arXiv subjects

Marek Bartkowiak

Publications and source records attributed to Marek Bartkowiak.

18 recordsLinked to original sources

arXiv2026

Single-ion anisotropy-stabilized short-period helimagnetism in frustrated chiral Co$_5$TeO$_8$

Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here we report that a cubic chiral insulator Co$_5$TeO$_8$ provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7-10 nm embedded in a complex phase diagram spanning eight distinct phases. Capacitance anomalies throughout the phase diagram indicate magneto-electric coupling, pointing to the possibility of future $E$-field control of these textures. The temperature- and field-dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, $ab\,initio$ calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co$_5$TeO$_8$ thus offers an experimental realization of sub-10 nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.

cond-mat.str-el↗

arXiv2026

Effect of Pressure and Oxygen-Isotope Substitution on Density-Wave Transitions in La$_4$Ni$_3$O$_{10}$

Understanding the interplay between magnetism and superconductivity in nickelate systems is a key objective in condensed matter physics. Here, we present a systematic muon-spin rotation/relaxation ($μ$SR) and resistivity study of the trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under ambient and applied pressure, combined with oxygen-isotope substitution. At ambient pressure, two incommensurate spin-density-wave (SDW) transitions are identified at $T_{SDW}\simeq132$ K and $T^\ast\simeq80-90$ K. Comparison of the internal magnetic fields with dipole-field calculations reveals a magnetic structure consistent with antiferromagnetically coupled SDW order on the outer two Ni layers, with smaller moments on the inner layer. Above $T^\ast$, the moments lie mainly in the $ab$ plane, whereas below this temperature they develop a $c$-axis component. The internal fields at the muon stopping sites appear abruptly at $T_{SDW}$, suggesting a first-order-like SDW transition closely linked to the charge-density-wave (CDW) order occurring at the same temperature ($T_{SDW}=T_{CDW}$). Under pressure, all transition temperatures -- $T_{SDW}$, $T^\ast$, and $T_{CDW}$ -- are suppressed at a nearly uniform rate of $\simeq-13$ K/GPa. This contrasts with bilayer La$_3$Ni$_2$O$_7$, where pressure enhances the separation between the SDW and CDW transitions. Oxygen-isotope substitution ($^{16}$O $\rightarrow$ $^{18}$O) shifts $T_{CDW}$ to higher values. The isotope effect on $T_{SDW}$ and $T^\ast$ differs markedly: when CDW and SDW are intertwined, a notable isotope effect is observed on $T_{SDW}$, yielding nearly identical isotope shifts for $T_{CDW}$ and $T_{SDW}$, whereas no isotope effect is detected at $T^\ast$, where the SDW transition occurs independently of the CDW.

cond-mat.supr-con↗

arXiv2025

Synthesis of a high intensity, superthermal muonium beam for gravity and laser spectroscopy experiments

The universality of free fall, a cornerstone of Einstein's theory of gravity, has so far only been tested with neutral composite states of first-generation Standard Model (SM) particles, such as atoms or neutrons, and, most recently, antihydrogen. Extending these gravitational measurements to other sectors of the SM requires the formation of neutral bound states using higher-generation, unstable particles. Muonium, the bound state of an antimuon ($μ^+$) and an electron ($e^-$), offers the possibility to probe gravity with second-generation (anti)leptons, in the absence of the strong interaction. However, the short $μ^+$ lifetime ($τ_μ\approx 2.2~μ$s) and the existing diffuse thermal muonium sources rendered such measurements unfeasible. Here, we report the synthesis of a high-brightness muonium beam, extracted from a thin layer of superfluid helium by exploiting its chemical potential and unique transport properties. The mean longitudinal velocity (${v}\approx 2180~\rm{m/s}$) and narrow distribution (${Δv}< 150 ~\rm{m/s}$) of the atoms characterise a superthermal beam, while yields are similar to the highest intensity diffuse sources. This new beam is expected to enable muonium interferometry and a percent-level measurement of its gravitational acceleration, providing the first direct test of the Weak Equivalence Principle with second-generation (anti)matter. Its unprecedented brightness also opens the way to sub-kHz 1S-2S spectroscopy, enabling precise determination of the muon mass and stringent tests of bound-state quantum electrodynamics.

