system. Our results show that the SSE is a powerful tool to investigate magnetic anisotropy at the interface.

cond-mat.mtrl-sci↗

arXiv2014

Photo-induced magnetization enhancement in two-dimensional weakly anisotropic Heisenberg magnets

By comparing the photo-induced magnetization dynamics in simple layered systems we show how light-induced modifications of the magnetic anisotropy directly enhance the magnetization. It is observed that the spin precession in (CH3NH3)2CuCl4, initiated by a light pulse, increases in amplitude at the critical temperature TC. The phenomenon is related to the dependence of the critical temperature on the axial magnetic anisotropy. The present results underline the possibility and the importance of the optical modifications of the anisotropy, opening new paths toward the control of the magnetization state for ultrafast memories.

cond-mat.mtrl-sci↗

arXiv2008

Magnetoelectric coupling in the cubic ferrimagnet Cu2OSeO3

We have investigated the magnetoelectric coupling in the lone pair containing piezoelectric ferrimagnet Cu2OSeO3. Significant magnetocapacitance develops in the magnetically ordered state (TC = 60 K). We find critical behavior near TC and a divergence near the metamagnetic transition at 500 Oe. High-resolution X-ray and neutron powder diffraction measurements show that Cu2OSeO3 is metrically cubic down to 10 K but that the ferrimagnetic ordering reduces the symmetry to rhombohedral R3. The metric cubic lattice dimensions exclude a magnetoelectric coupling mechanism involving spontaneous lattice strain, and this is unique among magnetoelectric and multiferroic materials.

cond-mat.mtrl-sci↗

arXiv2008

Dynamics of spin and orbital phase transitions in YVO3

YVO3 exhibits a well separated sequence of orbital and spin order transitions at 200 K and 116 K, followed by a combined spin-orbital reorientation at 77 K. It is shown that the spin order can be destroyed by a sufficiently strong optical pulse within less than 4 ps. In contrast, the orbital reordering transition from C-type to G-type orbital order is slower than 100 ps and goes via an intermediate nonthermal phase. We propose that the dynamics of phase transitions is subjected to symmetry relations between the associated phases.

cond-mat.str-el↗

arXiv2005

Symmetry changes at the ferroelectric transition in the multiferroic YMnO3

We have identified, for the first time, the change in symmetry at the ferroelectric transition TFE near 1023K of the ferroelectromagnet YMnO3. This transition takes place 300K below the transition to the centrosymmetric state at TIP. Single crystal synchrotron diffraction coupled to a group theoretical analysis show that the paraelectric intermediate phase between TIP and TFE has P63/mcm symmetry. This proves that YMnO3 is a proper ferroelectric and not an improper ferroelectric, as suggested by a previous group theoretical assignment. The origin of the ferroelectricity is caused by a correlated tilting of the MnO5 polyhedra along the (100), (110) and (010) directions

cond-mat.mtrl-sci↗

arXiv2005

Electronic Transport Properties of Pentacene Single Crystals upon Exposure to Air

We report the effect of air exposure on the electronic properties of pentacene single crystals. Air can diffuse reversibly in and out of the crystals and controls the physical properties. We discern two competing mechanisms that modulate the electronic transport. The presence of oxygen increases the hole conduction, as in dark four O2 molecules introduce one charge carrier. This effect is enhanced by the presence of visible light. Contrarily, water, present in ambient air, is incorporated in the crystal lattice and forms trapping sites for injected charges.

cond-mat.mtrl-sci↗

arXiv2004

The effect of impurities on the mobility of single crystal pentacene

We have obtained a hole mobility for the organic conductor pentacene of 35 cm2/Vs at room temperature increasing to 58 cm2/Vs at 225K. These high mobilities result from a purification process in which 6,13-pentacenequinone was removed by vacuum sublimation. The number of traps is reduced by two orders of magnitude compared with conventional methods. The temperature depandence of the mobility is consistent with the band model for electronic transport.

