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Papers by Peter Logg

We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_13 undergoes a structural transitio... more We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_13 undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I'-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T* can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been... more Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been identified within diverse structure families. Superconductivity in the layered iron germanide YFe$_2$Ge$_2$ was first reported in 2014. It stands out from the commonly known iron- based superconductor families for not containing either Group-V or Group-VI elements and has since been predicted to be an unconventional superconductor. The intermetallic ddd-electron system YFe$_2$Ge$_2$ exhibits an unusually high Sommerfeld coefficient of approxSI100millijoule/molekelvin2\approx \SI{100}{\milli\joule/\mole\kelvin^2}approxSI100millijoule/molekelvin2, signalling strong electronic correlations. Its low-temperature normal-state resistivity displays a T1.5T^{1.5}T1.5 power-law temperature dependence, which is an indication of non-Fermi-liquid behaviour. While superconductivity in YFe$_2$Ge$_2$ has been widely observed below TcapproxSI1.9kelvinT_c \approx \SI{1.9}{\kelvin}TcapproxSI1.9kelvin in electric transport measurements, evidence of a bulk superconducting transition has proved elusive. This has...

Physica Status Solidi-rapid Research Letters, 2014

In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of... more In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

In a many-body fermionic system, the suppression of continuous transitions to absolute zero can r... more In a many-body fermionic system, the suppression of continuous transitions to absolute zero can result in a low temperature quantum fluid which deviates strongly from typical metallic behaviour; unconventional superconductivity can be induced by the strange metal region surrounding the zero-temperature phase transition. In this thesis we focus on three systems which demonstrate a highly tunable phase transition, with the aim of pushing them toward the border of a zero-temperature phase transition, and potentially superconductivity. CeAgSb2 is a uniaxial 4f ferromagnet, where physical pressure or a transverse field may be used to tune the magnetic transition towards T = 0 K. Our investigations, however, did not reveal the presence of superconductivity. It is likely that the field tuned transition does not correspond to a true critical point, whilst the high pressure region may be occupied by an antiferromagnetic phase, with the true critical point at higher pressures. However, other interesting features emerge in the electrical resistivity and AC-susceptibility, along with novel thermodynamic signatures linking the magnetisation to the specific heat. The doping series Lu(1-x)YxFe2Ge2 shows an antiferromagnetic transition which is suppressed to absolute zero at a critical concentration x_c=0.2. YFe2Ge2 displays anomalous low temperature behaviour consistent with the proximity to quantum critical fluctuations, along with a superconducting transition which appears in the electrical resistivity beneath a critical temperature of T_c ~ 1.7 K. Using low temperature DC magnetisation measurements, we show that this is a bulk effect, and that the superconductivity in YFe2Ge2 is of type-II. The thermodynamic and BCS properties of the superconducting phase are analysed in line with the parameters we extract experimentally. The superconducting 3-4-13 stannides (Ca,Sr)3Ir4Sn13 show a high temperature structural transition which may be suppressed by the application of hydrostatic pressure or effective chemical pressure. A superc [...]

Ca0.5Sr0.5)3Rh4Sn13 is a member of the substitution series (CaxSr1−x)3Rh4Sn13 which has recently ... more Ca0.5Sr0.5)3Rh4Sn13 is a member of the substitution series (CaxSr1−x)3Rh4Sn13 which has recently been argued to feature a structural quantum critical point at xc = 0.9. In the stoichiometric compound Sr3Rh4Sn13, the structural transition at T * ≈ 138 K has been shown to be second-order. Moving towards xc, we examine the character of the structural transition in (Ca0.5Sr0.5)3Rh4Sn13 (i.e. x = 0.5, T * ≈ 55 K) using electrical resistivity, heat capacity and X-ray scattering. The absence of the thermal hysteresis in specific heat around T * , and the continuous evolution of the superlattice reflection detected by X-ray diffraction are consistent with the scenario that the structural transition associated with a modulation vector q = (0.5 0.5 0) in (Ca0.5Sr0.5)3Rh4Sn13 remains second-order on approaching the quantum critical point.