physics.atom-ph↗

arXiv2025

Strong electron-electron interactions in a dilute weakly-localized metal near a metal-to-insulator transition

Because it is easily switched from insulator to metal either via chemical doping or electrical gating, silicon is at the core of modern information technology and remains a candidate platform for quantum computing. The metal-to-insulator transition in this material has therefore been one of the most studied phenomena in condensed matter physics, and has been revisited with considerable profit each time a new fabrication technology has been introduced. Here we take advantage of recent advances in creating ultra-thin layers of Bohr-atom-like dopants to realize the two-dimensional disordered Hubbard model at half-filling and its metal-to-insulator transition (MIT) as a function of mean distance between atoms. We use gas-phase dosing of dopant precursor molecules on silicon to create arsenic and phosphorus $δ$-layers as thin as 0.4~nm and as dilute as 10$^{13}$~cm$^{-2}$. On approaching the insulating state, the conventional weak localization effects, prevalent at high dopant densities and due to orbital motion of the electrons in the plane, become dominated by electron-electron interaction contributions which obey a paramagnetic Zeeman scaling law. The latter make a negative contribution to the conductance, and thus cannot be interpreted in terms of an emergent Kondo regime near the MIT.

quant-ph↗

arXiv2025

Single-$q$ Cycloid and Double-$q$ Vortex Lattices in Layered Magnetic Semimetal EuAg$_4$Sb$_2$

Recently, a host of exotic magnetic textures such as topologically protected skyrmion lattices has been discovered in several bulk metallic lanthanide compounds. In addition to hosting skyrmion phases, a hallmark of this class of materials is the appearance of numerous spin textures characterized by a superposition of multi-$q$ magnetic modulations: spin moiré superlattices. The nuanced energy landscape thus motivates detailed studies to understand the underlying interactions. Here, we comprehensively characterize and model the three zero-field magnetic textures present in one such material, EuAg$_4$Sb$_2$. Systematic symmetry breaking experiments using magnetic field and strain determine that the ground state incommensurate magnetic phase (ICM1) is single-$q$. In contrast, ICM2 and ICM3 are both double-$q$, \textit{i.e.}, spin moiré superlattices. Further, through application of polarized small angle neutron scattering and spherical neutron polarimetry, we demonstrate that ICM1 is a single-$q$ cycloid and ICM2 and ICM3 are double-$q$ vortex lattices, with Eu moments lying in the $ab$-plane in zero field and with a ferromagnetic component at finite field. Despite the quasi-2D nature of EuAg$_4$Sb$_2$, the modulations propagate out of the \textit{ab}-plane, leading to a shift of the spin texture between triangle lattice planes. Further, the ICM3 to ICM2 transition includes an unusual 45$^\circ$ rotation of the magnetic vortex lattice. Motivated by the coexistence of such drastically different phases in this compound, we conclude by developing a phenomenological model to understand the stability of these states. Our experimental probes and theoretical modeling definitively characterize three different and tunable phases in one material, and provide insight for the design of new topological spin-texture materials.

cond-mat.mes-hall↗

arXiv2025

Pressure-enhanced splitting of density wave transitions in La$_3$Ni$_2$O$_{7-δ}$

The observation of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ under pressure, following the suppression of a high-temperature density wave state, has attracted considerable attention. The nature of this density wave order was not clearly identified. Here, we probe the magnetic response of the zero-pressure phase of La$_3$Ni$_2$O$_{7-δ}$ as hydrostatic pressure is applied and find that the apparent single density wave transition at zero applied pressure splits into two. The comparison of our muon-spin rotation and relaxation experiments with dipole-field numerical analysis reveals the magnetic structure's compatibility with a stripe-type arrangement of Ni moments, characterized by alternating lines of magnetic moments and nonmagnetic stripes at ambient pressure. When pressure is applied, the magnetic ordering temperature increases, while the unidentified density wave transition temperature falls. Our findings reveal that the ground state of the La$_3$Ni$_2$O$_{7-δ}$ system is characterized by the coexistence of two distinct orders -- a magnetically ordered spin density wave and a lower-temperature ordering that is most likely a charge density wave -- with a notable pressure-enhanced separation between them.