cond-mat.soft↗

arXiv2004

Influence of antiferromagnetic ordering on ferroelectric ordering in LuMnO3

We have studied the influence of antiferromagnetic ordering on the local dielectric moments of the MnO_5 and LuO_7 polyhedra by measuring neutron powder diffraction patterns of LuMnO_3 at temperatures near T_N. We show that the coupling is weak, because the magnetic exchange coupling is predominantly in the ab-plane of the MnO_5 trigonal bipyramids, and the electric dipole moments, originating in the LuO_7 polyhedra, are oriented along the hexagonal c-axis. Anomalies in the dielectric properties near T_N are thus caused by the geometric constraints between the MnO_5 and the LuO_7 polyhedra.

cond-mat.str-el↗

arXiv2003

Anisotropy of the Mobility of Pentacene from Frustration

The bandstructure of pentacene is calculated using first-principles density functional theory. A large anisotropy of the hole and electron effective masses within the molecular planes is found. The band dispersion of the HOMO and the LUMO is analyzed with the help of a tight-binding fit. The anisotropy is shown to be intimately related to the herringbone structure.

cond-mat.mtrl-sci↗

arXiv2002

6,13-dihydropentacene and pentacene single co-crystals

6,13-dihydropentacene and pentacene co-crystallise in a ratio of 2:1 during vapour transport of commercial pentacene in a gas flow. The crystal structure is monoclinic P2_1/n and contains one dihydropentacene molecule, and half a pentacene molecule in the asymmetric unit.

cond-mat↗

arXiv2002

Identification of polymorphs of pentacene

Pentacene crystallizes in a layered structure with a herringbone arrangement within the layers. The electronic properties depend strongly on the stacking of the molecules within the layers (Haddon et al., 2002). We have synthesized four different polymorphs of pentacene, identified by their layer periodicity, d(001): 14.1, 14.4, 15.0 and 15.4 A. Single crystals commonly adopt the 14.1 A structure, whereas all four polymorphs can be synthesized in thin film form, depending on growth conditions. We have identified part of the unit cell parameters of these polymorphs by X-ray and electron diffraction. The 15.0 and 15.4 A polymorphs transform at elevated temperature to the 14.1 and 14.4 A polymorphs, respectively. Using SCLC measurements, we determined the mobility of the 14.1 A polymorph to be 0.2 cm^2/Vs at room temperature.

cond-mat↗

arXiv2002

Structural response to O*-O' and magnetic transitions in orthorhombic perovskites

We present a temperature dependent single crystal x-ray diffraction study of twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These data show the evolution of the crystal structure from the ferromagnetic insulating state to the ferromagnetic metallic state. The data are modelled in space group Pnma with twin relations based on a distribution of the b axis over three perpendicular cubic axes. The twin model allows full structure determination in the presence of up to six twin fractions using the single crystal x-ray diffraction data.

cond-mat.mtrl-sci↗

arXiv2002

Orbital order induced metal-insulator transition in (La,Ca)MnO3

We present evidence that the insulator to metal transition in (La,Ca)MnO3 near x~0.2 is driven by the suppression of coherent Jahn-Teller distortions, originating from d type orbital ordering. The orbital ordered state is characterised by large long-range Q2 distortions below To*-o'. Above To*-o' we find evidence for coexistence between an orbital-ordered and -disordered state. This behaviour is discussed in terms of electronic phases of an orbital ordered insulating and orbital-disordered metallic states.

cond-mat.mtrl-sci↗

arXiv2001

Disorder induced hexagonal-orthorhombic transition in (Y,Gd)MnO3

We show that the transition in AMnO3 from the orthorhombic perovskite phase to the hexagonal phase is promoted by inducing disorder on the A-site. The gap between the orthorhombic and the hexagonal phase is widened for disordered, mixed yttrium-gadolinium manganite samples. At the cost of the orthorhombic phase a two phase region emerges. The phase separation exhibits very unusual thermodynamical behaviour. We also show that high pressure synthesis favours the orthorhombic phase. YMnO3 is formed in the orthorhombic phase at 15 kbar.

cond-mat.str-el↗