Journal of Physics: Conference Series, 2012

The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered heavy fer... more The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered heavy fermion system. Ferromagnetism sets in below the Curie temperature Tc=9.6 K at ambient pressure. We have investigated the magnetisation of CeAgSb2 under applied hydrostatic pressure of up to 45 kbar. Tc is suppressed rapidly, and at pressures > 35 kbar it is replaced by an unidentified ordered phase, possibly antiferromagnetism. The ordered magnetic moment in CeAgSb2 is aligned along the c-axis. We investigate the effect of transverse field tuning on Tc, and show that magnetic order at low temperature is suppressed by in-plane fields exceeding about 3 T.

physica status solidi (b)

The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered Kondo lat... more The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered Kondo lattice system. Ferromagnetism sets in below the Curie temperature TC = 9.6 K at ambient pressure and zero fields. We have investigated the sensitivity of the ferromagnetic phase to hydrostatic pressure and a transverse magnetic tuning field using a series of resistivity and magnetization measurements. With pressure, TC is suppressed rapidly, and extrapolates to zero at 35 kbar. The ferromagnetism arising in CeAgSb2 is uniaxial, with the ordered moment aligning along the tetragonal c-axis. The application of an in-plane tuning field similarly suppresses TC, and drives the transition to zero temperature by 2.8 T. The character of the phase transition remains continuous on the approach to T = 0 K, and it is found that the quadratic A coefficient from the resistivity is strongly enhanced near the critical field.

We show that the quasi-skutterudite superconductor Sr3Ir4Sn13 undergoes a structural transition f... more We show that the quasi-skutterudite superconductor Sr3Ir4Sn13 undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T * can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

Physica Status Solidi B-basic Solid State Physics, 2013

The intermetallic compound CeAgSb2 is a rare example of a ferromagnetically ordered Kondo lattice... more The intermetallic compound CeAgSb2 is a rare example of a ferromagnetically ordered Kondo lattice system. The transition temperature of 9.6 K in zero field is sensitive to the application of magnetic fields perpendicular to the easy axis. Therefore transverse field tuning could be used as an effective way to investigate quantum critical behaviour. In this work we present combined measurements of the specific heat and the magnetocaloric effect which allows a comprehensive thermodynamical study of the system. The entropy landscape is constructed as a function of temperature and field tuning.

We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_13 undergoes a structural transitio... more We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_13 undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I'-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T* can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been... more Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been identified within diverse structure families. Superconductivity in the layered iron germanide YFe$_2$Ge$_2$ was first reported in 2014. It stands out from the commonly known iron- based superconductor families for not containing either Group-V or Group-VI elements and has since been predicted to be an unconventional superconductor. The intermetallic ddd-electron system YFe$_2$Ge$_2$ exhibits an unusually high Sommerfeld coefficient of approxSI100millijoule/molekelvin2\approx \SI{100}{\milli\joule/\mole\kelvin^2}approxSI100millijoule/molekelvin2, signalling strong electronic correlations. Its low-temperature normal-state resistivity displays a T1.5T^{1.5}T1.5 power-law temperature dependence, which is an indication of non-Fermi-liquid behaviour. While superconductivity in YFe$_2$Ge$_2$ has been widely observed below TcapproxSI1.9kelvinT_c \approx \SI{1.9}{\kelvin}TcapproxSI1.9kelvin in electric transport measurements, evidence of a bulk superconducting transition has proved elusive. This has...

Physica Status Solidi-rapid Research Letters, 2014

In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of... more In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

In a many-body fermionic system, the suppression of continuous transitions to absolute zero can r... more In a many-body fermionic system, the suppression of continuous transitions to absolute zero can result in a low temperature quantum fluid which deviates strongly from typical metallic behaviour; unconventional superconductivity can be induced by the strange metal region surrounding the zero-temperature phase transition. In this thesis we focus on three systems which demonstrate a highly tunable phase transition, with the aim of pushing them toward the border of a zero-temperature phase transition, and potentially superconductivity. CeAgSb2 is a uniaxial 4f ferromagnet, where physical pressure or a transverse field may be used to tune the magnetic transition towards T = 0 K. Our investigations, however, did not reveal the presence of superconductivity. It is likely that the field tuned transition does not correspond to a true critical point, whilst the high pressure region may be occupied by an antiferromagnetic phase, with the true critical point at higher pressures. However, other interesting features emerge in the electrical resistivity and AC-susceptibility, along with novel thermodynamic signatures linking the magnetisation to the specific heat. The doping series Lu(1-x)YxFe2Ge2 shows an antiferromagnetic transition which is suppressed to absolute zero at a critical concentration x_c=0.2. YFe2Ge2 displays anomalous low temperature behaviour consistent with the proximity to quantum critical fluctuations, along with a superconducting transition which appears in the electrical resistivity beneath a critical temperature of T_c ~ 1.7 K. Using low temperature DC magnetisation measurements, we show that this is a bulk effect, and that the superconductivity in YFe2Ge2 is of type-II. The thermodynamic and BCS properties of the superconducting phase are analysed in line with the parameters we extract experimentally. The superconducting 3-4-13 stannides (Ca,Sr)3Ir4Sn13 show a high temperature structural transition which may be suppressed by the application of hydrostatic pressure or effective chemical pressure. A superc [...]