cond-mat.supr-con↗

arXiv2024

Correlation of Structural and Magnetic Properties of RFeO3 (R=Dy, Lu)

In orthoferrites the rare-earth (R) ion has a big impact on structural and magnetic properties in particular the ionic size influences the octahedral tilt and the R3+- Fe3+ interaction modifies properties like the spin reorientation. Growth induced strain in thin films is another means to modify materials properties since the sign of strain affects the bond length and therefore directly the orbital interaction. Our study focuses on epitaxially grown (010) oriented DyFeO3 and LuFeO3 thin films, thereby investigating the impact of compressive lattice strain on the magnetically active Dy3+ and magnetically inactive Lu3+ compared to uniaxially strained single crystal DyFeO3. The DyFeO3 films exhibits a shift of more than 20K in spin-reorientation temperatures, maintain the antiferromagnetic Γ4 phase of the Fe-lattice below the spin reorientation, and show double step hysteresis loops for both in-plane directions between 5 K and 390 K. This is the signature of an Fe-spin induced ferromagnetic Dy3+ lattice above the Néel temperature of the Dy. The observed shift in the film spin reorientation temperatures vs lattice strain is in good agreement with isostatic single crystal neutron diffraction experiments with a rate of 2 K/ kbar bar.

cond-mat.mtrl-sci↗

arXiv2024

Tuning of Charge Order by Uniaxial Stress in a Cuprate Superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La(2-x)BaxCuO4, around the x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at approximately 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to the critical stress, above which they stay constant. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller 'islands'. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

cond-mat.supr-con↗

arXiv2023

Non-destructive X-ray imaging of patterned delta-layer devices in silicon

The progress of miniaturisation in integrated electronics has led to atomic and nanometre-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as phosphorous and arsenic. However, the ability to non-destructively obtain atomic-species-specific images of the final structure, which would be an indispensable tool for building more complex nano-scale devices, such as quantum co-processors, remains an unresolved challenge. Here we exploit X-ray fluorescence to create an element-specific image of As dopants in silicon, with dopant densities in absolute units and a resolution limited by the beam focal size (here $\sim1~μ$m), without affecting the device's low temperature electronic properties. The As densities provided by the X-ray data are compared to those derived from Hall effect measurements as well as the standard non-repeatable, scanning tunnelling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, dominated by weak localisation, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the $1.5\times10^{10}$ Sv ($1.5\times10^{16}$ Rad/cm$^{-2}$) exposure of the device to X-rays, all transport data were unchanged to within experimental errors, corresponding to upper bounds of 0.2 Angstroms for the radiation-induced motion of the typical As atom and 3$\%$ for the loss of activated, carrier-contributing dopants. With next generation synchrotron radiation sources and more advanced optics, we foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm.

quant-ph↗

arXiv2023

Commissioning of the novel Continuous Angle Multi-Energy Analysis Spectrometer at the Paul Scherrer Institut

We report on the commissioning results of the cold neutron multiplexing secondary spectrometer CAMEA (\textbf{C}ontinuous \textbf{A}ngle \textbf{M}ulti-\textbf{E}nergy \textbf{A}nalysis) at the Swiss Spallation Neutron Source (SINQ) at the Paul Scherrer Institut, Switzerland. CAMEA is optimized for an efficient data acquisition of scattered neutrons in the horizontal scattering plane, allowing for detailed and rapid mapping of low-energy excitations under extreme sample environment conditions.

physics.ins-det↗

arXiv2022

Microscopic evidence for anisotropic multigap superconductivity in the CsV$_3$Sb$_5$ kagome superconductor

