About me
My research is focused on thermodynamic, transport, and structural characterization of materials with novel electronic properties at low temperatures. Particular focus is on superconductivity, magnetism, and other systems with electronic phase transitions, where the effects of a magnetic field and field direction are central. The main experimental technique is nanocalorimetry, sometimes combined with concurrent xray diffraction.
Publications
A selection from Stockholm University publication database
2018. M. P. Smylie (et al.). Physical Review B 98 (10)
We investigate the anisotropic superconducting and magnetic properties of singlecrystal RbEuFe(4)As(4 )using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of Tm = 15 K and a superconducting transition temperature of Tc = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of 70 and 42 kG/K for inplane and outofplane fields, respectively, and a surprisingly low superconducting anisotropy of Gamma = 1.7. GinzburgLandau parameters of Kc similar to 67 and Kab similar to 108 indicate extreme typeII behavior. These superconducting properties are in line with those commonly seen in optimally doped Febased superconductors. In contrast, Eu magnetism is quasitwo dimensional (2D), as evidenced by highly anisotropic inplane and outofplane exchange constants of 0.6 K and <0.04 K. A consequence of the quasi2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the CurieWeiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easyplane anisotropy with inplane and outofplane saturation fields of 2 and 4 kG.

2018. J. Kacmarcik (et al.). Physical Review Letters 121 (16)
We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field independent above a characteristic field HDOS, and that the HDOS (T) line displays an unusual inflection near the onset of the longrange 3D chargedensity wave order. The unusual S shape of HDOS (T) is suggestive of two mutually exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilization of a pairdensity wave state, which calls for further investigation in this region of the phase diagram.

2018. Donato Campanini, Zhu Diao, Andreas Rydh. Physical Review B 97 (18)
Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable betaGa with a critical temperature Tc = 6.04(5) K, while stable alphaGa has a much lower Tc < 1.2 K. Here we use a membranebased nanocalorimeter to initiate the transition from alphaGa to betaGa on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of alphaGa. After such treatment, the liquid supercools down to 232 K, where betaGa solidifies. We find that betaGa is a strongcoupling typeI superconductor with Delta(0)/k(B)T(c) = 2.00(5) and a Sommerfeld coefficient gamma(n) = 1.53(4) mJ/molK(2), 2.55 times higher than that in the alpha phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

2017. K. Willa (et al.). Review of Scientific Instruments 88 (12)
Recent advances in electronics and nanofabrication have enabled membranebased nanocalorimetry for measurements of the specific heat of microgramsized samples. We have integrated a nanocalorimeter platform into a 4.5 T splitpair verticalfield magnet to allow for the simultaneous measurement of the specific heat and xray scattering in magnetic fields and at temperatures as low as 4 K. This multimodal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the xray beam, and fast acquisition of xray scans over a wide temperature range without the need for timeconsuming recentering and realignment. Furthermore, on an YBa2Cu3O7delta crystal and a copper foil, we demonstrate a novel approach to xray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the firstorder magnetoelastic phase transition of Fe2P, a material that is of interest for magnetocaloric applications.

2016. Zhu Diao (et al.). Physical Review B. Condensed Matter and Materials Physics 93 (1)
We investigate the electronic specific heat of superoptimally substituted BaFe2(As1x Px(x))(2) single crystals in the superconducting state using highresolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Delta, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump Delta C proportional to Tc(3) , found in many ironbased superconductors, in this system originates from a Tcdependent ratio Delta/k(B)T(c) in combination with a substitutiondependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n(s) yields a penetration depth lambda that decreases with increasing Tc without sharp divergence at the quantum critical point. Uemura scaling indicates that Tc is governed by the Fermi temperature TF for this multiband system.

2015. I. Lukyanchuk (et al.). Nature Physics 11 (1), 2125
Depending on the GinzburgLandau parameter kappa, superconductors can either be fully diamagnetic if kappa < 1/root 2 (type I superconductors) or allow magnetic flux to penetrate through Abrikosov vortices if kappa > 1/root 2 (type II superconductors; refs 1,2). At the Bogomolny critical point, kappa = kappa(c) = 1/root 2, a state that is infinitely degenerate with respect to vortex spatial configurations arises(3,4). Despite indepth investigations of conventional type I and type II superconductors, a thorough understanding of the magnetic behaviour in the nearBogomolny critical regime at kappa similar to kappa(c) remains lacking. Here we report that in confined systems the critical regime expands over a finite interval of kappa forming a critical superconducting state. We show that in this state, in a sample with dimensions comparable to the vortex core size, vortices merge into a multiquanta droplet, which undergoes Rayleigh instability(5) on increasing kappa and decays by emitting single vortices. Superconducting vortices realize NielsenOlesen singular solutions of the Abelian Higgs model, which is pervasive in phenomena ranging from quantum electrodynamics to cosmology(69). Our study of the transient dynamics of AbrikosovNielsenOlesen vortices in systems with boundaries promises access to nontrivial effects in quantum field theory by means of benchtop laboratory experiments.

Article Superconducting gap evolution in overdoped BaFe2(As1xPx)(2) single crystals through nanocalorimetry2015. Donato Campanini (et al.). Physical Review B. Condensed Matter and Materials Physics 91 (24)
We report on specific heat measurements on clean overdoped BaFe2(As1xPx)(2) single crystals performed with a high resolution membranebased nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature gamma(r) = C/T backslash(T > 0) is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a twoband swave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zerotemperature gap of Delta(0) similar to 5.3 meV, corresponding to Delta(0)/k(B)T(c) similar to 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Delta(0) similar to 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on gamma(r), all samples however show similar behavior [gamma(r)(H) gamma(r)(H = 0) proportional to Hn, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

Article Thermodynamics around the firstorder ferromagnetic phase transition of Fe2P single crystals2014. M. Hudl (et al.). Physical Review B. Condensed Matter and Materials Physics 90 (14), 144432
The specific heat and thermodynamics of Fe2P single crystals around the firstorder paramagnetic to ferromagnetic (FM) phase transition at TC similar or equal to 217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived HT phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zerofield transition displays a textbook example of a firstorder transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

2012. Stella Tagliati, Vladimir M. Krasnov, Andreas Rydh. Review of Scientific Instruments 83 (5), 055107
A differential, membranebased nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to submu g in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1 K. The device has several distinctive features: (i) The resistive thermometer, made of a Ge1xAux alloy, displays a high dimensionless sensitivity dlnR/dlnT greater than or similar to 1 over the entire temperature range. (ii) The sample is placed in direct contact with the thermometer, which is allowed to selfheat. The thermometer can thus be operated at high dc current to increase the resolution. (iii) Data are acquired with a set of eight synchronized lockin amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. (iv) Absolute accuracy is achieved via a variablefrequencyfixedphase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 mu g piece of superconducting Pb in various magnetic fields.

2011. Stella Tagliati, Andreas Rydh. Thermochimica Acta 522 (12), 6671
To achieve accurate results in nanocalorimetry a detailed analysis and understanding of the behavior of the calorimetric system is required. There are especially two systemrelated aspects that should be taken in consideration: the properties of the empty cell and the effect of the thermal link between sample and cell. Here we study these two aspects for a membranebased system where heater and thermometer are both in good contact with each other and the center of the membrane. Practical, analytical expressions for describing the frequency dependence of heat capacity, thermal conductance, and temperature oscillation of the system are formulated and compared with measurements and numerical simulations. We finally discuss the experimental conditions for an optimal working frequency, where high resolution and good absolute accuracy are combined.