Ca0.5Sr0.5)3Rh4Sn13 is a member of the substitution series (CaxSr1−x)3Rh4Sn13 which has recently ... more Ca0.5Sr0.5)3Rh4Sn13 is a member of the substitution series (CaxSr1−x)3Rh4Sn13 which has recently been argued to feature a structural quantum critical point at xc = 0.9. In the stoichiometric compound Sr3Rh4Sn13, the structural transition at T * ≈ 138 K has been shown to be second-order. Moving towards xc, we examine the character of the structural transition in (Ca0.5Sr0.5)3Rh4Sn13 (i.e. x = 0.5, T * ≈ 55 K) using electrical resistivity, heat capacity and X-ray scattering. The absence of the thermal hysteresis in specific heat around T * , and the continuous evolution of the superlattice reflection detected by X-ray diffraction are consistent with the scenario that the structural transition associated with a modulation vector q = (0.5 0.5 0) in (Ca0.5Sr0.5)3Rh4Sn13 remains second-order on approaching the quantum critical point.

Journal of Physics: Conference Series, 2012

The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered heavy fer... more The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered heavy fermion system. Ferromagnetism sets in below the Curie temperature Tc=9.6 K at ambient pressure. We have investigated the magnetisation of CeAgSb2 under applied hydrostatic pressure of up to 45 kbar. Tc is suppressed rapidly, and at pressures > 35 kbar it is replaced by an unidentified ordered phase, possibly antiferromagnetism. The ordered magnetic moment in CeAgSb2 is aligned along the c-axis. We investigate the effect of transverse field tuning on Tc, and show that magnetic order at low temperature is suppressed by in-plane fields exceeding about 3 T.

physica status solidi (b)

The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered Kondo lat... more The intermetallic compound CeAgSb2 is an unusual example of a ferromagnetically ordered Kondo lattice system. Ferromagnetism sets in below the Curie temperature TC = 9.6 K at ambient pressure and zero fields. We have investigated the sensitivity of the ferromagnetic phase to hydrostatic pressure and a transverse magnetic tuning field using a series of resistivity and magnetization measurements. With pressure, TC is suppressed rapidly, and extrapolates to zero at 35 kbar. The ferromagnetism arising in CeAgSb2 is uniaxial, with the ordered moment aligning along the tetragonal c-axis. The application of an in-plane tuning field similarly suppresses TC, and drives the transition to zero temperature by 2.8 T. The character of the phase transition remains continuous on the approach to T = 0 K, and it is found that the quadratic A coefficient from the resistivity is strongly enhanced near the critical field.

We show that the quasi-skutterudite superconductor Sr3Ir4Sn13 undergoes a structural transition f... more We show that the quasi-skutterudite superconductor Sr3Ir4Sn13 undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T * can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

Physica Status Solidi B-basic Solid State Physics, 2013

The intermetallic compound CeAgSb2 is a rare example of a ferromagnetically ordered Kondo lattice... more The intermetallic compound CeAgSb2 is a rare example of a ferromagnetically ordered Kondo lattice system. The transition temperature of 9.6 K in zero field is sensitive to the application of magnetic fields perpendicular to the easy axis. Therefore transverse field tuning could be used as an effective way to investigate quantum critical behaviour. In this work we present combined measurements of the specific heat and the magnetocaloric effect which allows a comprehensive thermodynamical study of the system. The entropy landscape is constructed as a function of temperature and field tuning.