The recently discovered kagome superconductor CsV$_3$Sb$_5$ ($T_c \simeq 2.5$ K) has been found to host charge order as well as a non-trivial band topology, encompassing multiple Dirac points and probable surface states. Such a complex and phenomenologically rich system is, therefore, an ideal playground for observing unusual electronic phases. Here, we report on microscopic studies of its anisotropic superconducting properties by means of transverse-field muon spin rotation ($μ$SR) experiments. The temperature dependences of the in-plane and out-of-plane components of the magnetic penetration depth $λ_{ab}^{-2}(T)$ and $λ_{c}^{-2}(T)$ indicate that the superconducting order parameter exhibits a two-gap ($s+s$)-wave symmetry, reflecting the multiple Fermi surfaces of CsV3Sb5. The multiband nature of its superconductivity is further validated by the different temperature dependences of the anisotropic magnetic penetration depth $γ_λ(T)$ and upper critical field $γ_{\rm B_{c2}}(T)$, both in close analogy with the well known two-gap superconductor MgB$_2$. Remarkably, the high value of the $T_c/λ^{-2}(0)$ ratio in both field orientations strongly suggests the unconventional nature of superconductivity. The relaxation rates obtained from zero field $μ$SR experiments do not show noticeable change across the superconducting transition, indicating that superconductivity does not break time reversal symmetry.

cond-mat.supr-con↗

arXiv2022

Cu-doping effects on the ferromagnetic semimetal CeAuGe

We present a study of Cu-substitution effects in 4f-Ce intermetallic compound CeAu1-xCuxGe, with potentially unusual electronic states, in the whole concentration range (x = 0.0 - 1.0). The parent CeAuGe compound, crystallizing in a non-centrosymmetric hexagonal structure, is a ferromagnetic semimetal with Curie temperature 10 K. Cu-doping on Au-site of CeAuGe, CeAu1-xCuxGe, changes the crystal structure from the non-centrosymmetric (P63mc) to centrosymmetric (P63/mmc) space group at the concentration x ~ 0.5, where the c-lattice constant has a maximum value. Magnetic susceptibility and electrical resistivity measurements reveal that all Cu-doped compounds undergo magnetic phase transition near 10 K, with the maximum transition temperature of 12 K for x = 0.5. The neutron powder diffraction experiments show the ferromagnetic ordering of Ce3+ magnetic moments with a value of ~ 1.2 Bohr magneton at 1.8 K, oriented perpendicular to the hexagonal c-axis. By using symmetry analysis, we have found the solutions for the magnetic structure in the ferromagnetic Shubnikov space groups Cmc'21' and P21'/m' for x < 0.5 and x >= 0.5, respectively. Electrical resistivity exhibits a metallic temperature behaviour in all compounds. The resistivity has a local minimum in the paramagnetic state due to Kondo effects at high doping x = 0.8 and 1.0. At the small Cu-doping level, x = 0.2, the resistivity shows a broad feature at the ferromagnetic transition temperature and an additional transition-like peculiarity at 2.5 K in the ferromagnetic state.

cond-mat.str-el↗

arXiv2021

Optical setup for a piston-cylinder type pressure cell: a double-volume approach

Measurement of the absolute value of the applied pressure in high-pressure muon and neutron experiments is a complicated task. It requires both the presence of a calibration material inside the sample volume as well as additional time for refining the response of the calibrant. Here we describe the use of optical calibrants for precise determination of the pressure value inside the piston-cylinder clamp cells. Utilizing the concept of separate volumes for the sample and the optical media, a new setup for conducting in-situ pressure measurements has been successfully tested. Pressures in both the `sample' and the `optical' volumes were proved to be the same within experimental accuracy. The use of SrB$_{4}$O$_7$:(0.01 Sm$^{2+}$, 0.03 Eu$^{2+}$) as a pressure calibrant allows for a high accuracy of pressure determination by considering up to eight fluorescence lines.

cond-mat.other↗

arXiv2020

$Z_3$-vestigial nematic order due to superconducting fluctuations in the doped topological insulator Nb$_x$Bi$_2$Se$_3$ and Cu$_x$Bi$_2$Se$_3$

A state of matter with a multi-component order parameter can give rise to vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. Vestigial states due to primary spin and charge-density-wave order have been discussed in the context of iron-based and cuprate materials. Here we present the observation of a partially melted superconductor in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z3, i.e. three-state Potts-model symmetry. High-resolution thermal expansion, specific heat and magnetization measurements of the doped topological insulator NbxBi2Se3 reveal that this symmetry breaking occurs at Tnem=3.8 K above Tc=3.25 K, along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish long-range coherence at Tc, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a distortion of the crystalline lattice. Similar results are found for CuxBi2Se3.

cond-mat.supr-con↗

arXiv2017

Tuning magnetic spirals beyond room temperature with chemical disorder

In the past years, magnetism-driven ferroelectricity and gigantic magnetoelectric effects have been reported for a number of frustrated magnets with spiral magnetic orders. Such materials are of high current interest due to their potential for spintronics and low-power magnetoelectric devices. However, their low magnetic order temperatures (typically <100K) greatly restrict their fields of application. Here we demonstrate that the stability domain of the spiral phase in the perovskite YBaCuFeO5 can be enlarged by more than 150K through a controlled manipulation of the Fe/Cu chemical disorder. Moreover we show that this novel mechanism can stabilize the magnetic spiral state of YBaCuFeO5 above the symbolic value of 25°C at zero magnetic field. Our findings demonstrate that the properties of a magnetic spiral, including its wavelength and stability range, can be engineered through the control of chemical disorder, offering a great potential for the design of materials with magnetoelectric properties beyond room temperature.

cond-mat.str-el↗

arXiv2014

Switching of magnetic domains reveals evidence for spatially inhomogeneous superconductivity

The interplay of magnetic and charge fluctuations can lead to quantum phases with exceptional electronic properties. A case in point is magnetically-driven superconductivity, where magnetic correlations fundamentally affect the underlying symmetry and generate new physical properties. The superconducting wave-function in most known magnetic superconductors does not break translational symmetry. However, it has been predicted that modulated triplet p-wave superconductivity occurs in singlet d-wave superconductors with spin-density wave (SDW) order. Here we report evidence for the presence of a spatially inhomogeneous p-wave Cooper pair-density wave (PDW) in CeCoIn5. We show that the SDW domains can be switched completely by a tiny change of the magnetic field direction, which is naturally explained by the presence of triplet superconductivity. Further, the Q-phase emerges in a common magneto-superconducting quantum critical point. The Q-phase of CeCoIn5 thus represents an example where spatially modulated superconductivity is associated with SDW order.

cond-mat.supr-con↗

arXiv2011

Field induced phases in UPt2Si2

The tetragonal compound UPt2Si2 has been characterised as a moderately mass enhanced system with an antiferromagnetic ground state below T_N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt2Si2. Along the crystallographic a axis, at low temperatures, we find a metamagnetic-like transition in fields of the order 40 T, possibly indicating a first order transition. Along the crystallographic c axis, in magnetic fields of B>= ~24 T, we find distinct anomalies in both properties. From our analysis of the data we can distinguish new high field phases above the AFM ground state. We discuss the emergence of these new phases in the context of Fermi surface effects and the possible occurrence of a Lifshitz or electronic topological transition, this in contrast to previous modellings of UPt2Si2 based on crystal electric field effects.

cond-mat.str-el↗

arXiv2009

Two-gap and paramagnetic pair-breaking effects on upper critical field of SmFeAsO$_{0.85}$ and SmFeAsO$_{0.8}$F$_{0.2}$ single crystals

We investigated the temperature dependence of the upper critical field [$H_{c2}(T)$] of fluorine-free SmFeAsO$_{0.85}$ and fluorine-doped SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by measuring the resistive transition in low static magnetic fields and in pulsed fields up to 60 T. Both crystals show that $H_{c2}(T)$'s along the c axis [$H_{c2}^c(T)$] and in an $ab$-planar direction [$H_{c2}^{ab}(T)$] exhibit a linear and a sublinear increase, respectively, with decreasing temperature below the superconducting transition. $H_{c2}(T)$'s in both directions deviate from the conventional one-gap Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A two-gap nature and the paramagnetic pair-breaking effect are shown to be responsible for the temperature-dependent behavior of $H_{c2}^c$ and $H_{c2}^{ab}$, respectively.

cond-mat.supr-con↗